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The classic high-T-low-P metamorphism of west-central Maine: Is it post-tectonic or syntectonic? Evidence from porphyroblast-matrix relations: Reply
Abstract
Devonian polymetamorphism of the Central Maine belt of sedimentary rocks has been regarded as 'static', the result of advective heat due to sequential emplacement of 'post-tectonic' granite plutons. The following evidence is cited in support of pluton-driven static metamorphism: (1) a spatial relationship between higher-grade metamorphic zones and pluton margins, and (2) a reported random orientation of euhedral porphyroblasts within matrix fabrics. In contrast, our observations of porphyroblast-matrix relations show that regional metamorphism was synchronous with progressive accumulation of plastic strain. Metasedimentary rocks have penetrative grain-shape fabrics defined by mica and quartz. Spatial variations in the style of matrix fabrics reflect contrasts in rheology as a function of lithology, stratigraphy and metamorphic grade. Strain partitioned into zones characterized by a high degree of parallelism between steeply oriented compositional layering and foliation (higher-strain zones) that surround zones in which variably oriented, moderately dipping foliation is not as strongly developed (lower-strain zones). A well-developed moderately to steeply northeast-plunging mineral-elongation lineation is pervasive in both types of structure, and is defined by the same mineral assemblage at the same grade of metamorphism. Thus, we interpret mineral growth to record the accumulation of plastic strain. Biotite 'fish' and quartz-dominated polycrystalline aggregates in asymmetrical pressure-shadows around porphyroblasts, which are elongate in the direction of mineral elongation, show consistent dextral-reverse displacement along the mineral-elongation lineation in the plane of the foliation. Andalusite and staurolite porphyroblasts have preferred orientations, statistically parallel to matrix fabrics, and garnet and staurolite porphyroblasts include a foliation (S(i)) that is discontinuous with matrix foliation (S(e)). In staurolite, the obliquity of S(i) with respect to S(e) decreases through up to three textural zones from core to rim, to record episodic interkinematic growth of porphyroblasts and progressive modification of the matrix. In lower-strain zones, granite plutons are associated with retrogressive replacement of andalusite and staurolite, which reflects preferential contact metamorphism in these structures.
... 147 I), who noted that "metamorphic textures suggest that this metamorphism took place in a nearly static environment or locally in a weak stress environment that was unrelated to that which produced the slaty cleavage (S,)." Solar and Brown (1999) later providcd microstructural observations in support of a very late syntectonic peak of M2 metamorphism. ...
... Gravity and geologic observations suggest that the Mooselookmeguntic pluton is a sheet-like body, approximately 1-2 km thick, gently dipping -5" to the northcast (Carnese, 198 1 ;Bothner and Kucks, 1993). This inferred geometry is also supported by gently-dipping M3 metamorphic isograds (Guidotti, 1970(Guidotti, , 1974, pluton-country rock contacts observed in the field, and by the orientation of the magmatic fabric preserved in the pluton (Moench, 1966(Moench, , 1970Solar, 1999;Solar and Brown, 1999). The currently exposed aureole above the Mooselookmeguntic pluton experienced contact metamorphic conditions consistent with a depth of approximatcly 14-17 km (e.g., ...
... Bladed porphyroblasts with c-axes perpendicular to the surrounding foliation have quartz-dominated strain-shadows parallel to the elongation lineation. In contrast, biotite porphyroblasts with c-axes oriented sub-parallel to the muscovite foliation possess a 'pull-apart' microstructure (Solar and Brown, 1999). The biotite 'pull-aparts' are characterized by an overall rod- The grain size and shape of the included quartz, plagioclase and ilmenite is nearly identical to the size and shape of the samc phases in the matrix. ...
The porphyroblastic pelitic schists of western Maine are ideal for studying the deformation and metamorphism that accompanies pluton emplacement. A regional lowpressure, high-temperature metamorphic event affected the areally extensive Silurian sediments with the thermal peak occuning ca. 404 Ma. This metamorphism produced the widespread assemblage staurolite + andalusite + biotite +I- garnet, and occurred during the late stages of development of a northeast-trending, steeply-dipping axial-surface foliation. A period of extensive plutonism accompanied and followed this Acadian-aged deformation and metamorphism. This study focuses specifically on the contact aureole of the Mooselookmeguntic pluton emplaced ca. 370 Ma. Contact metamorphism accompanied the development of a gently northeast-dipping, crnplacement-related, foliation that varies from a crenulation cleavage in the middle and outer portions of the aureole, to an intense pervasive foliation proximal to the pluton margin. The deformation and metamorphic aureole of this pluton provides a rare opportunity to study the progressive development of crenulation cleavage, and to evaluate porphyroblast kinematics across a clearly defined deformation gradient. Staurolite porphyroblasts in the pelitic country rocks preserve spectacular inclusion trails, the three-dimensional orientations of which can be used to evaluate the geometric and kinematic development of the crenulation cleavage. Outside the pluton aureole, staurolite porphyroblasts overgrow the regional axial surface foliation, preserving straight inclusion trails that are continuous with the matrix foliation. This microstructure is consistent over a large area, and as such can be used as a "control" to evaluate the structural and metamorphic effects of crenulation cleavage development along transects across the pluton-related thermal gradient. Porphyroblast inclusion trails vary systematically from areas of the "control" microstructure outside the aureole through the deformation gradient towards the pluton. This variation provided an opportunity to evaluate the relations among porphyroblast kinematics, crenulation cleavage development and the dominant deformation mechanisms during progressive foliation development. The kinematic behavior of porphyroblasts during crenulation cleavage development has been examined in previous studies, but this thesis represents the first detailed examination across a clearly defined deformation gradient. This study provides new results that demonstrate conclusively that porphyroblasts rotate relative to one another during crenulation cleavage development, and that their kinematic behavior varies in relation to a change in the dominant deformation mechanism at specific stages of crenulation cleavage development. Rotation of porphyroblasts during crenulation cleavage development is intimately linked to the mechanisms of cleavagc formation. Strain in the early stages of crenulation cleavage development is accommodated by dissolution-precipitation creep of quartz and feldspar. Consequently, there is little to no relative rotation of porphyroblasts during the onset of crenulation cleavage. When the crenulation cleavage evolves to the stage where there are no longer significant amounts of quartz and feldspar to dissolve from the phyllosilicate-domains the principle deformation mechanism probably changes to a combination of dislocation creep and diffusion creep. This change in dominant deformation mechanism lead to an increased stress couple between the porphyroblast and the deforming matrix, resulting in the relative rotation of porphyroblasts during the later stages of fabric development.
... Two types of structural zones have been identified within this belt: NE-SW-trending zones of apparent flattening or high strain, and zones of apparent constriction or low strain Solar, 1998a,b, 1999;Solar and Brown, 1999;Solar and Brown, 2001a). ...
... The Central Maine Belt, a major regional unit ( Fig. 1 . Two types of structural zones, trending NW-SW, have been identified within this belt; zones of apparent flattening or high strain, and zones of apparent constriction or low strain (Brown and Solar, 1998 a, b;1999;Solar and Brown, 1999;Solar and Brown, 2001). ...
... Two of them concentrate around 420 Ma and 405 Ma whereas the third one spreads over the 400-350 Ma interval. Porphyroblast-matrix relationships suggest that deformation and metamorphism were synchronous (e.g., Bell et al., 1998;Solar and Brown, 1999;Bell and Welch, 2002;Bell and Newman, 2006), whereas pluton emplacement was deformation-controlled (e.g., Solar et al., 1998;Solar and Brown, 2001). The two distinct peaks, prior to 400 Ma probably indicate that the deformation was more intense and evenly partitioned across the orogen. ...
... Methods and the resulting data lie in separate paragraphs in the following sections. Fig. 2 shows a structural geologic map of NW Maine surrounding the Swift River area in Fig. 1b (Brown and Solar, 1998a,b;Brown, 1999, 2001). Solar and Brown (1999) mapped foliations in detail across this region and constructed closely spaced form lines from their distribution. ...
... Fig. 2 shows a structural geologic map of NW Maine surrounding the Swift River area in Fig. 1b (Brown and Solar, 1998a,b;Brown, 1999, 2001). Solar and Brown (1999) mapped foliations in detail across this region and constructed closely spaced form lines from their distribution. This map (Fig. 2) shows zones of complex folding separated by curvilinear belts where the foliation tends to appear more simply deformed suggesting that significant partitioning of the deformation has taken place. ...
... Eighty-five spatially oriented samples were collected across the boundary between a zone of complex folding and a curvilinear belt Solar and Brown (1999) showing their form lines for foliations. The axial planes of all folds that can be reasonably defined are marked with similar length lines. ...
Regional distributions of axial plane trends retain information on the orientation in which successive generations formed because multi-scale partitioning results in most orogenic belts preserving subsequently undeformed portions of all large-scale folds. At depths greater than ∼10 km within orogens, successions of regional folds are accompanied by the sequential development of crenulation hinges in pelites, which are commonly overgrown early during their development by successive generations of porphyroblasts. Consequently, the original trends of the axial planes of these folds are preserved within the distribution of foliation inflection/intersection axes within porphyroblasts (FIAs). Peaks in the distribution of FIA trends in western Maine predominantly coincide with peaks in the distribution of trends of the axial planes of macroscopic and regional folds. The WNW–ESE (∼420 Ma), N–S (408 ± 10 Ma), W–E (388 ± 9 Ma), WSW–ENE (372 ± 5 Ma), SW–NE (353 ± 4 Ma) succession of FIA peaks defines the sequence of folds and accords with map scale overprinting relationships. This quantitative approach to interpreting fold successions in multiply deformed terrains resolves timing where overprinting criteria are rare, uncertain or obliterated by younger events in portions of the orogen. Significantly, lengthy detailed histories of structural development can be extracted from a small area containing porphyroblastic rocks and applied to very large-scale regions.
... In addition to questions about the details of tectonic assembly of a continental margin that was active for more than 100 Ma, the northern Appalachians present longstanding problems regarding the heat sources for the generation of abundant peraluminous granite bodies (DeYoreo et al., 1989; Chamberlain and Sonder, 1990; Brown and Solar, 1999). Furthermore, orogens control the mechanics of interactions between converging plates; thus, it is important to understand both weakening and hardening mechanisms. ...
... The area of study covers part of the western side of the Central Maine belt where it is in contact with the Bronson Hill belt to the northwest (Figs. 1 and 2). The CMB comprises Siluro-Devonian metasedimentary rocks (metaturbidite) of the bRangeley stratigraphic sequenceQ (Moench, 1971; Moench et al., 1995; Solar and Brown, 2001a), migmatites (Brown and Solar, 1998a, 1999; Solar and Brown, 2001b) and predominantly granitic plutons of various volumes and shapes (e.g., Moench et al., 1995; Brown and Solar, 1998b, 1999; Pressley and Brown, 1999). To the northwest, the CMB is bounded by a Devonian tectonite zone (Fig. 1; Solar and Brown, 2001a) across which the belt is juxtaposed against the BHB. ...
... Oblique (dextral) southeast-side up contraction of the belt was largely accommodated within the broad Central Maine belt shear zone system (Fig. 1; Brown and Solar, 1998a). Solar and Brown (1999, 2001a) and Brown and Solar (1998a,b, 1999) subdivided the region into kilometerscale alternating NE–SW-trending structural zones of apparent flattening (AFZ in Fig. 2) and apparent constrictional finite strain as defined by the bulk rock fabrics, supported by the style and intensity of folds of the metasedimentary rock layers (tighter in the apparent flattening zones). The grade of Devonian regional metamorphism within the CMB ranges from greenschist to upper amphibolite facies; granulite facies assemblages occur in Massachusetts (Chamberlain and Robinson, 1989). ...
Radiogenic isotope data (initial Nd, Pb) and elemental concentrations for the Mooselookmeguntic igneous complex, a suite of mainly granitic intrusions in New Hampshire and western Maine, are used to evaluate petrogenesis and crustal variations across a mid-Paleozoic suture zone. The complex comprises an areally subordinate monzodiorite suite [377±2 Ma; εNd (at 370 Ma)=−2.7 to −0.7; initial 207Pb/204Pb=15.56–15.58] and an areally dominant granite [370±2 Ma; εNd (at 370 Ma)=−7.0 to −0.6; initial 207Pb/204Pb=15.55–15.63]. The granite contains meter-scale enclaves of monzodiorite, petrographically similar to but older than that of the rest of the complex [389±2 Ma; εNd (at 370 Ma)=−2.6 to +0.3; initial 207Pb/204Pb ∼15.58, with one exception]. Other granite complexes in western Maine and New Hampshire are ∼30 Ma older than the Mooselookmeguntic igneous complex granite, but possess similar isotopic signatures.
... Geochemical methods enable comparison between migmatites and plutonic granites (Brown and D'Lemos 1991;Williamson et al. 1997;Pressley and Brown 1999;Brown and Pressley 1999;Solar and Brown 2001b), and between exposed crust and lower crust brought up in xenolith suites in volcanic rocks (Rudnick and Presper 1990;Rudnick 1992;Rudnick and Fountain 1995), although such suites may be biased in favor of better preserved mafic rock types. Further, migmatite terranes may be compared with plu- tonic granites to evaluate the question of whether migmatites are feeder zones for granites (Brown and Solar 1999;Del Moro et al. 1999;Solar and Brown 2001b). There is also the issue of whether metasomatism may play a role in the chemical and rheological evolution of the crust (Franz and Harlov 1998;Harlov et al. 1998). ...
... Heat advected with mantle-derived magmas is important in the development of continental arcs, like the Andes, but the apparent smaller volume of mafic plutonic rocks exposed in ancient collisional belts means the role of mantle-derived magma in the thermal evolution of these belts is not well constrained. A combination of these processes strongly perturbs the geothermal gradient by displacing isotherms toward the surface, creating a thermal antiform with a near-isothermal core (Royden 1993;Thompson et al. 1997;Huerta et al. 1998;Jamieson et al. 1998), and generating temperatures exceeding the wet solidus and the stability of mica and amphi- bole in common crustal rocks (Brown and Solar 1999). ...
... The maximum thermal effect of pluton-driven metamorphism in the upper crust occurs when intrusions are con- temporaneous, but significant heating will occur even when intrusions are temporally separate (Barton and Hanson 1989). If the granites are crustally derived, however, the first order problem is what causes crustal melting, and we should avoid follow- ing casually an implied cause and effect (Brown and Solar 1999;Gerdes et al. 2000). ...
The type ofP-T-t path and availability of fluid (H2O-rich metamorphic volatile phase or melt) are important variables in metamorphism. Collisional orogens are characterized
by clockwiseP-T evolution, which means that in the core, where temperatures exceed the wet solidus for common crustal rocks, melt may be
present throughout a significant portion of the evolution. Field observations of eroded orogens show that lower crust is migmatitic,
and geophysical observations have been interpreted to suggest the presence of melt in active orogens. A consequence of these
results is that orogenic collapse in mature orogens may be controlled by a partially-molten layer that decouples weak crust
from subducting lithosphere, and such a weak layer may enable exhumation of deeply buried crust. Migmatites provide a record
of melt segregation in partially molten crustal materials and syn-anatectic deformation under natural conditions. Grain boundary
flow and intra-and inter-grain fracture flow are the principal grain scale melt flow mechanisms. Field observations of migmatites
in ancient orogens show that leucosomes occur oriented in the metamorphic fabrics or are located in dilational sites. These
observations are interpreted to suggest that melt segregation and extraction are syntectonic processes, and that melt migration
pathways commonly relate to rock fabrics and structures. Thus, leucosomes in depleted migmatites record the remnant permeability
network, but evolution of permeability networks and amplification of anomalies are poorly understood. Deformation of partially
molten rocks is accommodated by melt-enhanced granular flow, and volumetric strain is accommodated by melt loss. Melt segregation
and extraction may be cyclic or continuous, depending on the level of applied differential stress and rate of melt pressure
buildup. During clockwiseP-T evolution, H2O is transferred from protolith to melt as rocks cross dehydration melting reactions, and H2O may be evolved above the solidus at lowP by crossing supra-solidus decompression-dehydration reactions if micas are still present in the depleted protolith. H2O dissolved in melt is transported through the crust to be exsolved on crystallization. This recycled H2O may promote wet melting at supra-solidus conditions and retrogression at subsolidus conditions. The common growth of ‘late’
muscovite over sillimanite in migmatite may be the result of this process, and influx of exogenous H2O may not be necessary. However, in general, metasomatism in the evolution of the crust remains a contentious issue. Processes
in the lower-most crust may be inferred from studies of xenolith suites brought to the surface in lavas. Based on geochemical
data, we can use statistical methods and modeling to evaluate whether migmatites are sources or feeder zones for granites,
or simply segregated melt that was stagnant in residue, and to compare xenoliths of inferred lower crust with exposed deep
crust. Upper-crustal granites are a necessary complement to melt-depleted granulites common in the lower crust, but the role
of mafic magma in crustal melting remains uncertain. Plutons occur at various depths above and below the brittle-to-viscous
transition in the crust and have a variety of 3-D shapes that may vary systematically with depth. The switch from ascent to
emplacement may be caused by amplification of instabilities within (permeability, magma flow rate) or surrounding (strength
or state of stress) the ascent column, or by the ascending magma intersecting some discontinuity in the crust that enables
horizontal magma emplacement followed by thickening during pluton inflation. Feedback relations between rates of pluton filling,
magma ascent and melt extraction maintain compatibility among these processes.
... In the northern Appalachian orogen, deformation was partitioned heterogeneously during dextral transpression in response to Early Devonian oblique convergence (van Staal and de Roo, 1995; van Staal et al., 1998 ). Dextral±SE-sideup displacement was accommodated within the CMB shear zone system (Brown and Solar, 1998a; Solar et al., 1998; Solar, 1999; Solar and Brown, 1999 ) while dextral±transcurrent displacement was accommodated within the Norumbega shear zone system (Swanson, 1992; West and Hubbard, 1997; West, 1999) along the southeastern side of the CMB (Fig. 1). By the Carboniferous, deformation had ceased within the CMB shear zone system and strain had localized into the Norumbega shear zone system (Hubbard et al., 1995; West and Hubbard, 1997; Ludman, 1998; West, 1999). ...
... Thus, the metamorphism is of high-T±low-P type, although the metamorphic ®eld gradient is the product of polymetamorphism (e.g. Guidotti, 1989 ) related to plutondriven thermal pulses (De Yoreo et al., 1991) overprinted on a regionally elevated thermal gradient that resulted from transpression (Brown and Solar, 1999; Solar and Brown, 1999). ...
... The principal structures of western Maine are: (1) the kilometer scale open to tight folds of the stratigraphic succession, de®ned by the orientation of centimeter to decimeter scale psammite±pelite compositional layers (Fig. 1; seeFig. 4 , section A±A 0 ); and, (2) the kilometer scale alternation of structural zones, de®ned by the NE±SW-striking and steeply SE dipping domainal structure of the CMB shear zone system and the characteristic fabrics of the alternating zones (Figs. 2 and 3; Solar and Brown, 1999). The regional structure is well illustrated by the foliation form line map ofFig. ...
Transpressive deformation was distributed heterogeneously within the Central Maine belt shear zone system, which formed in response to Early Devonian oblique convergence during the Acadian orogeny in the northern Appalachians. ‘Straight’ belts are characterized by tight folds, S>L fabrics and sub-parallel form lines, and asymmetric structures that together indicate dextral–SE-side-up kinematics. In contrast, intervening zones between ‘straight’ belts are characterized by open folds and L≫S fabrics. Within both types of zone, metasedimentary rocks have fabrics defined by the same minerals at the same metamorphic grade, including a penetrative, moderately to steeply NE-plunging mineral lineation. Thus, we interpret accumulation of plastic deformation and regional metamorphic (re-) crystallization to have been synchronous across the Central Maine belt shear zone system. Discordance between inclusion trails in regionally developed porphyroblasts of garnet and staurolite and matrix fabrics in ‘straight’ belt rocks records shortening by tightening of folds and greater reorientation of matrix fabrics with respect to porphyroblasts. Kinematic partitioning of flow was responsible for the contrasting states of finite deformation recorded in the Central Maine belt shear zone system. Perturbations in the flow were caused by serially developed thrust-ramp anticlines in the stratigraphic succession immediately above the Avalon-like basement, at which décollement of the shear zone system was initially rooted. General shear deformation at the ramps involved strain softening with an enhanced component of noncoaxial flow. In contrast, deformation during extrusion in the intervening zones involved strain hardening with a greater component of coaxial flow. Part of the thickening stratigraphic succession exceeded Tsolidus, reflected by the occurrence of migmatites and granites. The latter were partly sourced from the underlying Avalon-like basement that was involved in the deformation and melting.
... Pelite layers are staurolite bearing throughout, locally andalusite bearing (pseudomorphs), and show distinct centimeterscale 'P'-and 'Q'-domains defined by matrix minerals (Solar and Brown, 2001a). The long-axis orientations of staurolite porphyroblasts were measured at two locations here, and those data are discussed in Solar and Brown (1999;see their Figures 4b and 4c). ...
... This "new" growth is due to thermal aureole effects of the nearby Mooselookmeguntic pluton (west of this location; Figures 2 and 11, and STOPS 9 and 10). The long-axis orientations of the pseudomorphs were measured here, and those data are discussed in Solar and Brown (1999;see their Figure 4h). ...
... Indeed, in the metamorphic realm, the intimate rela- tionship between deformation and metamorphism has made the specialties of petrology and struc- tural geology essentially seamless (e.g. Vernon, 1976;Williams, 1994;Johnson and Vernon, 1995;Hodges, 1998;Solar and Brown, 1999). ...
... For illustrative purpose, I take as an example the Devonian Acadian metamorphism of the Northern Appalachians, USA (see Brown and Solar, 1998a, b;Solar and Brown, 1999, 2001a. The Acadian metamorphic belt is characterized by elevated modern-day heat flow ( $65 m Wm À2 ) and high heat production ($3.5Â10 À6 Wm À3 ). ...
Thirty-five years ago the introduction of the plate tectonics paradigm led to a new understanding of orogeny. Subsequently, the development of advanced instruments for remote collection of information and for analysis of elemental and isotopic composition of materials, and the increases in computing power have enabled an unprecedented number of high-precision data about the Earth to be collected, analyzed, modelled and displayed. Within this revolution in global tectonics, the metamorphic petrologist has developed methods to unravel the depth, thermal, temporal and deformational history of orogens using detailed observations at map, hand sample and thin-section scales in combination with elemental and isotope data, and using inverse and forward modelling. Two exciting new directions in metamorphic petrology in relation to geodynamics concern the kinship between earthquakes and metamorphic reactions in subduction zones, and the petrology of the Earth’s mantle. Evidence of the changes in pressure (P) and temperature (T) in the Earth’s crust and upper mantle during the break up, movement, and collision of pieces of the continental lithosphere is sporadically recorded by the mineralogy and microstructures preserved in rocks exhumed to the surface. Better calibration of phase equilibria, the use of internally-consistent thermodynamic data sets and the development of techniques to retrieve close-to-peak P–T conditions from metamorphic rocks have yielded more precise P–T data that enhance our ability to characterize the path followed by individual rocks in P–T space. An improved ability to date segments of the P–T path, and to separate the length of time associated with the prograde (increasing T) evolution from the age of close-to-peak P–T conditions has enabled better understanding of the rates and processes involved in lithosphere thickening. At the same time, better constraints on the retrograde thermal history have contributed to our knowledge of the several tectonic processes that may operate during exhumation, although these are less well understood. The expanding database of key information, combined with predictions from modelling, has allowed the identification of characteristic P–T–t evolutions expected for rocks that have undergone distinct tectono-metamorphic histories. However, relating structural events recorded by rocks to specific points along the P–T evolution remains problematic, particularly regarding complex overprinting patterns of inclusion trails in porphyroblasts. These advances have improved our understanding of the tectonic evolution of orogens. At the extreme of conditions for crustal metamorphism are the recently discovered ultra-high pressure (UHP) and ultra-high temperature (UHT) facies of metamorphism. Both are problematic given our limited knowledge of processes at these conditions, particularly the return of UHP rocks from peak-P conditions and the mechanism for extreme heat in the crust in UHT metamorphism. The extreme depth inferred for metamorphism in some UHP terranes raises the issue of whether theoretically plausible tectonic overpressures can be dynamically maintained to affect metamorphic reactions. If the pressure gradient recorded by UHP rocks is greater than lithostatic, the UHP metamorphism may have occurred at depths shallower than currently believed. These studies have provided a reliable first-order framework for the comparison of rocks of ancient suture zones where the plate tectonics situation is less certain. However, orogens are spatially and temporally extended nonlinear systems with feedback relations. Such complex systems generate apparently simple behavior by self-organization, and the influence of unique histories must be respected.
... Cordierite of regional metamorphic origin is present in sulfide-rich rocks of the Small Falls Formation (e.g., Henry 1981; Holdaway et al. 1982; Guidotti and Holdaway 1993; Guidotti et al. 1996). The growth of porphyroblasts attributed to regional metamorphism and, by inference, the regional metamorphic peak, was synchronous with the terminal stages of accumulation of plastic strain during Acadian orogenesis (i.e., late-syntectonic; Solar and Brown 1999, 2001a). Syn-metamorphic shortening of the CMB is interpreted to have been accommodated preferentially within the " straight " belts by tightening of folds and southeast-side up (reverse) shear (Solar and Brown 2001a). ...
... Holdaway et al. (1982) and Guidotti et al. (1996) postulated that such a P increase may have been the result of post-tectonic extrusion of volcanic rocks or pluton emplacement at levels above that of the current exposure. Given the evidence from porphyroblast-matrix relations for the ongoing accumulation of plastic strain at and immediately after the metamorphic peak (e.g., Solar and Brown 1999, 2001a), we suggest that an equally plausible explanation for the P increase is the final increment of tectonic thickening during the waning stage of Acadian orogenesis. Furthermore, following the peak of Acadian regional metamorphism, the brittle-viscous transition zone will be displaced downwards as the orogen cools (Handy et al. 2001 ). ...
The peak of regional metamorphism in western Maine was reached at ca. 404 Ma during the waning stage of Devonian Acadian deformation. Regional metamorphic mineral assemblages in metapelitic rocks range from greenschist to upper amphibolite facies. Subsolidus rocks are characterized by the association andalusite + staurolite; at the highest grades, anatectic migmatites are developed. Results of thermodynamic modeling in the MnNCKFMASH system are consistent with field data and imply a metamorphic field gradient that extends from 3.5-4.0 kbar at lower grades (500-520 °C) to > 4.5 kbar at suprasolidus temperatures that exceeded 700 °C. Regional isotherms that are inferred to have been shallowly inclined at lower grades are closely spaced around synmetamorphic granites and at the migmatite front, consistent with advection-controlled intracrustal redistribution of heat within the regionally extensive thermal high. Peak pressures vary both along and across the strike of the Central Maine belt, which is interpreted to record differential thickening during syntectonic metamorphism. Contact metamorphism associated with the Mooselookmeguntic igneous complex occurred ca. 35 million years after the regional metamorphic peak, and records higher pressure conditions than the regional event. We suggest that the final increment of late-Acadian thickening accounts for the pressure increase, consistent with regional cooling prior to the emplacement of the Mooselookmeguntic igneous complex. Pluton emplacement at deeper levels ca. 35 million years after the peak of Acadian metamorphism reflects lowering of the brittle-viscous transition zone, a level at which ascending magma is trapped, consequent on regional cooling and a steeper geotherm. An overall counter-clockwise P-T-t evolution is implied in the Central Maine belt, consistent with that proposed for Acadian metamorphism in western New Hampshire.
... larger tectonic displacements within the AFZs. The pelite layers exhibit porphyroblast–matrix microstructures that The Central Maine belt (CMB) of the northern Apsuggest syntectonic growth of porphyroblast minerals palachians is the principal tectonostratigraphic unit of during progressive tightening of regional-scale folds (Solar the eastern part of New England and New Brunswick & Brown, 1999, 2000). Because a well-developed (Fig. 1). ...
... elevated thermal gradient (Brown & Solar, 1999). Sep-syntectonically (Solar & Brown, 1999, 2000), it follows that both metamorphism and plutonism were synarate periods of metamorphism (e.g. Guidotti, 1963 Guidotti, , chronous with deformation. ...
In Maine, Siluro-Devonian turbidites were metamorphosed under high- T –low- P facies series conditions during deformation within a Devonian crustal-scale shear zone system, defined by kilometer-scale straight belts of apparent flattening strain that anastomose around lozenges of apparent constrictional strain. At upper amphibolite facies grade, metapelites are partially melted, the onset of which is recorded by a migmatite front. The resulting migmatites are stromatic or heterogeneous, and smaller-volume granites form sheets or cylinders according to the structural zone in which they occur, suggesting that migmatites and granites record syntectonic melt flow through the deforming crust. Common leucogranite of the nearby coeval Phillips pluton, which was emplaced syntectonically, was sourced from crustal rocks with geochemical characteristics similar to those of the host Siluro-Devonian succession. Migmatites have melt-depleted compositions relative to metapelites. Leucosomes are peraluminous and represent the cumulate products of fractional crystallization and variable loss of evolved fractionated liquid. Among the heterogeneous migmatites are schlieric granites, the geochemistry of which suggests melt accumulation before fractional crystallization and loss of the evolved liquid. Smaller-volume granites are peraluminous with a range of chemistries that reflect variable entrainment of residual plagioclase and biotite, accumulation of products of fractional crystallization and loss of most of the evolved liquid. Common leucogranite of the Phillips pluton and larger granites in the migmatites have compositions that suggest crystallization of evolved liquids derived by fractional crystallization of primary muscovite dehydration melts. We infer that the leucogranite represents the crystallized fugitive liquid from a migmatite source similar to that exposed nearby. Water transported through the shear zone system dissolved in melt was exsolved at the wet solidus to cause retrogression in sub-solidus rocks and retrograde muscovite growth in migmatites.
... The voluminous magmatic intrusions together with the extensive correlation between the pluton boundaries and metamorphic isograd led some authors to give the term regional thermal metamorphism to describe these LP-HT metamorphisms (e.g. Solar and Brown 1999;Holdaway 1982). Porphyroblastic andalusite and/or cordierite are the main metamorphic phases to grow in pelitic and calc-pelitic rocks metamorphosed at LP-HT metamorphic conditions (Bucher and Frey 1994). ...
Low-pressure metamorphic assemblages are very common in many orogenic belts. Characteristically, they are related to the thermal metamorphism caused by post-orogenic rift-related plutonism. However, it has proven correct that magmatic activity at any stage of the orogen can provide the required temperature to initiate and drive the metamorphism. The Arabian–Nubian Shield (ANS), the northern part of the Neoproterozoic East African Orogen (EAO), is characterized by widespread exposures of the pelites-dominated metasedimentary terranes originated in different tectonic settings. Low-pressure metamorphic belts constitute important components, controversial in places, of some of these terranes. The current chapter documents the field characterization of four multiply deformed and metamorphosed low-pressure/medium- to high-temperature metamorphic belts located in the Eastern Desert and Midyan terranes in the extreme north-eastern tip of the ANS.
... M2 porphyroblast microstructures show a progressive growth evolution in relation to the regional Acadian deformation (e.g., Moench, 1970;Solar and Brown, 1999;Johnson et al., 2006), with syntectonic biotite and garnet porphyroblasts overgrown by very late syntectonic staurolite porphyroblasts (Fig. 6). M2 staurolite porphyroblasts at the outer edge of the deformation aureole typically show inclusion trails with only slight angular discordance with the matrix foliation ( Fig. 6), indicating that the metasediments immediately outside of the deformation aureole in Figure 3 were not pervasively affected in a measurable way by post-M2 regional deformation. ...
The 370–377 Ma Mooselookmeguntic igneous complex was emplaced at ~14 km depth into steeply dipping metaturbidites that were folded and metamorphosed ca. 400–405 Ma during the Acadian orog-eny. Gravity and drill-hole data, isograd geometry, thermal modeling, and struc-tural measurements all indicate that the eastern aureole of the complex represents an areally extensive roof above the gently east-dipping, tabular-shaped intrusion. This roof preserves a classic example of low-pressure, high-temperature metamorphism caused by the underlying intrusion. Ther-mal modeling indicates that the roof is no more than 1000 m thick over an area exceed-ing 100 km 2 . Unlike other roofs described in the literature, the Mooselookmeguntic igneous complex roof preserves a thick (~600 m) emplacement-related strain gradi-ent with a deformational fabric that evolved through all stages of crenulation cleavage to become an intense, nondifferentiated folia-tion approximately parallel to the intrusive contact and perpendicular to the steeply dip-ping, regional Acadian-age foliation. Fabric evolution through all stages of crenulation cleavage requires the vertical growth of the tabular intrusion to have been largely accommodated by dissolution-precipitation creep, which is a linear viscous deformation mechanism that can occur at lower differ-ential stress than, for example, dislocation creep. As the crenulation cleavage evolves, a stage is reached where further deformation requires a change in deformation mecha-nism, which may lead to "hardening" of the rock. Development of crenulation cleavage requires a mica-rich foliation at an initially high angle to the emplacement-related fl at-tening plane, and this confi guration may be the primary reason why the strain aureole in the Mooselookmeguntic igneous complex roof is so well developed compared to other published examples of midcrustal roofs.
... In other examples, temperature increase leads eventually to partial melting and the generation of migmatites that are found in the axial zone of some orogenic belts but also along the edges of orogenic plateaus (see (Vanderhaeghe, 2009) for a review). Some of these migmatites display steep foliation planes and are interpreted to have recorded vertical extrusion as exemplified by the northern Appalachians (Solar and Brown, 1999) or the Bohemian massif (Racek et al., 2006;Schulmann et al., 2008 Fig. 6. P, T, t of Alpine orogenic belts. ...
Convergent plate boundaries are characterized by the development of
crustal orogenic wedges and orogenic plateaus but also by gravitational
collapse of previously thickened crust leading to the opening of
intermontane and eventually oceanic back-arc basins. Foreland and
extensional sedimentary basins in the plate boundary region are filled
by the erosional products of the orogenic crust. Metamorphic rocks
forming orogenic crust attest to burial and exhumation under contrasted
geothermal gradients. These features portray the crustal orogenic cycle
and are first-order indicators of the thermal and mechanical evolution
of the crust within the plate boundary region. This evolution is
controlled by complex interactions among (i) the dynamic balance among
forces that arise from plate-tectonic, gravitational potential energy,
and buoyancy, (ii) the thermal balance between deformation-induced and
radioactive heat production and heat advection related to subduction,
orogenic deformation, and magma transfer, and (iii) the mass transfer
balance between uplift and erosion. To account for these geological
characteristics, a generic model, that integrates results from physical
modeling, is proposed for the thermal-mechanical evolution of crustal
orogenic belts and for its implication in controlling the transition
between the different phases of the orogenic cycle. In this model, the
transition from low to high geothermal gradient is associated with
increased heat production in the thickened crust owing to radioactive
decay and deformation. Partial melting and rheologic weakening of the
thermally mature thickened crust triggers gravity-driven lateral flow of
the lower crust and controls the transition from wedge to orogenic
plateau. Destruction of the orogenic crust is achieved in part by
erosion but mostly by gravitational collapse. The style of extension is
controlled by the rheology of the crust at the onset of gravitational
collapse and its evolution as the crust thins and cools. Gravitational
collapse is permitted by a modification of lithosphere dynamics in the
convergence zone and might eventually lead to opening of a new oceanic
basin if collapse is followed by thinning of the lithospheric mantle.
... Inclusion trails within porphyroblasts (which represent a matrix foliation that was overgrown by the porphyroblast) reveal the inter-relationships between multiple long-lived deformation and Tectonophysics 587 (2013) [133][134][135][136][137][138][139][140][141][142][143][144][145] metamorphic events (e.g., Aerden, 1994;Bakker et al., 1989;Bell and Johnson, 1989;Bell and Rubenach, 1983;Ilg and Karlstrom, 2000;Johnson, 1999;Solar and Brown, 1999;Williams, 1994;Zwart, 1962), and provide information on the kinematics of paleo-tectonic movements (e.g., Aerden and Sayab, 2008;Bell et al., 1995;Shah et al., 2011). Such inclusion trails, with various geometries, are found in metasedimentary rocks of the northeastern Taebaeksan Basin, within porphyroblasts of andalusite, chloritoid, garnet, and staurolite. ...
The Permo-Triassic Songrim (Indosinian) orogeny in South Korea was a major tectonic event involving complicated continental collisions at the eastern margin of Eurasia. Previous studies have examined the structural and metamorphic features of the Songrim orogeny in each of the Paleozoic terranes of the orogenic belt (i.e., the Taebaeksan Basin, the Okcheon Basin, and the Imjingang Belt), but correlations of these features among the terranes remain uncertain. The aim of this paper is to reveal deformation history including bulk crustal shortening directions in the Taebaeksan Basin, and to correlate the tectono-metamorphic evolution of the Taebaeksan Basin with other Phanerozoic mobile belts in eastern Asia based on a combined analysis of foliation intersection/inflection axes (FIA) trends and metamorphic P–T and T–t (time) paths. The orientations and relative timing of FIA preserved as inclusion trails within porphyroblasts of andalusite, chloritoid, garnet, and staurolite reveal two age groups of inclusion trails in the Pyeongan Supergroup at the northeastern margin of the Taebaeksan Basin. These microstructures indicate the development of early NNW–NNE-trending structures and fabrics, followed by later E–W-trending ones. These observations suggest a change in the orientation of bulk crustal shortening from E–W to N–S during the Songrim orogeny. Based on the similar microstructures and temperature–time paths of the three Paleozoic terranes, we interpret that the E–W bulk crustal shortening influenced the eastern part of the Korean Peninsula during the early stages of the Songrim orogeny, presumably related to amalgamation between the proto-Japan terrane and the eastern margin of Eurasia, whereas the N–S bulk crustal shortening was stronger in the western part of the peninsula during the later stages of the orogeny, related to collision between the South and North China blocks.
... However, three recent studies have argued for passive porphyroblast rotation with fold limbs (e.g.,Fig. 10) during macroscale folding (Kraus and Williams 1998; Solar and Brown 1999) or pluton emplacement (Morgan et al. 1998 ). Further studies of this sort would be valuable, particularly if inclusion-trail orientation data could be gathered from both limbs of a well-exposed macroscale fold unaffected by deformation events that post-date porphyroblast growth. ...
Many recent papers show how porphyroblast microstructures play an important role in a wide range of structural and metamorphic
studies. This paper reviews ten current applications of these microstructures: (1) porphyroblast growth-timing criteria; (2)
tracking progressive foliation development relative to changing metamorphic conditions; (3) timing of pluton emplacement relative
to deformation and metamorphism; (4) finite longitudinal strain determinations; (5) kinematics and porphyroblast rotation;
(6) use of linear fabrics preserved in porphyroblasts; (7) porphyroblasts and folding mechanisms; (8) inclusion-trail orientations
and orogenic processes; (9) inferring shear-strain rates from porphyroblast growth rates; and (10) in-situ age determinations.
Although there is still no concensus on the interpretation of some porphyroblast microstructures, a bright future lies ahead
as traditional and newly developed techniques of microstructural analysis are combined with modern chemical and microprobe
techniques to provide an increased understanding of the relationships between deformation and metamorphism in a wide range
of metamorphic settings.
... abundant granitoids) are rarely observed in the immediate vicinity of these terrains and that they can be explained in terms of conductive geotherms in unusual tectonic settings (e.g. Harley, 1989;Sandiford & Hand, 1998;Solar & Brown, 2000). ...
Migmatitic granulites and arc-related felsic intrusives of Pan-African age form the bedrock in the Rio de Janeiro area, SE Brazil. These rocks preserve a partial record of three parageneses. The earliest assemblage (M1) grew during Fabric formation in the rocks (D1 and is characterized by the mineral assemblage Pl + Bt + Sil + Kfs + Qtz. Peak metamorphic conditions (M2) are characterized by the assemblage Bt + Crd + Kfs + Pl + Grt + liq + Qtz and are inferred to have developed during D2 folding of the rocks at T = 750-800°C and P = 7 kbar. M3 reaction textures overprint the M2 assemblage and comprise symplectitic intergrowth of cordierite(II) and quartz that formed after garnet, whereas secondary biotite formed as a result of reactions between garnet and K-feldspar. By comparing the observed modal abundances with modal contours of garnet, cordierite and quartz on the relevant pseudosection a post M 2 P-T vector indicating contemporaneous cooling and decompression can be deduced. The inferred equilibrium assemblage and reaction textures are interpreted to reflect a clockwise P-T path involving heating followed by post-peak decompression and associated cooling. We infer that metamorphism occurred in response to advective heating by the abundant syn-collisional (arc-related) I-type granitoids in the region, consistent with the unusually high peak T/P ratio.
... As an alternative to bulk magmatic flow, Miyazaki (2004) calculated that pervasive flow of melt (cf. Brown & Solar , 1999 ; Weinberg , 1999 ; Leitch & Weinberg , 2002) could raise the temperature of a region to con - ditions suitable for low-pressure metamorphism and lowpressure anatexis . Thermal modelling indicates that temperatures of 600 – 800 °C at 15 km depth are attainable ( Miyazaki , 2004 ) . ...
Low-pressure anatexis, whereby rocks melt in place after passing through the andalusite stability field, develops under more restricted conditions than does low-pressure metamorphism. Our thermal modelling and review of published work indicate that the following mechanisms, operating alone, may induce anatexis in typical pelitic rocks without inducing wholesale melting in the lower crust: (i) magmatic advection by pervasive flow; (ii) crustal-scale detachment faulting; and (iii) the presence of a high heat-producing layer. Of these, only magmatic advection by pervasive flow and crustal-scale detachment faulting have been shown quantitatively to provide sufficient heat to cause widespread melting. Combinations of the above mechanisms with pluton-scale magmatic advection, shear heating, removal of the lithospheric mantle, or with each other provide additional means of developing suitable high temperatures at shallow crustal levels to generate low-pressure anatexis.
... The study area covers an approximately 20-km 2 portion of the wall rocks east of the Mooselookmeguntic pluton, in the Central Maine Belt of the northern Appalachians (Fig. 2). The distributions of stratigraphic units, structures and metamorphic zones throughout the area of interest are well known from the regional mapping and structural studies of Moench (1966, 1970, 1971), Moench & Hildreth (1976), Brown & Solar (1998a,b, 1999) and Solar & Brown (1999, 2001) and the petrological and microstructural work of Guidotti (1970a,b, 1974), Conatore (1974), Solar & Brown (1999, 2001), Guidotti & Johnson (2002) and Johnson et al. (2003). The pluton is generally considered to have a gently east-dipping contact, a conclusion based both on geological observations (e.g. ...
In the low-pressure, high-temperature metamorphic rocks of western Maine, USA, staurolite porphyroblasts grew at c. 400 Ma, very late during the regional orogenesis. These porphyroblasts, which preserve straight inclusion trails with small thin-section-scale variation in pitch, were subsequently involved in the strain and metamorphic aureole of the c. 370 Ma Mooselookmeguntic pluton. The aureole shows a progressive fabric intensity gradient from effectively zero emplacement-related deformation at the outer edge of the aureole ∼2900 m (map distance) from the pluton margin to the development of a pervasive emplacement-related foliation adjacent to the pluton. The development of this pervasive foliation spanned all stages of crenulation cleavage development, which are preserved at different distances from the pluton. The spread of inclusion-trail pitches in the staurolite porphyroblasts, as measured in two-dimensional (2-D) thin sections, increases nonlinearly from ∼16° to 75° with increasing strain in the aureole. These data provide clear evidence for rotation of the staurolite porphyroblasts relative to one another and to the developing crenulation cleavage. The data spread is qualitatively modelled for both pure and simple shear, and both solutions match the data reasonably well. The spread of inclusion-trail orientations (40–75°) in the moderately to highly strained rocks is similar to the spread reported in several previous studies. We consider it likely that the sample-scale spread in these previous studies is also the result of porphyroblast rotation relative to one another. However, the average inclusion-trail orientation for a single sample may, in at least some instances, reflect the original orientation of the overgrown foliation.
... The X-and Z-dimensions of our rheological heterogeneity are intended to represent the size of the zone of metamorphic strengthening in eastern New Hamsphire and Western Maine during the early Acadian Orogeny, prior to overprinting by extensive migmatization (e.g. Guidotti, 1989;Solar & Brown, 1999). The 100-km Y-dimension is somewhat arbitrary and chosen to eliminate boundary effects when evaluating mechanical data in X-Z cross-sections. ...
Numerical models are used to examine the effects of porphyroblast growth on the rheology of compositionally layered rocks (metapelites and metapsammites) and by extension the middle crust during prograde metamorphism. As porphyroblast abundance increases during prograde metamorphism, metapelitic layers will strengthen relative to porphyroblast-free metapelitic units, and potentially relative to quartzofeldspathic metapsammitic units. As metapelitic layers become stronger, the integrated strength of compositionally layered successions increases, potentially causing large volumes of mid-crustal rock to strengthen, altering the strain-rate distribution in the middle crust and affecting the geodynamic evolution of an orogenic belt. The growth of effectively rigid porphyroblasts creates strength heterogeneities in the layer undergoing porphyroblast growth, which leads to complex strain-rate distributions within the layer. At the orogen scale, the strengthening of large crustal volumes (on the order of thousands of cubic kilometres) changes the strain-rate distribution, which may change exhumation rates of high-grade metamorphic rocks, the geothermal structure and the topography of the orogen. The presence of a strong zone in the middle crust causes strain-rate partitioning around the zone, suppressed uplift rates within and above the zone and leads to the development of a basin on the surface.
... A strong pervasive matrix foliation striking NE is crosscut by an axial plane crenulation cleavage. Two types of structural zones (Fig.1) trending NE-SW has been identified within this region; zones of apparent flattening or high strain, and zones of apparent constriction or low strain (Brown and Solar, 1998;Solar and Brown, 1999;Solar and Brown, 2001). The metamorphism occurred at low pressures and high temperatures with partial melting at higher grades forming migmatites within the Tumbledown and Weld anatectict domains (Brown and Solar, 1998). ...
Microstructural measurements of FIAs in staurolite reveal at least 3 periods of growth in the Proterozoic Colorado Front Range
and 5 in the Paleozoic Western Maine. Dated monazite inclusions in staurolite have an absolute age of 1760±12 Ma (FIA 1),
1720±7 Ma (FIA 2), 1682±18 Ma (FIA 3) in Colorado, and 408±10 Ma (FIA 2), 388±8 Ma (FIA 3), 372±6 Ma (FIA 4), 352±4 Ma (FIA
5) in Maine, supporting the multiple periods of deformation and metamorphism indicated by the FIA succession in each region.
Multiple phases of growth by similar reactions in the same as well as in diverse adjacent rocks in both regions suggest that
PT and X are not the only factors controlling the commencement and cessation of metamorphic reactions. The FIAs preserved
by the staurolite porphyroblasts indicate that the local partitioning of deformation at the scale of a porphyroblast was the
eventual controlling factor on whether or not the staurolite forming reactions took place.
KeywordsFIAs–Staurolite–Monazite dating–Metamorphism–Thermocalc
Titanite is a potentially powerful U‐Pb petrochronometer that may record metamorphism, metasomatism, and deformation. Titanite may also incorporate significant inherited Pb, which may lead to inaccurate and geologically ambiguous U‐Pb dates if a proper correction is not or cannot be applied. Here we present laser ablation inductively coupled mass spectrometry (LA‐ICP‐MS)‐derived titanite U‐Pb dates and trace element concentrations for two banded calc‐silicate gneisses from south‐central Maine, USA (SSP18‐1A & ‐1B). Single spot common Pb‐corrected dates range from 400 to 280 Ma with ±12–20 Ma propagated 2SE. Titanite grains in sample SSP18‐1B exhibit regular core‐to‐rim variations in texture, composition, and date. We identify four titanite populations: 1) 397 ± 5 Ma (95% CL) low Y + HREE cores and mottled grains, 2) 370 ± 7 Ma high Y + REE mantles and cores, 3) 342 ± 6 Ma cores with high Y + REE and no Eu anomaly, and 4) 295 ± 6 Ma LREE‐depleted rims. We interpret the increase in titanite Y + HREE between ca. 397 and ca. 370 Ma to constrain the timing of diopside fracturing and recrystallization and amphibole breakdown. Apparent Zr‐in‐titanite temperatures (803 ± 36°C at 0.5 ± 0.2 GPa) and increased XDi suggest a thermal maximum at ca. 370 Ma. Population 3 domains dated to ca. 342 Ma exhibit no Eu anomaly and are observed only in compositional bands dominated by diopside (>80 vol%), suggesting limited equilibrium between titanite and plagioclase. Finally, low LREE and high U/Th in Population 4 titanite date the formation of hydrous phases, such as allanite, during high XH2O fluid infiltration at ca. 295 Ma. In contrast to the well‐defined date‐composition‐texture relationships observed for titanite from SSP18‐1B, titanite grains from sample SSP18‐1A exhibit complex zoning patterns and little correlation between texture, composition, and date. We hypothesize that the incorporation of variable amounts of radiogenic Pb from dissolved titanite into recrystallized domains resulted in mixed ages spanning 380–330 Ma. Although titanite may reliably record multiple phases of metamorphism, these data highlight the importance of considering U‐Pb data along with chemical and textural data to screen for inherited radiogenic Pb.
Titanite is a potentially powerful U-Pb petrochronometer that may record metamorphism, metasomatism, and deformation. Titanite may also incorporate significant inherited Pb, the correction for which may introduce inaccuracies and result in geologically ambiguous U-Pb dates. Here we present laser ablation inductively coupled mass spectrometry (LA-ICP-MS)-derived titanite U-Pb dates and trace element concentrations for two banded calc-silicate gneisses from south-central Maine, USA (SSP18-1A &-1B). Single spot common Pb-corrected dates range from 400 to 280 Ma with 12-20 Ma propagated 2SE. Titanite in sample SSP18-1B exhibit regular core-to-rim variations in texture, composition, and date. We identify four titanite populations: 1) 399 ± 5 Ma (95 % CL) low Y + HREE cores and mottled grains, 2) 372 ± 7 Ma high Y + REE mantles and cores, 3) 342 ± 6 Ma cores with high Y + REE and no Eu anomaly, and 4) 295 ± 6 Ma LREE-depleted rims. We interpret the increase in titanite Y + HREE between ca. 400 and ca. 372 Ma to constrain the timing of diopside fracturing and recrystallization and amphibole breakdown. Apparent Zr-in-titanite temperatures (803 ± 36 °C at 0.5 ± 0.2 GPa) and increased XDi suggest a thermal maximum at ca. 372 Ma. Population 3 domains dated to ca. 342 Ma exhibit no Eu anomaly and are observed only in compositional bands dominated by diopside (> 80 vol %), suggesting limited equilibrium between titanite and plagioclase. Finally, low LREE and high U/Th in Population 4 titanite date the formation of hydrous phases, such as allanite, during high XH2O fluid infiltration at ca. 295 Ma. In contrast to the well-defined date-composition-texture relationships observed for titanite from SSP18-1B, titanite grains from sample SSP18-1A exhibit complex zoning patterns and little correlation between texture, composition, and date. We hypothesize that the incorporation of variable amounts of radiogenic Pb from dissolved titanite into recrystallized domains resulted in mixed ages spanning 380-330 Ma. Although titanite may reliably record multiple phases of metamorphism, these data highlight the importance of considering U-Pb data along with chemical and textural data to screen for inherited radiogenic Pb. 2
Preservation of partially completed metamorphic reactions in the form of partial pseudomorphs is very important as it provides direct insight onto the reaction mechanism and the phases involved in the reaction. The staurolite and andalusite grade rocks in western Maine, USA, contain cordierite porphyroblasts partly pseudomorphed by coarse‐grained muscovite and biotite. The pseudomorphs consist of a cordierite core surrounded by a reaction rim. Modal mineralogy, calculated using the ImageJ processing software based on backscatter images and x‐ray compositional maps, reveals that the core consists of cordierite (53.5%), muscovite (22.8%), biotite (9.1%), quartz (1 0.4%), plagioclase (3.1%) and ilmenite/pyrrhotite and apatite (1.1%) whereas the reaction rim consists of cordierite (1.8%), muscovite (51.6%), biotite (30.4%), quartz (4.3%), plagioclase (10%), garnet (1.2%), ilmenite/pyrrhotite and apatite (0.8%). The net effect of the cordierite breakdown reaction is an increase of 226% in muscovite, 334% in biotite and 323% in plagioclase content and a decrease of 97% in cordierite. The reaction involved exchange of components with the matrix requiring addition of H2O, K+, Na+ and Ti4+ and removal of SiO2, Mg2+ and PO43‐ from the reaction site. PT estimates using the garnet‐biotite, Ti‐in‐biotite, Na‐in‐cordierite thermometers and the garnet‐biotite‐Al2O5‐quartz barometer indicate that cordierite breakdown occurred at ~550°C and 3.5 kbar. THERMOCALC modeling using the bulk rock composition suggests that cordierite is not stable at these conditions, whereas modeling using a thin section‐derived bulk composition indicates that cordierite stability extends to higher pressures, and most likely that the cordierite breakdown was not PT‐dependent. The incorporation of Na (up to 0.18 a.f.u.) into the cordierite structure has the effect of stabilizing the cordierite under a variety of H2O activity and limiting the role of fluids into destabilizing it. The cordierite cores contain evidence of plastic and brittle deformation in the form of subgrains and microcracks, which facilitated the infiltration of fluids that destabilized cordierite at constant PT conditions by leaching Na and introducing K. New mica growth along these structural heterogeneities suggests that deformation played an important role promoting breakdown of cordierite to muscovite and biotite.
The geology and petrogenesis of a Neoproterozoic migmatitic rock association (MRA) at Wadi Abu Higlig, Hafafit region in the Eastern Desert of Egypt were studied. Diatexites and schlieric granites occupy the core of a domal structure flanked by metatexites and preserved metagabbros. This migmatitic association was intruded later by syenogranite and mylonitized along local shear zones. The rocks have experienced three main deformational events (D 1-D 3) contemporaneous with three main metamorphic phases (M 1-M 3). An early medium pressure-high temperature prograde metamorphism (M 1) during regional scale deformation (D 1), leading to migmatization under water-rich conditions within the P-T conditions of the amphibolite facies. The dynamothermal metamorphic phase (M 2) was contemporaneous with D 2 and confined to the diapiric uplift of the anatectic solid-melt mixture in the core of the dome. D 1 and D 2 events pertain to a single orogeny during crustal shortening and are the most pervasive in the study area. Plagioclase, clinopyroxene, hornblende, garnet and biotite show compositional variability as a consequence of the composition of protoliths and prevailing P-T conditions of metamorphism. The earlier mineral phases in metatexites are enriched in Mg and Ca and depleted in Fe, Mn and Na relative to the refractory minerals in diatexites and schlieric granites. Hornblende, garnet and biotite occur as xenocrysts in syenogranite and are enriched in Na, Fe, and Mn relative to those in the migmatitic rock association. Thermobarometry based on composition of coexisting mineral pairs indicate that peak metamorphism and partial melting occurred at ~ 750 o C and ~ 5 kb at high H 2 O activity for the metatexite. The obtained metamorphic history indicates a clockwise P-T path. The studied migmatitic rock association provides an example of the close relation between orogeny, metamorphism and granite generation and emplacement. The present rock association is interpreted as Pan-African synorogenic anatectic migmatites and granitoid intrusives developed under Andean-type continental margin conditions. INTRODUCTION The processes of melt generation and segregation, and magma extraction, ascent and emplacement lead to differentiation of the continental crust into a depleted lower crust and an enriched upper crust (Brown et al; 1995). Migmatites and granites generated by crustal anatexis form the core of a number of eroded orogenic belts of various ages. Migmatites are commonly divided into two main parts, the 'palaeosome' and the 'neosome' (e.g. Mehnert, 1968). The palaeosome is described as the parent rock of a migmate while the neosome is the newly formed part of a migmatite. The neosome is divided into the mafic 'melanosome' and the felsic 'leucosome'. Migmates are subdivided into metatexites and diatexites (Brown, 1973). Where migmatitic banding is present, the rock is called a metatexite, where the banding is disrupted due to higher melt proportions, it is called a diatexite. Metatexites represent small to moderate degrees of partial melting, in which a pervasive melt fracon does not develop throughout the rock (Brown, 1994; Sawyer, 1998). In contrast to metatexites, diatexites have granitic compositions, considerable amounts pore melt and variable amounts of restite. Pre-migmatization structures are destroyed due to homogenization and coarsening of the mineral grains. The process of migmatisation is generally accompanied by prograde rections that either produces a water-rich vapour phase (subsolidus migmates; e.g. Sawyer and Barnes, 1988) or melt (anatecc migmatites; e.g. Spear et al., 1999). Kriegsman (2001) proposed that anatecc migmates commonly show prograde and retrograde reactions between minerals and melt. Felsic migmatites may be generated by melt infiltration from an external source into banded orthogneiss during deformation (Hasalova et al., 2008).
The magmatic history of batholiths formed as the result of tectonic accretion mirrors the evolution of the underlying crust from island arcs to accretion and collapse. Chemical composition, age and structure give information about the magma source and emplacement depths. The target of this study is the Central Finland batholith (CFGC) that provides a magmatic record after terrane accretion during the Svecofennian orogeny followed by orogenic collapse. The crustal source of partial melting has changed during the crustal evolution and differentiation after the terrane accretion, and when the stress regime has changed from contraction to extension.
A Google Earth-based virtual field trip, part of an introductory geology class, has been developed to illustrate the geology of the Presidential Range, New Hampshire. During a class field trip to Mt. Washington, the highest peak in the Northeast, students record GPS locations of exposures and collect information in the form of field notes and digital images from outcrops. Students upload the GPS waypoints into Google Earth and their images into a class PicasaWeb album, and they also make video clips that are uploaded into a class YouTube account. In Google Earth, the students embed and geologically annotate their images and embed their video clips. The final product is a Google Earth .kmz file or what is termed a mashup. The mashup provides a permanent record of the excursion and, if made available on the Internet, allows any user the ability to easily view the geology at any time. Constructing the mashup from the real field trip initiated reflective, independent, student-motivated learning and group work using technology that the students regularly use and enjoy doing. The resulting mashups have been very good, with an appropriate level of geologic content for an introductory course. Grading, which normally is onerous, is actually enjoyable, entertaining, and easy.
Apatite fission track (AFT) ages in samples collected along the 5000 foot relief (1500 m) exposed at Mt. Washington in the Presidential Range of New Hampshire are used to constrain the Cretaceous cooling history of this area in the northern Appalachians. Nine AFT ages for samples of the Littleton and Rangeley formations collected along this profile range in age from ~ 150 Ma at the highest elevations of ~ 1900 m to ~ 100 Ma at the base (~ 500 m). Thermal histories based on these results and on model time–temperature histories based on the distribution of fission track lengths in the higher elevations samples show three stages of cooling, from 1.5–2.0 °C/m.y. (170–130 Ma) to 0.2 °C (130–65 Ma), to ~ 0.6 °C/m.y. (65 Ma to the present). In contrast, the thermal history calculated for the lower elevations sample shows cooling delayed until ~ 120 Ma at ~ 2 °C/m.y. (120–100 Ma), followed by monotonic cooling of ~ 0.6 °C/m.y. from 100 Ma to the present. The convergence of these histories from high, intermediate, and low elevations suggests a common cooling history independent of elevation differences of > 1 km. Structural/tectonic explanations for this thermal convergence are implausible, and we conclude that the most likely explanation for the common cooling history across > 1 km of relief is that the relief was established by the end of the Cretaceous and has persisted with steady-state topography through the Tertiary to the present. The AFT results are consistent with an earlier relief method study employing 40Ar/39Ar muscovite cooling ages. Geothermal gradients calculated from the results of both studies yield ~ 40 °C/km suggesting that this gradient persisted throughout Permian and Mesozoic times.
This article has been retracted at the request of the Editor-in-Chief and Author. Please see Elsevier Policy on Article Withdrawal (http://www.elsevier.com/locate/withdrawalpolicy).
Reason: This article was essentially a duplication of a paper that had already appeared in Egypt. J. Geol., 52 (2008) 25–54. The author would like to apologize for a misunderstanding on his part that led him to believe that the publication of the paper in a local journal and Lithos, without prior agreement with both journals and clear notification, did not represent duplicate publication.
aBStraCt A crystal-chemical study of thirteen biotite (twelve of 1M polytype and one of 2M 1 polytype) and four muscovite samples was made. The biotite coexists with the muscovite. Samples are from metamorphic terranes and from granitic and granodioritic bodies occurring in three areas of western Maine. The metamorphic mineral zones identified by mineral compatibilities are, in order of increas-ing metamorphic grade: the Lower Sillimanite Zone (LSZ), the Upper Sillimanite Zone (USZ), and the K-feldspar + Sillimanite Zone (K + SZ). The muscovite compositions cluster near ideal muscovite and display a small celadonite substitu-tion and a small, but variable, paragonite substitution. The biotite composition has values of VI Mg 2+ / VI (Mg 2+ + Fe 2+) ranging from 0.26 to 0.54 and significant octahedral Al content (0.48 ≤ VI Al ≤ 0.72 apfu in metamorphic biotite samples, 0.51 ≤ VI Al ≤ 0.67 in those from granites). In trioctahedral micas from western Maine and especially in those with graphite, there are a greater number of interlayer vacancies than in common micas. Interlayer vacancies have an increase in interlayer cation-basal oxygen atom distances and a decrease in tetrahedral flattening angle τ, thus suggesting a reduced interlayer charge. With a few exceptions, tetrahedral rotation angle α is related to crystallization temperature. In particular, α decreases with a temperature increase, and α is also related to octahedral chemical substitutions. Results tentatively suggest, for micas from metamorphic environments, a direct influence of genetic parameters (T and f O 2) on mica crystal structure, and not just chemical composition.
Regional metamorphism occurs in plate boundary zones. Accretionary orogenic systems form at subduction boundaries in the absence of continent collision, whereas collisional orogenic systems form where ocean basins close and subduction steps back and flips (arc collisions), simply steps back and continues with the same polarity (block and terrane collisions) or ultimately ceases (continental collisions). As a result, collisional orogenic systems may be superimposed on accretionary orogenic systems. Metamorphism associated with orogenesis provides a mineral record that may be inverted to yield apparent thermal gradients for different metamorphic belts, which in turn may be used to infer tectonic setting. Potentially, peak mineral assemblages are robust recorders of metamorphic P and T, particularly at high P-T conditions, because prograde dehydration and melting with melt loss produce nominally anhydrous mineral assemblages that are difficult to retrogress or overprint without fluid influx. Currently on Earth, lower thermal gradients are associated with subduction (and early stages of collision) whereas higher thermal gradients are characteristic of back-arcs and orogenic hinterlands. This duality of thermal regimes is the hallmark of asymmetric or one-sided subduction and plate tectonics on modern Earth, and a duality of metamorphic belts will be the characteristic imprint of asymmetric or one-sided subduction in the geological record. Accretionary orogenic systems may exhibit retreating trench-advancing trench cycles, associated with high (>750 °C GPa-1) thermal gradient type of metamorphism, or advancing trench-retreating trench cycles, associated with low (>350 °C GPa-1) to intermediate (350-750 °C GPa-1) thermal gradient types of metamorphism. Whether the subducting boundary advances or retreats determines the mode of evolution. Accretionary orogenic systems may involve accretion of allochthonous and/or para-autochthonous elements to continental margins at subduction boundaries. Paired metamorphic belts, sensu Miyashiro, comprising a low thermal gradient metamorphic belt outboard and a high thermal gradient metamorphic belt inboard, are characteristic and may record orogen-parallel terrane migration and juxtaposition by accretion of contemporary belts of contrasting type. A wider definition of 'paired' metamorphism is proposed to incorporate all types of dual metamorphic belts. An additional feature is ridge subduction, which may be reflected in the pattern of high dT/dP metamorphism and associated magmatism. Apparent thermal gradients derived from inversion of age-constrained metamorphic P-T data are used to identify tectonic settings of ancient metamorphism, to evaluate the age distribution of metamorphism in the rock record from the Neoarchaean Era to the Cenozoic Era, and to consider how this relates to the supercontinent cycle and the process of terrane export and accretion. In addition, I speculate about metamorphism and tectonics before the Mesoarchaean Era.
Foliation intersection/inflexion axes combined with pseudosections and garnet-core isopleths reveal only 1.5 kbar variation in P–T conditions while plutons were emplaced regionally and deformation and metamorphism continued during orogenesis lasting 70 Myr. Tectonism ended with slight decompression into the cordierite stability field. Garnet growth was always overstepped by up to 100 °C occurring at conditions that staurolite growth was also possible. Episodic start, stop, start growth behaviour of both of these phases throughout this period did not result from the effects of bulk composition on their stability fields. Different porphyroblast growth patterns in same bulk composition and outcrop samples reveals reaction start/stop behaviour was controlled by the manner in which deformation partitioned through an outcrop. The regional isograds were established during the first period of bulk shortening near orthogonal to the orogen trend. They did not migrate across lower grade rocks during each of the subsequent periods of metamorphism in spite of dramatic changes in the direction of bulk shortening; rather they contracted slightly. During the youngest periods of orogenesis directed at a high angle to the current orogen trend the isograds were folded about axial planes parallel to the fold belt. The regional distribution of these isograds directly reflects the oldest period of pluton emplacement, with both controlled by orogen-scale partitioning of bulk shortening at a high angle to the current orogen trend relative to intervening zones of transform-like shear.
Porphyroblast assemblage of andalusite, garnet, staurolite and cordierite in the metasedimentary rocks of the Palaeoproterozoic Mahakoshal supracrustal belt suggests low pressure/medium temperature metamorphism. These porphyroblasts exhibit both dynamic (syn-kinematic) and static (post-kinematic) growth. Field and petrographic studies indicate that the growth of andalusite and garnet porphyroblasts initiated during the early stages of D2 (S2 cleavage) deformation and outlasted it. The staurolite porphyroblasts appeared during the late stages of D2 deformation and continued after it. Restricted occurrence of these syn-kinematic porphyroblasts to the vicinity of the shear zone suggest that intense deformation and repetitive cleavage development, apart from the elevated temperature conditions, enhanced the growth of the porphyroblasts. The cordierite porphyroblasts, whose growth is controlled essentially by the lithological composition, exhibit static growth, post-dating D1 (S1 cleavage) deformation. The mineralogical assemblages in the metasedimentary rocks suggest a peak P and T around 3.5 kb and 550–600°C respectively, which was attained during the D2 deformation. The D2 deformation was coeval with the development of reverse-oblique slip ductile shear zone and emplacement of linear, syn-kinematic granites along the southern margin of the belt. This indicates a contractional tectonic regime, wherein rising granitic plutons caused advective heat transfer to the middle and upper crust and created relatively higher temperature conditions at lower pressure.
Emplacement of the Mooselookmeguntic pluton, located in the western Maine region of the northern Appalachians, was thought to have occurred towards the end of the Acadian deformation at around 370 Ma. Crystallization ages from different parts of the pluton suggest a more sequential emplacement history over a period of c. 20 Myr. Foliation inflection/intersection axes (FIAs) within porphyroblasts from its aureole reveal at least five periods of garnet and staurolite growth. The orientation of FIAs in both garnet and staurolite porphyroblasts trend successively from ESE–WNW, NNW–SSE, E–W, ENE–WSW to NE–SW. Electron probe microanalysis dating of monazite grains included in staurolite porphyroblasts containing one of these five periods of FIA development reveals a succession of apparent ages from 410 Ma to 345 Ma. A similar spread of crystallization ages can be observed for plutons from Maine and adjacent regions. This succession indicates that deformation and metamorphism began well before and continued long after what is classically regarded as the Acadian orogeny. The thermal structure of the orogen progressively evolved to enable pluton emplacement, and it continued to develop afterwards with magmatic fluids still forming at depth.
We propose a model for syntectonic ascent and emplacement of granite magma based on structural relations in part of the northern Appalachians. In the study area in western Maine, strain was distributed heterogeneously during Devonian Acadian transpression. Metasedimentary rocks (migmatites at high grades) record two contrasting types of finite strain in zones that alternate across strike. Rocks in both types of zones have a penetrative, moderately-to-steeply NE-plunging mineral elongation lineation defined by bladed muscovite (fibrolite/sillimanite at high grades). In `straight' belts of enhanced deformation rocks have S > L fabrics that record apparent flattening-to-plane strain (apparent flattening zones, AFZs), but rocks between these belts have L > S fabrics that record apparent constriction (apparent constriction zones, ACZs). At metamorphic grades above the contemporary solidus, rocks in AFZs developed stromatic structure in migmatite, which suggests that percolative flow of melt occurred along the evolving flattening fabric. Stromatic migmatites are intruded by concordant to weakly discordant, m-scale composite sheet-like bodies of granite to suggest magma transport in planar conduits through the AFZ rocks. Inhomogeneous migmatite is found in the intervening ACZs, which suggests migration of partially molten material through these zones en masse, probably by melt-assisted granular flow. Inhomogeneous migmatites are intruded by irregular m-scale bodies of granite that vary from elongate to sub-circular in plan view and seem cylindrical in three dimensions. These bodies apparently plunge to the northeast, parallel to the regional mineral elongation lineation, to suggest magma transport in pipe-like conduits through the ACZ rocks. We postulate that the form of magma ascent conduits was deformation-controlled, and was governed by the contemporaneous strain partitioning. Magma ascent in planar and pipe-like conduits through migmatites is possible because oblique translation during contraction displaces isotherms upward in the orogenic crust to form a thermal antiform. Within this hot corridor, it is the difference in temperature between melt-producing reactions in the anatectic zone and the wet solidus for granite melt that enables magma to migrate pervasively up through the orogenic crust without congealing. Heat advected with the migrating melt promotes amplification of the thermal antiform in a feedback relation that extends the zone of plastic deformation and pervasive melt migration to shallower levels in the crust. At the wet solidus, we suggest melt flows obliquely toward axial culminations in the thermal antiform, which are sites of melt accumulation and perturbations from which magma may escape to form plutons. Batches of melt that escape from these perturbations may be trapped by tectonic structures higher in the crust, or ascent may become inhibited with decreasing depth by thermal arrest and solidification. If the rate of arrival of subsequent melt batches exceeds the rate of crystallization at the site of pluton construction, melt pressure ultimately may lead to (sub-)horizontal magma fracture, or viscous flow of wall rocks may allow lateral spreading. The resultant plutons have (sub-)horizontal tabular geometries with floors that slope down to the ascent conduit. As the thermal antiform decays, the height of sub-solidus crust that separates the deepest part of the pluton from the anatectic zone increases. Consequently, pluton inflation declines and solidification leads to infilling of the magma feeder channel to form a root zone that passes downward into migmatite, which may explain the difficulty of determining precisely the floor to these deeper segments of large plutons using gravimetry.
Observational data and geochronology show synchronous deformation, metamorphism and intrusion of granite in many obliquely convergent (transpressive) orogenic belts. In these belts, melt extraction and transport was by syntectonic pulsed flow in structurally-controlled channels through migmatites, and granite plutons were constructed by aggregation of multiple melt batches. Compositional information in the granites provides clues to unraveling the petrological processes by which their parental magmas were generated and extracted from the crust, but we need to read the information with sufficient insight to enable prudent interpretation, because we cannot see granite in process of formation! In this paper, we evaluate the role of source processes in granite petrogenesis using results from west-central Maine, in the northern part of the Appalachian orogenic belt of eastern North America. In this area, precise U-Pb zircon/monazite crystallization ages of schlieric granite in migmatite, metric to decametric sheets of granite and kilometric granite plutons are in the range c. 408-404 Ma, within 1 Ma at 95% confidence limits, and in one granite, the Phillips pluton, multiple samples of leucogranite and granodiorite yield synchronous ages. If the results of this study are representative, they suggest that in restricted segments of orogenic belts regionally-significant crustal melting in nature occurs within short timescales (~ 107a), and melt extraction and transport, and pluton construction are fast processes. In the Phillips pluton, petrographic features and geochemistry of leucogranite samples, including K2O contents, ratios, covariation in Rb, Sr and Ba distributions, and chondritenormalized REE patterns, are inconsistent with fractional crystallization, but compatible with eutectic high-to-moderate a(H2O) muscovite-dehydration melting of a predominantly pelite source. Petrographic features and geochemistry of granodiorite samples, including chondrite-normalized REE patterns and ratios, also suggest derivation from a metasedimentary source, but by non-eutectic (minimum) moderate-to-low a(H2O) biotite-dehydration melting of greywacke. In this pluton, the two types of granite s.l. show heterogeneity in Nd isotope compositions (granodiorite εNd (404 Ma) of + 0.1 – −1.8; leucogranite εNd (404 Ma) of −5.3 –−8.0), which we interpret to reflect derivation of the two granites from isotopically-distinct sources, to preserve withinsource heterogeneity and to imply efficient extraction, ascent and emplacement of melt without significant interaction between individual batches.
New U-Pb geochronology constrains the timing of the Acadian orogeny in the Central Maine Terrane of northern New Hampshire. Sixteen fractions of one to six grains each of zircon or monazite have been analyzed from six samples: (1) an early syntectonic diorite that records the onset of the Acadian; (2) a schist, a migmatite, and two granites that together record the peak of the Acadian; and (3) a postkinematic pluton that records the end of the Acadian. Zircon from the syntectonic Wamsutta Diorite gives a 207Pb/206Pb age of circa 408 Ma, the time at which the boundary between the deforming orogenic wedge and the foreland basin was in the vicinity of the Presidential Range. This age agrees well with the Emsian position of the northwest migrating Acadian orogenic front and records the beginning of the Acadian in this part of the Central Maine Terrane. We propose a possible Acadian tectonic model that incorporates the geochronologic, structural, and stratigraphic data. Monazite from the schist, migmatite, Bigelow Lawn Granite, and Slide Peak Granite gives 207Pb/206U ages, suggesting the peak of Acadian metamorphism and intrusion of two-mica granites occurred at circa 402-405 Ma, the main pulse of Acadian orogenesis. Previously reported monazite ages from schists that likely record the peak metamorphism in the Central Maine Terrane of New Hampshire and western Maine range from circa 406-384 Ma, with younger ages in southeastern New Hampshire and progressively older ages to the west, north, and northeast. Acadian orogenesis in the Presidential Range had ended by circa 355 Ma, the 207Pb/235U age of monazite from the Peabody River Granite. From 408 to perhaps at least 394 Ma, Acadian orogenesis in the Presidential Range was typical of the tectonic style, dominated by synkinematic metamorphism, seen in central and southern New Hampshire, Massachusetts, and Connecticut. From no earlier than 394 Ma to as late as 355 Ma, the orogenesis was typical of the style in parts of Maine dominated by postkinematic metamorphism.
Studies of structure, metamorphism, and geochronology provide evidence that the Norumbega Fault Zone represents a transition from mid- to shallow crustal levels in a dextral, transcurrent shear zone within the northern Appalachian Orogen. A younging trend in 40Ar/39Ar cooling ages toward the northeast, together with the deformational fabrics and metamorphic features, are interpreted to represent exhumation of the southwestern section of the Norumbega Fault Zone from mid-crustal levels during the polyphase history of this transcurrent zone. The Norumbega Fault Zone may therefore serve as a model for deformational processes at mid- to shallow crustal levels in active strike-slip systems. -from Authors
Garnets with an inclusion pattern of cylindrical quartz intergrowths develop exclusively in the presence of graphite. The quartz rods 1-5 mu m in diameter originate at the sector-zone interfaces in the garnet with the long axes normal to the (110) garnet faces. The interphase boundaries may be epitaxially related, with new material added to the tube as the crystal face of the garnet grew. In the presence of a C-O-H fluid at 6.5 kbar and 500oC, the amount of CO2 present would restrict the solubility of SiO2 in the intergranular fluid phase and hence restrict its ease of transport, resulting in an excess of SiO2 at the site of garnet growth. -R.A.H.
Abstract The mid-Tertiary blueschists, eclogites and eclogitic gneisses of northern New Caledonia are the products of four phases of regional metamorphism and deformation (D1–D4). Omphacite, lawsonite and Mn-rich garnet isogradic surfaces were developed during the second deformation (D2) under prograde pressure and temperature conditions. Subsequent deformations (D3–D4) folded these D2 isogradic surfaces. However, within the P-retrograde, T-prograde metamorphic environment of the D4 phase, omphacite altered to albite and chlorite; as a result, a late-stage sub-horizontal isogradic surface developed for omphacite-out where this mineral preserved as relics within syn-D4 albite porphyroblasts. Other minerals that crystallized for the first time (epidote) or had rim additions (almandine phengite) during D4, also form nearly horizontal isogradic surfaces. Porphyroblastic garnet and albite contain inclusion trails, which allow their microstructural development and crystallization of the matrix to be traced from D2 to D4.
Late syn-D4 the temperature increased markedly in association with an extensive exothermic decarbonation, even though the rocks were in a state of pressure retrogression. This caused considerable neocrystallization, recrystallization and growth of mattix and porphyroblasts such that, although S2 foliation crenulated by D3 and D4 is readily observable, almost all signs of stored strain due to D3 and D4 have been removed, and the deeper schists and eclogitic gneisses superficially appear to have undergone a drastic annealing recrystallization, post-dating deformation.
The Early to Middle Paleozoic stratigraphic, structural, and metamorphic histories of central Newfoundland and New Brunswick show remarkable similarities. These characteristics should be reflected in zonal or terrane subdivisions of the Canadian Appalachians, and we therefore recombine the Dunnage and Gander zones/terranes into the Central Mobile Belt (CMB). In our interpretation, the CMB is a deformed Ordovician ocean basin (Iapetus II), that was formed in between the Taconic magmatic arc and the Avalon continent during Mariana-type subduction of the Iapetus I ocean. The generalized stratigraphic sequence of the CMB records the formation of ophiolitic crust and continental margin sediments of Iapetus II in Early Ordovician times, which was followed by a period of relative quiescence when spreading stopped (limestone and shale deposition). Late Ordovician to Silurian closure of Iapetus II was accompanied by the deposition of a thick clastic sequence and locally volcanics. A complex history of thrusting, regional folding, and strike-slip faulting has been recognized. Mineral assemblages in the area are representative of two distinct metamorphic events. The first event produced medium to high pressure assemblages and is associated with the formation of the accretionary complex. The second, low pressure event is correlated with the widespread intrusion of granitic bodies. -from Authors
Six stages of crenulation cleavage development during a single deformation event (D2) can be recognised in both matrix and porphyroblast inclusion trails in the Robertson River Formation, N.E. Australia. These stages progress from undeformed to crenulation and differentiated crenulation of S1, through development of differentiated crenulation cleavage (S2) and differentiated schistosity (S2), to homogeneous foliation (S2).These rocks underwent prograde metamorphism during D2 and chloritoid, garnet, staurolite, andalusite, and sillimanite isograds have been mapped. Most rocks are rich in porphyroblasts, which commonly contain well-defined inclusion trails. The geometry of these trails varies from one mineral type to the next, depending on the timing of porphyroblast growth relative to the stage of crenulation cleavage development in the schistose matrix. The deformation history involved progressive, bulk, inhomogeneous shortening and the strain is very heterogeneous on all scales, partly as a consequence of this. Hence each of the six stages of crenulation cleavage development formed locally at various times during D2. Therefore, if a porphyroblast grew late in the deformation, it could overgrow all stages of crenulation cleavage development, including the very early ones.The temperature increased during metamorphism in any one locality and consequently some porphyroblasts overgrew others that were unstable in the new conditions, preserving them from destruction. Hence the precise timing of mineralogical reactions (deduced from isograds and mineral chemistry) relative to the stages in schistosity development can also be determined. In the andalusite zone the sequence of porphyroblasts, in decreasing order of age, is biotite-garnet-biotite-staurolite-andalusite. Dissolution of garnet occurred during growth of staurolite and especially andalusite, and dissolution of staurolite accompanied andalusite growth.
In regional metamorphic rocks, the partitioning of deformation into progressive shearing and progressive shortening components results in strain and strain‐rate gradients across the boundaries between the partitioned zones. These generate dislocation density gradients and hence chemical potential gradients that drive dissolution and solution transfer. Phyllosilicates and graphite are well adapted to accommodating progressive shearing without necessarily building up large dislocation density gradients within a grain, because of their uniquely layered crystal structure. However, most silicates and oxides cannot accommodate strain transitions within grains without associated dislocation density gradients, and hence are susceptible to dissolution and solution transfer.
As a consequence, zones of progressive shearing become zones of dissolution of most minerals, and of concentration of phyllosilicates and graphite. Exceptions are mylonites, where strain‐rates are commonly high enough for plastic deformation to dominate over diffusion rates and therefore over dissolution and solution transfer. Porphyroblastic minerals cannot nucleate and grow in zones of active progressive shearing, as they would be dissolved by the effects of shearing strain on their boundaries. However, they can nucleate and grow in zones of progressive shortening and this is aided by the propensity for microfracturing in these zones, which allows rapid access of fluids carrying the material presumed to be necessary for nucleation and growth. Zones of progessive shortening also have a number of characteristics that help to lower the activation energy barrier for nucleation, this includes a build up of stored strain‐energy relative to zones of progressive shearing, in which dissolution is occuring.
Porphyroblast growth is generally syndeformational, and previously accepted criteria for static growth are not valid when the role of deformation partitioning is taken into account.
Porphyroblasts in a contact aureole do not grow statically either, as microfracturing, associated with emplacement, allows access of fluids in a fashion that is similar to microfracturing in zones of progressive shortening.
The criteria used for porphyroblast timing can be readily accommodated in terms of deformation partitioning, reactivation of deforming foliations, and a general lack of rotation of porphyroblasts, with the spectacular exception of genuinely spiralling garnet porphyroblasts.
Thermal models of collisional orogens generally predict temperature structures that are much cooler than those recovered by thermobarometric studies. Here we demonstrate that high-temperature, low-pressure metamorphism and the development of inverted geotherms within collisional belts may be the result of accretion and erosion acting on crust enriched with heat-producing elements. A new two-dimensional finite difference model, described here, incorporates the subduction of lithosphere with heat-producing material in the upper crust, accretion of crustal material from the subducting plate to the upper plate, and surface erosion of the upper plate. These processes result in the development of a wedge of heat-producing material within the upper plate. The rate of heat production within the wedge and maximum depth of the wedge are the most important parameters controlling the magnitude of upper plate temperatures. Our model yields inverted upper plate geotherms when heat production rates exceed 0.75 μW/m3 and the heat-producing wedge extends to a depth greater than 35 km. Temperatures in excess of 500°C at depths of 20–30 km are computed when heat production rates are greater than ∼1.75 μW/m3 and the wedge extends to a depth >50 km. Other processes, such as shear heating, fluid flow, or mantle delamination, need not be invoked to explain geologic evidence of high temperatures or inverted thermal gradients in collisional systems.
Extensive examination of large numbers of spatially orientated thin sections of orientated samples from orogens of all ages around the world has demonstrated that porphyroblasts do not rotate relative to geographical coordinates during highly non-coaxial ductile deformation of the matrix subsequent to their growth. This has been demonstrated for all tectonic environments so far investigated. The work also has provided new insights and data on metamorphic, structural and tectonic processes including: (1) the intimate control of deformation partitioning on metamorphic reactions; (2) solutions to the lack of correlation between lineations that indicate the direction of movement within thrusts and shear zones, and relative plate motion; and (3) a possible technique for determining the direction of relative plate motion that caused orogenesis in ancient orogens.
In the contact metamorphic aureole of the Tinaroo Batholith (north Queensland, Australia), mylonitic rocks were metamorphosed during a regional folding/crenulation event ( D 2 ) synchronous with the emplacement of muscovite‐bearing granitoids. Prismatic and skeletal andalusite porphyoblasts grew in carbonaceous schists, mainly from the dissolution of staurolite. Muscovite, quartz and biotite played a dual role in this reaction, acting in a catalytic capacity as well as reactants or products. Staurolite was replaced by coarse‐grained muscovite ± biotite, whereas andalusite locally replaced quartz ± muscovite ± biotite, with diffusion of H, Al, Si, Mg, Fe and K ionic species linking sites of dissolution and growth.
Graphite contributed to the reaction mechanism in a number of ways. Accumulations of graphite in front of advancing andalusite crystal faces led to skeletal growth and the formation of chiastolite structure, where incremental growth occurred on adjacent {110} faces, with subsequent filling in and inclusion of graphite along the diagonal zones. The presence of graphite in some layers in the schist matrix prevented recrystallization of strained muscovite grains. The muscovite grains in these layers, in contrast to adjacent thin non‐graphitic layers, were preferentially replaced by quartz. This resulted in muscovite‐depletion haloes in graphitic layers around andalusite porphyroblasts. Somewhat arcuate zones of graphite, concentrated during dissolution of quartz along a crenulation cleavage, occur on some andalusite faces. Reactivation of the mylonitic foliation during the formation of D 2 crenulations led to a preferential dissolution of quartz in zones of progressive shearing localized near andalusite porphyroblasts and hence the accumulation of graphite.
Lack of deflection of the pre‐existing mylonitic foliation and anastomosing of the axial planes of D 2 crenulations around andalusite porphyroblasts demonstrate not only the timing of growth, but also that growing porphyroblasts do not push aside existing foliations.
Crustal temperatures within collisional orogens are anomalously high compared with temperatures at comparable depths in stable
continents, which is evidence of thermal processes that are fundamental to orogenesis. These temperatures can be explained
by the redistribution of crust enriched in heat-producing elements through the accretion of crust from the down-going plate
to the upper plate and surface erosion. With the use of geologically reasonable rates, the model results predict high temperatures
(over 600°C) and inverted upper-plate geotherms (about 100°C over 20 kilometers) at shallow depths (20 to 40 kilometers) by
25 to 35 million years after collision. This study emphasizes the interdependence of deformational, surficial, and thermal
processes.
This report attempts to synthesize the results of recent geologic mapping and petrologic studies in western Maine, an area that lies on the transition zone between the sillimanitic portion of the Merrimack synclinorium in New Hampshire and the greenschist-facies portion in central Maine. Results of two fundamentally different deformations are recognized in the transition zone. The first or early deformation produced northeast-trending tight, upright passive-flow folds; longitudinal premetamorphic faults of large displacement; and slaty cleavage formed also by the first metamorphism (M-l). These features were produced throughout the whole geosyncline that was ancestral to the Merrimack synclinorium; they characterize the structure of the greenschist terrane. But southwestward across the transition zone, they are increasingly blurred by younger superposed structural and metamorphic features. Inferred origins are geosynclinally controlled slump deformation and tectonic dewatering culminating in low-grade metamorphism. The younger features collectively define the late deformation and at least two late metamorphic events (M-2 and M-3). The late deformation produced a complex pattern of slip cleavages; schistosity derived from slip cleavage, small recumbent folds, large flexural folds, and domes of varied shapes and trends; and conspicuous northwest-trending flexural cross folds. In the northeast part of the transition zone, M-2 and M-3 metamorphic events produced overlapping metamorphic zones approximately concentric to granitic plutons. To the southwest, where metamorphic grade is regionally in the sillimanite or potassium feldspar plus sillimanite zones, M-2 and M-3 zones coalesce and become indistinguishable. Many structural features are directly magma-generated, but the northwest-trending cross folds and related cleavages probably express regional strain that is only temporally magma-related. Nine samples of granite and aplite have yielded an isotopic age of 379 ± 6 m.y. by the Rb-Sr whole-rock method. This age dates the youngest intrusive rocks of the New Hampshire Plutonic Series in the report area, M-3 metamorphism, and the late deformations that accompanied M-3. Because all the plutons intrude previously deformed Devonian and older strata, the isotopic age narrowly restricts the duration of the early and late deformations, plutonism, and M-l to M-3 metamorphisms. We speculate that the Mooselookmeguntic pluton and the larger Sebago pluton farther south are irregular but broadly arched subhorizontal sheets only a few kilometres thick that were emplaced at depths of 11 to 15 km. The transition zone is where these and possibly other sheetlike bodies dive monoclinelike to the northeast. Northeast of the transition zone, the tops of the sheets are now a few kilometres below the surface. Associated with them are many of the late structural and metamorphic features that characterize the transition zone and the sillimanite terrane. Above them is a partly eroded thick slate layer. Numerous plutons now exposed northeast of the transition zone are cupolas and possibly isolated bodies that rose above the sheets. In the sillimanite terrane southwest of the transition zone, a subhorizontal granitic sheet complex is exposed. The thick slate layer that is now exposed only in the northeast once extended over the sillimanite terrane.
A sequence of distinctive stratigraphic units has been defined in a succession of sillimanite-grade metasedimentary schists, gneisses, and granulites in eastern and central New Hampshire north of the 44th parallel. The units are correlated and continuous with the formations of the Rangeley area, Maine, to the northeast. The same sequence of units has been mapped immediately south of latitude 44 degree and is firmly correlated southward with a succession of units in south-central New Hampshire.
Heterogeneous biotite in regard to Ti occurs in the Ryoke metamorphic rocks in the Yanai district, southwest Japan. Where this biotite is associated with reversely zoned garnet, in which the Mn content increases towards the periphery, there is a decrease of the Ti content of the biotite towards the garnet. The change in composition of the biotite approaching the reversely zoned garnet is expressed by the substitution, Ti2□viAlvi-2R²⁺-1. The presence of Ti-poor biotite in contact with the embayed parts of garnet suggest that the formation of Ti-poor biotite involved the consumption of this garnet during retrograde metamorphism. -from Author
Following the Middle Devonian Acadian deformation an extensive belt of high grade metamorphism was formed in New England. In south-western Maine, at the northern end of this belt, there occurs a transition along the strike from regional low-pressure/high-temperature metamorphism to contact metamorphism in low-grade rocks. Petrological studies indicate that this transition occurs along a surface plunging to the north-east at about 3.5°, with respect to the Middle-to-Late Devonian erosion surface. In addition, detailed petrological mapping has defined a history of temporally separate, localized metamorphic events associated with plutonism and occurring at increasingly deeper levels to the south-west. Geochronological studies constrain ambient temperatures in the transition zone at the time of metamorphism to be less than 300° C in the north-east and between 350° C and 500° C in the south-west. They also establish a pattern of diachronous cooling due to differential uplift and erosion, with cooling occurring later and most rapidly to the south-west. Geophysical evidence suggests that along with this spatial variation in metamorphic style the shapes of the plutons in Maine undergo a transition from laterally extensive sheet-like bodies in the high grade terrane to more equant-shaped bodies in the low-grade terrane. Using the results of these petrological, geochronological and geophysical studies, as well as those of stratigraphical and structural studies we construct a thermal model for the transition zone. The model suggests that the Acadian metamorphism in south-western Maine is a result of deep-level contact metamorphism near laterally extensive granitic sills dipping to the north-east with respect to the present erosion surface. The plutons themselves are interpreted to be a result of lower crustal melting in response to crustal thickening in the presence of normal or slightly augmented mantle heat flux.
Thirty-two samples of a series of metamorphosed Silurian (?) pelitic schists in the greenschist and amphibolite facies from N.W. Maine have been analyzed for their rare-earth element (REE) content. The REE contents of these samples do not change as a function of metamorphic grade. Two different metasedimentary formations have been sampled, and they differ significantly in their light REE content. The absolute and relative distribution of the REE in the Bangeley Formation are quite similar to the composites of N. American, European and Russian shales that have been determined thus far (e.g. median La/Lu ratio of the Bangeley Formation normalized to chondrites = 8.7 ± 3.0). Samples from the Perry Mt. Formation show large depletions in the light REE compared to the Bangeley Formation and previously analyzed shale and metamorphosed shale samples, but heavy REE concentrations that are quite similar to the other samples (e.g. median La/Lu ratio of the Perry Mt. normalized to chondrites = 1.2 ± 0.6). These differences in light REE content between the Perry Mt. Formation and other sedimentary rocks are probably due to differences in the original clay mineral compositions as modified by weathering and/or depositional environments.
Zusammenfassung Porphyroblasten werden in der Literatur dann gemeinhin als interkinematisch bezeichnet, wenn sie ein planares Interngefüge (Si) haben, das sich durchgehend in der Schistosität der Matrix (Se) — mit oder ohne Winkel zwischen Si und Se - verfolgen läßt und wo Se die Porphyroblasten umschmiegt. Die vorliegende Arbeit zeigt, daß diese Porphyroblasten sehr wohl synkinematisch sein können. Es wird dargelegt, daß die Porphyroblasten im allgemeinen rasch wachsen verglichen mit der Verformungsgeschwindigkeit der Gesteine, in denen sie sich befinden. Das Handstück eines eng gefalteten Glimmerschiefers mit Granat- und Biotit-Porphyroblasten wird en detail beschrieben. Untersuchungen im regionalen Maßstab deuten darauf hin, daß sich die Falte entwickelte aus einem wenigel eng gefalteten Phyllit (?) mit Schergleit-Schieferung parallel zur Achsenebene. Zunächst wuchsen die Porphyroblasten rasch; einige der größeren enthalten intern gefältelte, andere planare Einschlüsse; dann wurde die Faltung zunehmend enger und eine neue Schistosität bildete sich aus parallel zur Achsenebene. Nach der Faltung sehen folglich einige Porphyroblasten aus, als ob sie interkinematisch wären. In Wirklichkeit sind sie synkinematisch.
Careful observation and interpretation of suitably oriented sections indicates that many porphyroblasts with curved inclusion trails grow during the development of a crenulation foliation. Inclusion trails that are approximately parallel in separate porphyroblasts, or that outline axial surfaces of crenulations that are parallel in separate porphyroblasts, suggest that many porphyroblasts do not rotate, with respect to geographic coordinates, during later deformation. Moreover, where such rotation has not occurred, inclusion trails in porphyroblasts may indicate former orientations of S-surfaces that have been obliterated from the matrix.
In ideal situations, porphyroblast-matrix microstructural relationships can be used, with more confidence than before, to provide useful information on metamorphic/deformation histories and on the details of metamorphic reactions. Porphyroblast-matrix relationships are also useful in contact metamorphic situations and can provide evidence of deformation accompanying contact metamorphism. Though mostly applied to medium-grade metamorphic rocks, porphyroblast-matrix relationships can also be useful in some high-grade situations.
A model suggested by Bell & Rubenach and Bell et al. postulates that porphyroblasts grow in zones of relatively weak coaxial deformation between anastomosing zones of strong non-coaxial deformation during crenulation folding. Many observations appear to be consistent with this model, but it is difficult to verify, and much detailed observation and testing of critical aspects of the model are needed. Regardless of whether or not the model is accepted, curved inclusion trails may be used to relate growth of porphyroblasts to generations and stages of crenulation development.
Bruno Sander has proposed that the fabric symmetry of a deformed rock reflects the kinematic symmetry of its deformation. In order to place this symmetry principle on a firmer basis, the background of the symmetry theory of fabrics is here reviewed. From very general notions of symmetry as a starting point, fabric symmetry is shown to be a statistical space symmetry consisting of a point group of symmetry operations combined with arbitrary translations in all directions. Where likely restrictions are placed upon the point groups to be expected in homogeneously deformed rocks, the usual types of symmetry observed in fabrics of deformed rocks (namely, spherical, axial, orthorhombic, monoclinic, and triclinic) remain as possible types. The general derivation demonstrates that apart from pseudocrystallographic symmetries defined by some crystallographic fabric elements, no other types of fabric symmetry can be expected in homogeneously deformed rocks. Attention is drawn to the relevance to the views of Sander of Curie's principles governing the symmetries of "cause" and "effect" in physical phenomena. The features of a deformed rock that define its fabric are found to be a three dimensionally ordered array of discontinuities in structure (lattice planes and lines in crystals, grain boundaries, foliations, lineations, folds, and so on) which may be viewed as the "effect" of deformation. These surfaces and lines of discontinuity in structure are generally sites of surfaces and lines of discontinuity in deformation - implicit in Sander's concept of componental movements - which collectively define the "movement picture" (Bewegungsbild) of the deformation. An attempt is made to state more precisely the significance of a movement picture in terms of analysis of local heterogeneities and perturbations in deformation that reflect or are responsible for the development of fabric features in a statistically homogeneously deformed aggregate. The kinds of symmetry observed in tectonite fabrics are found also to be the only kinds possible in movement pictures of homogeneous deformations. Curie's principles may now be restated in a form directly applicable in the interpretation of tectonite fabrics, thereby amplifying Sander's principle. Without qualification, the most important principle may be restated as follows: whatever the nature of the factors contributing to a deformation may be, the symmetry that is common to them cannot be higher than the symmetry of the deformed fabric, and symmetry elements absent in this fabric must be absent in at least one of the contributing factors.
The South Mountain fold is a large asymmetrical overturned anticline. Its axial plane dips to the southeast, and its crest is the western slope of South Mountain. Cleavage dips steeper in the upper than the lower limb thus forming a fan which opens to the northwest. All parts of this fold participate and reveal an identical deformation plan: fold axes are nearly horizontal, cleavage dips southeast, lineation is in the cleavage plane normal to the fold axes, also dipping east. All formations including the volcanics participate.
Deformation axes have been determined at several hundred localities by systematic measurement of oöid distortions in Cambero-Ordovician oölites. The directions are shown in maps and diagrams.
Intensity of deformation varies greatly within the fold depending on (1) physical properties of materials (2) location within the fold and (3) geographical location.
Undeformed crystalline micropebbles and detrital carbonate grains within highly distorted oölites seem to indicate that deformation affected rather soft and pliable and maybe little consolidated rocks. Undistorted growth aprons on deformed oöids and crystallization of matrix are postkinematic and serve to date deformation and consolidation.
Approaching South Mountain from the west intensity grows gradually, is strongest in the lower limb, decreases toward the crest and the upper limb. Abrupt changes as would be expected in large-scale thrusting were not observed, the deformation seems to have been absorbed within a much wider complex.
Cleavage is invariably defined by the maximum (a) and mean (b) axis of distortion and not by a shear plane. This also points toward flowage as distinct from shearing of solids.
The fold is interpreted as a large “shear” fold as distinct from flexures. Deformation is thought to be due to laminar flow on subparallel planes. The presence of flow planes and the fact that oöids are extended at large angles to bedding show that stratigraphic thicknesses as now seen are not equivalent to depths of deposition. Calculations indicate that the latter amounted to less than one half of the present thickness.
Analysis of South Mountain fold requires reconstructions not usually applied in tectonic analysis. If deformation affected soft strata, exaggeration of thicknesses would go unnoticed but for painstaking structural observations, not commonly undertaken in stratigraphic research at present.
Careful microscopic investigation of the constituents of the oölites permits the dating of deformation, crystallization, consolidation, and, generally, the relation between diagenesis and deformation. Lateral shortening cannot be determined by straightening the strata into the horizontal if cleavage is present, but reconstruction becomes much more complicated in a “shear” fold. The author has attempted such a reconstruction using oöid distortion.
In the Yanai district, southwest Japan, garnet crystals in pelitic and siliceous rocks are normally zoned, reversely zoned or homogeneous. Normally zoned crystals show a continuous decrease in Mn content from the core towards the margin, whereas reversely zoned crystals show enrichment of Mn at the margin. Normally zoned garnet grew by one of two distinct coarsening mechanisms, either nucleation and growth or impingement and overgrowth.In pelitic rocks, there is a systematic change from normal zoning to homogeneous and reverse zoning with increasing metamorphic grade. This can be ascribed to the enhancement of internal diffusion with increasing metamorphic temperature. In the cordierite-I zone, with an estimated metamorphic temperature of 560 ± 30°, garnet in pelitic rocks with a grain size larger than 0.2 mm preserves its zoning pattern, whereas garnet in siliceous rocks with a similar grain size is homogeneous. This suggests that both rock type and grain size are factors that influence the degree to whole growth zoning of garnet is homogenized. The relative paucity of biotite in siliceous rock can explain the development of fine-grained garnet in this rock type, which is, therefore, easily homogenized. Coarse homogeneous garnet in the siliceous rocks was formed by impingement of small grains during prograde metamorphism. Such aggregates are easily homogenized by both internal and grain-boundary diffusion.
There is an association between the development of cleavage domes, a texture reflecting the displacement of insoluble matrix grains by porphyroblasts growing under a bulk hydrostatic stress, and textural sector-zoning. This has been found in garnet, staurolite, chiastolite, pyrite and possibly emerald porphyroblasts. Sector-zoned porphyroblasts form by lineage growth normal to the crystal faces. This causes several distinctive textures (type 1 inclusions and type 2 intergrowths, inclusion bands, growth prongs), all of which are directly or indirectly related to displacement growth. Graphite or other carbonaceous material is ubiquitous in samples showing textural sector-zoning.
Models of the Appalachian-Caledonian Orogen infer oblique collision and an enigmatic reversal from sinistral (Silurian) to dextral (Devonian) transpression. The thrust complex is part of the Appalachian Central Mobile Belt and is partitioned into steep belts and flat belts according to the attitude of the schistosity. The steep belts are coeval with Late Silurian plutonism and low-pressure metamorphism, representing transpressional zones (D2) in which the extension lineation of D1 thrusting was tilted twice. Flat belts represent a later flattening (D3) that probably resulted from extensional collapse and preceded early Devonian dextral transpression (D4) along the Central Mobile Belt. -from Authors
The Rangeley area of western Maine is underlain by a thick sequence of dominantly eugeosynclinal metasedimentary rocks of Ordovician, Silurian, and Devonian age. The dominant structural pattern of these rocks is defined by tight, upright, northeast-trending passive flow folds and by three major normal faults along which younger rocks on the southeast are down-faulted against older rocks on the northwest. Each normal fault, together with a major syncline and a complementary anticline farther southeast, defines a geometrically related fault-fold unit. In best-exposed units, displacement along the faults increases in the direction of plunge of the synclines and of increasing structural relief in the syncline-anticline pairs. A genetic relation between normal faulting and folding is inferred.
The dominant fault-fold pattern represents the oldest recognized deformation in the area. Slaty or phyllitic cleavage of this deformation is typically subparallel to the axial surfaces of folds, but locally crosses the faults and the axial surfaces of tight folds at low angles. Metamorphosed clastic dikes along the cleavage suggest that cleavage formation was in part a diagenetic dewatering process. This process probably graded, however, into low-grade metamorphism at depth. It was quickly followed by emplacement of large plutons, local superposed passive slip and flexural slip folding, and by two recognized events of greenschist and amphibolite facies metamorphism. Porphyroblasts of these events have grown across slip cleavages as well as older phyllitic cleavage, and metamorphic zones cross the dominant fault-fold pattern.
Deformation, as well as sedimentation, is considered to have been controlled by the ancestral Merrimack synclinorium—a strongly linear two-sided trough that persisted at least from Late Ordovician through Early Devonian time. The fault-fold pattern is inferred to have evolved over a long period of time, as follows: (1) Rapid deposition of 15,000 to 20,000 ft of nearly-impermeable clastic sediments in Late Ordovician and Early Silurian time on the southeast-dipping slope of the sedimentary trough; mass weakened in depth by excess fluid pressure. (2) Continuing sedimentation, down-to-basin creep with associated slump faulting and folding, probably beginning in Middle Silurian time; faults flattened basinward in depth along lower boundary of zone of excess fluid pressure. (3) Horizontal compression developed parallel to slide direction as mass piled against material in the trough; incipient slaty cleavage developed normal to compression, improving vertical permeability. (4) Pore fluids expelled vertically, permitting the slumping mass to compact horizontally, and fold with at least 25 percent shortening. The process culminated in Early Devonian time, during and after deposition of the youngest exposed rocks in the area.
Wickham and Oxburgh1 recently proposed that low-pressure/high-temperature (low-P/high-T) metamorphism in the eastern Pyrenees, and possibly all low-P/high-T metamorphic belts, resulted from anomalously high mantle heat flow brought about by rifting. Their model is largely constrained by the presence of nearby synmetamorphic rift-related sedimentary rocks and the interpretation that the migmatites and granites are the product of in situ melting in the presence of an anomalously steep geotherm. Here we present an alternative model, in which low-P/high-T metamorphism (pro-grade reactions at pressures near or below the Al2SiO5 triple point) results from contact effects near sill-like igneous intrusions at intermediate crustal levels. Low-P/high-T conditions can be achieved through this process in regions of continent–continent collision with normal mantle heat flux as well as in zones of extension. Our model is based on studies of the low-P/high-T metamorphic terrane in the New England Appalachians.
Textural patterns characteristic of supposed pre-, post- and to a lesser extent syn-tectonic porphyroblasts are reviewed. Difficulties of interpretation arise where an Si (internal schistosity) is absent, and Se (external schistosity) behaviour adjacent to porphyroblast margins is the only available criterion; examples may be found showing truncation of Se by pre-Se porphyroblasts. Truncation and/or deflection textures are not by themselves sufficient criteria for determining the time relations of particular phases of deformation and crystallization.
Differential exhumation in Maine exposes nearly half of the crustal profile of the Norumbega fault zone (NFZ), a major transcurrent fault zone of the Northern Appalachians. A large exposure of turbidites in eastern Maine records a multi-stage deformation history that provides insight into the epizonal evolution of the NFZ. Early deformation (Stage 1) involved episodic bed-parallel faulting with fluids playing a progressively smaller role. Strain was broadly distributed at the exposure but was localized by lithic anisotropy in pelitic layers. Little Stage 1 deformation occurred in the dominant wacke beds (by diffusive mass transfer); most took place in subordinate pelitic horizons by combined diffusive mass transfer and crystal plastic mechanisms. Later (Stage 2) activity was almost entirely brittle, producing isolated NE-trending faults and two deformation zones filled with cataclasite. The latter comprise complexly anastomosing fault strands with multiple slickenline generations indicating episodic faulting.Regional-scale NFZ structures and history mirror those at outcrop scale. Late brittle faulting was superimposed on a broad region that had experienced largely ductile shearing, and the regional-scale brittle deformation was concentrated in three deformation zones characterized by anastomosing fault strands. It is the Stage 2 features that are mapped elsewhere in the Northern Appalachians as the Norumbega fault zone.
A study of the occurrence of and relations between rare-earth element (REE) minerals in pelitic schists indicates that monazite forms at or near the P and T of the staurolite isograd. Samples at staurolite grade from the Silurian Perry Mountain Formation in the Rumford quadrangle of Maine yield monazite in sufficient quantities to permit accurate dating of the metamorphic events forming the monazites. The bulk chemistry of the metapelites, as seen in the major element abundances and REE patterns, does not vary significantly across the study area. Thus the appearance and disappearance of REE phases is assumed to reflect changes in metamorphic grade. In a sample from the biotite zone, scanning electron microscope and microprobe studies show allanite and monazite intimately associated on a 10 m scale. The texture suggest that metastable detrital monazite breaks down, distributing its REE components to allanite. From samples below staurolite grade in which monazite is not present, our observations suggest that REEs are partitioned into allanite. At or near the staurolite isograd monazite forms as a metamorphic mineral, initiating its role as a geochronometer. Garnet-biotite geothermometry on samples at this grade from this and other studies places constraints on the minimum temperature necessary to form monazite: 525 C25C at 3.10.25 kbar. A total of 15 separates from nine schist samples ranging up to sillimanite grade have been dated. Each date is remarkably concordant, even though petrologic and textural studies by previous workers have shown that the rocks in the area have been affected by at least three metamorphic episodes. Calculations indicate insignificant Th disequilibrium in these monazites. The conditions associated with the metamorphic events suggest that monazite remains closed to lead loss provided that subsequent metamorphisms are at or below sillimanite grade. Two distinct metamorphic events are resolved, one at around 400 Ma and one at about 370 Ma. The latter was due to thermal effects of a nearby pluton that yields concordant monazite ages of 363 Ma. This work suggests that in addition to dating plutonism and high-grade metamorphism, monazite should be viewed as a reliable geochronometer for moderate metamorphism of pelitic schists.
Neodymium and lead isotope and elemental data are presented for the Sebago batholith (293±2 Ma), the largest exposed granite in New England. The batholith is lithologically homogeneous, yet internally heterogeneous with respect to rare earth elements (REE) and Nd isotopic composition. Two-mica granites in the southern/central portion of the batholith (group 1) are characterized by REE patterns with uniform shapes [CeN/YbN (chondrite normalized) = 9.4–19 and Eu/Eu* (Eu anomaly) = 0.27–0.42] and ɛ
Nd(t) = −3.1 to −2.1. Peripheral two-mica granites (group 2), spatially associated with stromatic and schlieric migmatites, have a wider range of total REE contents and patterns with variable shapes (CeN/YbN = 6.1–67, Eu/Eu* = 0.20–0.46) and ɛ
Nd(t) = −5.6 to −2.8. The heterogeneous REE character of the group 2 granites records the effects of magmatic differentiation that involved monazite. Coarse-grained leucogranites and aplites have kinked REE patterns and low total REE, but have Nd isotope systematics similar to group 2 granites with ɛ
Nd(t) = −5.5 to −4.7. Rare biotite granites have steep REE patterns (CeN/YbN = 51–61, Eu/Eu* = 0.32–0.84) and ɛ
Nd(t) = −4.6 to −3.8. The two-mica granites have a restricted range in initial Pb isotopic composition (206Pb/204Pb = 18.41–18.75; 207Pb/204Pb = 15.60–15.68; 208Pb/204Pb = 38.21–38.55), requiring and old, high U/Pb (but not Th/U) source component. The Nd isotope data are consistent with magma derivation from two sources: Avalon-like crust (ɛ
Nd>−3), and Central Maine Belt metasedimentary rocks (ɛ
Nd<−4), without material input from the mantle. The variations in isotope systematics and REE patterns are inconsistent with models of disequilibrium melting which involved monazite.
The Phillips pluton (age of 403.8±1.3 Ma) was assembled at a crustal level below the contemporary brittle-plastic transition during regional dextral-reverse transpressive deformation. The pluton is composed dominantly of medium- to coarse-grained leucogranite sensu lato (s.l.), but within its bounds includes decametric massive outcrop of fine- to medium-grained granodiorite (s.l.). In places, the leucogranite contains centimetric enclaves apparently of the granodiorite. Granodiorite is host to more biotite than muscovite, and more calcic, oscillatory-zoned plagioclase, compared to the leucogranite. Pegmatitic granite and composite pegmatite–aplite occur as metric sheets within the pluton and as larger bodies outside the pluton to the SW. Magmatic fabrics, defined by biotite schlieren, occur locally in the leucogranite; the attitude of these fabrics and layering within the leucogranite are concordant with the NE-striking, steeply-dipping country rock foliation. K2O contents, Rb/Sr ratios, Rb, Sr and Ba covariations, and chondrite-normalized rare earth element (REE) patterns of leucogranite are consistent with high-to-moderate a(H2O) muscovite dehydration equilibrium eutectic melting of a predominantly pelite source similar to metasedimentary rocks of the surrounding central Maine belt (CMB). The REE patterns and Rb/Sr ratios of granodiorite also suggest derivation from a metasedimentary source, but more likely by moderate-to-low a(H2O) (muscovite-) biotite dehydration equilibrium eutectic to non-eutectic (minimum) melting of a protolith dominated by greywacke in which garnet and plagioclase were residual phases. Both granite (s.l.) types have heterogeneous initial Nd isotope compositions. Samples of granodiorite define a range in εNd (404 Ma) of −1.8 to +0.1 (±0.3 2σ uncertainty), and samples of leucogranite define a range in εNd (404 Ma) of −8.0 to −5.3 (±0.3 2σ uncertainty). This bimodal distribution suggests that melts were derived from a minimum of two sources. The data are consistent with these sources being CMB metasedimentary rocks (εNd (404 Ma)<−4) for the leucogranite, and Avalon-like (peri-Gondwanan) metasedimentary crust (εNd (404 Ma)>−4) for the granodiorite. The range of Nd isotope compositions within each granite type most likely reflects isotopic heterogeneity inherited from the source. These data imply that the integrity of individual melt batches was maintained during ascent, and that extensive mixing of melt batches during emplacement at this level in the pluton did not occur, although centimetric enclaves have intermediate Nd isotope compositions consistent with small-scale interactions between magmas. We infer that the Phillips pluton represents the root of a larger pluton, and that what remains of this larger pluton is the feeder constructed from multiple melt batches arrested during waning flow of granite magma through a crustal-scale shear zone system.
Low-pressure/high-temperature metamorphic belts occur throughout the world. They display a characteristic metamorphic style which has remained unchanged with time. They are found in disparate tectonic settings, including magmatic arcs, regions of extension, regions of thickened crust in continent-continent collision zones, and accretionary wedges. They are commonly characterized by an abundance of granitoid plutons. Pressures of metamorphism are primarily less than or equal to that of the aluminosilicate triple point (approximately 4 kbar), while temperatures are in the range 500–750 °C. With rare exceptions, inferred PTt paths are nearly isobaric and recrystallization was rapid and prograde. Metamorphic thermal gradients are commonly in the range 60–150 °C/km. These conditions necessitate the presence of a large component of advective heat transfer and/or anomalously high basal heat flow during metamorphism.The large magmatic fluxes associated with the formation of magmatic arcs provide an adequate heat source for low pressure metamorphism (LPM). Lithospheric extension is likely to produce LPM only if the degree of thinning is large (β > 3) and there is additional heat from magmas. The generation of LPM by crustal thickening requires either a large component of advective heating by magmas, anomalously high basal heat flow, or pre-thickening lithospheric extension. LPM in this environment must necessarily be accompanied by plutonic activity. Advection of heat by aqueous fluids may provide a significant contribution to the thermal budget but, acting along, is unlikely to produce LPM.Regional LPM induced by advective sources, rarely occurs along a geotherm which increases monotonically with depth, or synchronously across the region at a given depth; it occurs over time as a series of overlapping temporally separate and spatially localized thermal events.
In a rock pile undergoing prograde regional metamorphism, the isotherms corresponding to certain metamorphic reactions shift continuously, while the same rock volume develops progressive deformation fabrics as a result of increasing strain. Porphyroblasts in pelitic schists can provide crucial information on both these aspects of metamorphism. Not only do they reflect the pressure-temperature evolution of that terrain, but their inclusion trails are clues to the state of deformation at the time of porphyroblast growth. Microstructural relationships of inclusion trail patterns versus matrix foliations, combined with petrological data on prograde reaction sequences, can be used to reconstruct temperature-time relationships in different zones within metamorphic terrains.In two Proterozoic areas in northern Queensland, the western Mary Kathleen Fold Belt and the Robertson River Subgroup, a systematic variation of inclusion trail patterns has been found across metamorphic zones. Each porphyroblast species contains more advanced stages of foliation development towards its isograd, compared with higher-grade zones in which these porphyroblasts are present. The combined microstructural-petrographic observations allow to place constraints on the T−t evolution of the various zones. The regional distribution of inclusion trail patterns in the two study areas indicates that deformation affected high- and low-grade zones simultaneously.
We propose a model for syntectonic ascent and emplacement of granite magma based on structural relations in part of the northern Appalachians. In the study area in western Maine, strain was distributed heterogeneously during Devonian Acadian transpression. Metasedimentary rocks (migmatites at high grades) record two contrasting types of finite strain in zones that alternate across strike. Rocks in both types of zones have a penetrative, moderately-to-steeply NE-plunging mineral elongation lineation defined by bladed muscovite (fibrolite/sillimanite at high grades). In `straight' belts of enhanced deformation rocks have S > L fabrics that record apparent flattening-to-plane strain (apparent flattening zones, AFZs), but rocks between these belts have L > S fabrics that record apparent constriction (apparent constriction zones, ACZs). At metamorphic grades above the contemporary solidus, rocks in AFZs developed stromatic structure in migmatite, which suggests that percolative flow of melt occurred along the evolving flattening fabric. Stromatic migmatites are intruded by concordant to weakly discordant, m-scale composite sheet-like bodies of granite to suggest magma transport in planar conduits through the AFZ rocks. Inhomogeneous migmatite is found in the intervening ACZs, which suggests migration of partially molten material through these zones en masse, probably by melt-assisted granular flow. Inhomogeneous migmatites are intruded by irregular m-scale bodies of granite that vary from elongate to sub-circular in plan view and seem cylindrical in three dimensions. These bodies apparently plunge to the northeast, parallel to the regional mineral elongation lineation, to suggest magma transport in pipe-like conduits through the ACZ rocks. We postulate that the form of magma ascent conduits was deformation-controlled, and was governed by the contemporaneous strain partitioning. Magma ascent in planar and pipe-like conduits through migmatites is possible because oblique translation during contraction displaces isotherms upward in the orogenic crust to form a thermal antiform. Within this hot corridor, it is the difference in temperature between melt-producing reactions in the anatectic zone and the wet solidus for granite melt that enables magma to migrate pervasively up through the orogenic crust without congealing. Heat advected with the migrating melt promotes amplification of the thermal antiform in a feedback relation that extends the zone of plastic deformation and pervasive melt migration to shallower levels in the crust. At the wet solidus, we suggest melt flows obliquely toward axial culminations in the thermal antiform, which are sites of melt accumulation and perturbations from which magma may escape to form plutons. Batches of melt that escape from these perturbations may be trapped by tectonic structures higher in the crust, or ascent may become inhibited with decreasing depth by thermal arrest and solidification. If the rate of arrival of subsequent melt batches exceeds the rate of crystallization at the site of pluton construction, melt pressure ultimately may lead to (sub-)horizontal magma fracture, or viscous flow of wall rocks may allow lateral spreading. The resultant plutons have (sub-)horizontal tabular geometries with floors that slope down to the ascent conduit. As the thermal antiform decays, the height of sub-solidus crust that separates the deepest part of the pluton from the anatectic zone increases. Consequently, pluton inflation declines and solidification leads to infilling of the magma feeder channel to form a root zone that passes downward into migmatite, which may explain the difficulty of determining precisely the floor to these deeper segments of large plutons using gravimetry.
In orogenic belts, the common spatial and temporal association of granites with crustal-scale shear-zone systems suggests melt transfer from source to upper crust was the result of a feedback relation. In this relation, the presence of melt in the crust profoundly affects the rheology, and induces localization of strain within shear-zone systems. Consequently, melt is moved out of the source preferentially along high-strain zones, which helps the system to accommodate strain. Because actively deforming orogenic belts are non-equilibrium systems, they may generate dissipative structure by self-organization; we interpret crustal-scale shear-zone systems and their associated granites as the manifestation of this self-organization. The architecture and permeability structure are controlled by the type of shear-zone system (transcurrent, normal, reverse or oblique); this is the primary control on melt transfer in orogenic belts. During active deformation, movement of melt is by percolative flow and melt essentially is pumped through the system parallel to the maximum principal finite elongation direction. If a build-up of melt pressure occurs, melt-enhanced embrittlement enables tensile and dilatant shear fracturing, and transfer of melt is by channelized flow.
Based on a case study in the Central Maine Belt of west-central Maine, U.S.A., it is proposed that crustal-scale shear zone systems provide an effective focussing mechanism for transfer of granite melt through the crust in convergent orogens. During contractional deformation, flow of melt in crustal materials at depths below the brittle–plastic transition is coupled with plastic deformation of these materials. The flow is driven by pressure gradients generated by buoyancy forces and tectonic stresses. Within the oblique-reverse Central Maine Belt shear zone system, stromatic migmatite and concordant to weakly discordant irregular granite sheets occur in zones of higher strain, which suggests percolative flow of melt to form the migmatite leucosomes and viscous flow of melt channelized in sheet-like bodies, possibly along fractures. Cyclic fluctuations of melt pressure may cause instantaneous changes in the effective permeability of the flow network if self-propagating melt-filled tensile and/or dilatant shear fractures are produced due to melt-enhanced embrittlement. Inhomogeneous migmatite and schlieric granite occur in zones of lower strain, which suggests migration of partially-molten material through these zones en masse by granular flow, and channelized flow of melt carrying entrained residue. Founded on the Central Maine Belt case study, we develop a model of melt extraction and ascent using the driving forces, stress conditions and crustal rheologies in convergent, especially transpressive orogens. Ascent of melt becomes inhibited with decreasing depth as the solidus is approached. For intermediate a(H2O) muscovite-dehydration melting, the water-saturated solidus occurs between 400 and 200 MPa, near the brittle–plastic transition during high-T–low-P metamorphism, where the balance of forces favors (sub-) horizontal fracture propagation. Emplacement of melt may be accommodated by ductile flow and/or stoping of wall rock, and inflation may be accommodated by lifting of the roof at shallower crustal levels and/or sinking of the pluton floor. The resultant plutons have (sub-) horizontal tabular geometries with floors that slope down to the ascent conduits. Although these plutons may have locally discordant relations with country rock structures, when viewed at the crustal-scale, granite ascent and emplacement in convergent orogens are syn-tectonic processes.
Observational data and geochronology show synchronous deformation, metamorphism and intrusion of granite in many obliquely convergent (transpressive) orogenic belts. In these belts, melt extraction and transport was by syntectonic pulsed flow in structurally-controlled channels through migmatites, and granite plutons were constructed by aggregation of multiple melt batches. Compositional information in the granites provides clues to unraveling the petrological processes by which their parental magmas were generated and extracted from the crust, but we need to read the information with sufficient insight to enable prudent interpretation, because we cannot see granite in process of formation! In this paper, we evaluate the role of source processes in granite petrogenesis using results from west-central Maine, in the northern part of the Appalachian orogenic belt of eastern North America. In this area, precise U-Pb zircon/monazite crystallization ages of schlieric granite in migmatite, metric to decametric sheets of granite and kilometric granite plutons are in the range c. 408-404 Ma, within 1 Ma at 95% confidence limits, and in one granite, the Phillips pluton, multiple samples of leucogranite and granodiorite yield synchronous ages. If the results of this study are representative, they suggest that in restricted segments of orogenic belts regionally-significant crustal melting in nature occurs within short timescales (~ 107a), and melt extraction and transport, and pluton construction are fast processes. In the Phillips pluton, petrographic features and geochemistry of leucogranite samples, including K2O contents, ratios, covariation in Rb, Sr and Ba distributions, and chondritenormalized REE patterns, are inconsistent with fractional crystallization, but compatible with eutectic high-to-moderate a(H2O) muscovite-dehydration melting of a predominantly pelite source. Petrographic features and geochemistry of granodiorite samples, including chondrite-normalized REE patterns and ratios, also suggest derivation from a metasedimentary source, but by non-eutectic (minimum) moderate-to-low a(H2O) biotite-dehydration melting of greywacke. In this pluton, the two types of granite s.l. show heterogeneity in Nd isotope compositions (granodiorite εNd (404 Ma) of + 0.1 – −1.8; leucogranite εNd (404 Ma) of −5.3 –−8.0), which we interpret to reflect derivation of the two granites from isotopically-distinct sources, to preserve withinsource heterogeneity and to imply efficient extraction, ascent and emplacement of melt without significant interaction between individual batches.
A method to demonstrate the simultaneous development of different minerals is discussed. Minerals are contemporaneous when they have formed before and after a point in time. Points in time are supplied by phases of deformation. The relations between deformation and crystallization can be deduced from the internal and external structures of porphyroblasts and matrix.
This method is applied to unravel the metamorphic history of the Bosost area. Four metamorphic zones succeed one another: the biotite-zone, stauroliteandalusite-cordierite-zone, andalusite-cordierite-zone and cordierite-sillimanitezone. This zonal distribution represents a progressive metamorphic series and each higher metamorphic zone has passed through one or more lower metamorphic zones. Even within one zone a further subdivision of the crystallization history can be made.
The described mineral associations are the result of low pressure metamorphism. They can be assigned to a low pressure faciesgroup of the amphibolite facies. The geothermal gradient during metamorphism was approximately 15° C pro 100 meter.
The character of sedimentary basins, before they are deformed and metamorphosed, may strongly influence the thermal and baric patterns of metamorphic belts. Crustal thickening of anoxic sedimentary basins and subsequent thermal reequilibration may produce large areas of high-grade metamorphic rocks and granites because the concentrations of the heat-producing elements are high in such basins. In New England there is a spatial association among granites and high-grade metasedimentary rocks rich in U and Th that now form the Central Maine terrane. The high content of heat-producing elements in these rocks is attributed to fixing of U and Th in highly reduced sediments that were deposited in an anoxic basin that formed in the Silurian. When the basin was thickened during the Devonian Acadian orogeny, the thermal energy generated by the U- and Th-rich sediments produced the observed broad zone of high-grade rocks and anatectic granites. This hypothesis was tested with thermal calculations that reproduce most of the first-order thermal and baric patterns of the Acadian Appalachians, if pretectonic lateral variations in heat production are assumed.