FIGURE 5 - uploaded by Gary S. Solar
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MnNCKFMASH bathozones (cf. Carmichael 1978). Emboldened lines are the bathograd-defining equilibria.
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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,...
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... an example, Figure 5 shows phase relations for the aver- age metapelite composition with the important bathograd-de- fining equilibria highlighted. Six bathozones are shown and numbered to be consistent with the original notation of Carmichael (1978). ...
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... action 4 remain essentially invariant. Reaction 5 is a continu- ous reaction that runs over 5-10 ∞C at fixed P (Figs. 2 and 5). Consequently, bathograds that are constrained by reaction 5, [i.e., separating bathozones 2 from 3 and 5 from 6 (Carmichael 1978)] are also continuous, and define a narrow P range (i.e., transitional bathograd 2-3t in Fig. 5). The bathograd separat- ing bathozones 3 and 4 defines whether rocks followed a P-T evolution that passed at P lower than or greater than the Al 2 SiO 5 triple point. In addition to the original bathgrads, we have subdivided bathozone 2 on the basis of the essentially compositionally independent low-variance point "IP1" (Fig. 2), ...
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... IV is characteristic of bathozone 2 in the KFMASH system (Carmichael 1978), but may also occur within the transition between bathozones 2 and 3 in the expanded sys- tem (bathozone 2-3t in Figs. 5 and 6a). Assemblage Va is diag- nostic of bathozone 2b, whereas assemblage Vb is diagnostic of bathozone 2-3t and 3. Consequently, at the lower Al-silicate to upper Al-silicate transition (Figs. 1 and 6a), P conditions in the northeast of the study area occur entirely within bathozone 2 (P < 3.9 kbar, below the star in Fig. 6a). In the south, ...
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Citations
... It is composed of interlayered pelite and psammite rocks, metamorphosed to greenschist and amphibolite facies during the Acadian Orogeny (Solar and Brown 2001). Peak metamorphic conditions occurred at ca. 408-404 Ma with temperatures of around 500-520 °C for lower-grade metapelitic rocks (Solar et al. 1998;Johnson et al. 2003). U-Pb age determination from apatite and cassiterite yielded ages of 270.4 ± 8.1 Ma and 270-265 Ma, respectively, while 40 Ar/ 39 Ar age determinations of muscovite yielded ages of 250.4 ± 2.3 Ma, for the Emmons pegmatite, (Bradley et al. 2016;Felch et al. 2022). ...
The anisotropic textures, including unidirectional solidification textures and graphic intergrowths, characteristic for pegmatites, are interpreted to result from disequilibrium crystallization at high degrees of undercooling. Experimental studies have revealed the existence of thin boundary layers surrounding the rapidly growing crystals. Here, tourmaline-bearing samples from the outer zones of the Emmons pegmatite (Maine, U.S.A.) are used to examine if a boundary layer can also occur in natural samples. Crystal morphology is linked with geochemistry to understand the evolution of pegmatite melts and to constrain disequilibrium conditions at large degrees of undercooling. Petrographic studies and semiquantitative micro-X-ray fluorescence element mapping were conducted to identify crystal morphology and zonation, complemented with electron microprobe analyses to determine major and minor element compositions and LA-ICP-MS analyses of selected trace elements. Three textural groups were identified: comb-like tourmaline, quartz-tourmaline intergrowths, and radiating tourmaline. The intergrowths are optically coherent and are split into three different morphologies: central, second tier, and skeletal tourmaline. Most tourmaline is schorl, but chemical variation occurs on three different scales: between textural groups, between different morphologies, and intracrystalline. The largest scale geochemical variation is caused by the progressive evolution of the melt as it crystallized from the borders inwards, while the intracrystalline variations are attributed to sector zoning. A model is suggested where the systematic variation of Mg, Mn, and Fe within individual intergrowths is proposed to be the result of crystallization from a boundary layer, rich in water and other fluxing elements (e.g., Li, P, B), formed around the rapidly growing central tourmaline. Here, we show the first examples of boundary layers in natural pegmatites. Furthermore, the results bring into question whether boundary layer tourmaline can be used as a bulk melt indicator in pegmatitic melts.
... Although chloritoids are more commonly associated with higher-grade metamorphic index minerals, such as andalusite, kyanite, and staurolite [31], hydrothermal activity has been reported to induce the growth of chloritoid minerals [32]. The low-grade metamorphic ensemble of chloritoid, chlorite, and muscovite of the upper unit of the Yedong Member, Socheong Formation indicates Fe enrichment and high contents of Al [30,[33][34][35], which can be explained by the addition of hydrothermal iron to Al-rich pelitic rocks. ...
During the latest Mesoproterozoic–Early Neoproterozoic era, extensional regimes generated a number of sedimentary basins in various regions in the Sino-Korean Craton. Mantle-plume emplacements are widely recognized in the sedimentary strata as mafic dikes and sills of the Dashigou Large Igneous Province (LIP). The occurrence of Fe-rich redbeds is first reported in the Neoproterozoic Socheong Formation of the Sangwon Supergroup in the Pyeongnam Basin. Their geochemical and mineralogical characteristics indicate basin-wide Fe enrichment due to hydrothermal fluid input. The episodic yet repetitive hydrothermal injection into the basin generated short-lived anoxia, recorded as greenish-gray coloration in the ferruginous beds. This hydrothermal fluid was likely sourced from the mafic igneous activities involved in the Dashigou LIP. The redbeds can be utilized as key beds for intra- or inter-basinal stratigraphic correlation and to study the negative carbon isotope excursions that occurred in the genetically related basins in the region (e.g., the Sangwon, Xu-Huai, and Dalian basins).
... All data has been verified by comparison with P-T pseudosection modelling for similar bulk compositions in the literature (e.g. Tinkham and Ghent, 2005;White et al., 2001;Johnson et al., 2003;Stipska et al., 2006). Data is listed as the average and standard deviation of the population, with the number (n) of data listed. ...
... kbar based on Si-in-white mica geobarometry (Supplementary Data, Table S3; Massone and Schreyer, 1987). All pressure estimates from the northern flank are largely consistent with the stability fields of garnet-free, cordierite-biotite ± sillimanite metapelite parageneses, which are limited to 4.0-4.5 kbar (Johnson et al., 2003;Tinkham and Ghent, 2005). The pooled average of all Pestimates from the northern flank is 4.3 ± 0.5 kbar (n = 13), which is similar to the South Ugab Zone (Table 2; Supplementary Data, Table S2). ...
... Less accurate averagePT calculations using garnet cores give slightly higher pressures of $5.4-6.0 kbar that may be representative of prograde conditions. Prograde garnet was probably not in equilibrium with matrix cordierite, which formed later during decompression below $4.5-5.0 kbar (White et al., 2001;Johnson et al., 2003;Tinkham and Ghent, 2005;Stipska et al., 2006). Highresolution LA-MC-ICP-MS spot analysis of monazite in sample BG18-191a yield typical prograde and peak metamorphic 206 Pb/ 238 U age dates of 545 ± 5 Ma (n = 3) and 537 ± 6 Ma (n = 2), that are consistent with relatively deep burial of the leading edge early in the Damaran orogenic cycle (Table 3; Supplementary Data, Table S4). ...
Collision, high-angle contraction, crustal thickening and heating at 555–516 Ma, primed the Damara Belt ready for crustal collapse, which was triggered by a transition to ENE–WSW contraction along the length of the belt in response to orogenic events in east Gondwana at 516–505 Ma. Along-orogen shortening reworked and thickened the high-grade core of the belt, increasing gravitational instability, and establishing an NW–SE extension direction across the belt that was conducive to reactivation of pre-existing structures and eventual collapse. This extension direction persisted, and the switch to vertical σ1 and collapse was signalled by decompression melting at ∼502 Ma and subsequent rapid cooling. Collapse was focused on the high-grade core of the belt that was exhumed as a ∼170 km wide, semi-coherent massif-type metamorphic core complex with steep extensional shear zones and faults in the marginal flanks. During exhumation the core complex was reactivated by oblique-slip extensional shear zones that responded to external transient stress fields. Reactivation by middle- to lower-amphibolite facies dextral-normal shear zones at ∼500–495 Ma and ∼495–490 Ma, involved E–W to ENE–WSW shortening consistent with accretionary events in the west Gondwana margin during the Pampean Orogeny. Reactivation by greenschist facies sinistral-normal shear zones at ∼485 Ma, involved N–S shortening consistent with accretionary events in the south Gondwana margin during the Famatinian Orogen. Early stages of exhumation involved decompression melting, flattening folds and ductile ultramylonite zones within carbonate that formed by NW–SE extension. Late stage exhumation in the brittle field from ∼480 Ma onwards, involved a stress-switch to radial extension directions dominated by NE–SW. This stage involved flat-lying breccia, inclined faults, vertical fractures, and oxidizing fluids partitioned into the top of the lower-levels of the massif. Ongoing exhumation of the core complex drove localized NW–SE shortening within the flanking margins and hanging-wall, and produced low-strain reverse structures that straddle the ductile to brittle transition. The pressure difference between exhumed massif (4.8–5.5 kbar) and hanging-wall margins (3.9–4.2 kbar), indicate that ∼3.2–4.6 km of crust was stripped from above the core complex.
... 400 Ma throughout western and central Maine and New Hampshire are consistent with large scale high-T Acadian metamorphism (Gerbi & West, 2007;Moecher et al., 2021;Pyle et al., 2005;Sanislav, 2011;Smith & Barreiro, 1990;Solar et al., 1998;Wing et al., 2003). In the region surrounding the Mooselookmeguntic Igneous Complex (Figure 1a), Acadian metamorphic conditions are thought to have exceeded 450 MPa and 700 C in the migmatites, resulting in syntectonic melt generation, percolative flow, and emplacement of granite magmas Johnson et al., 2003). ...
... These data, in conjunction with the pooled ca. 376 Ma age of zircons from migmatites to the west of our field area , suggest that anatexis and deformation in this region occurred 20-30 Ma later than in the migmatites in the vicinity of the Mooselookmeguntic pluton ( Figure 1a) Johnson et al., 2003). Partial melting ceased prior to ca. 295 Ma, F I G U R E 1 2 Percent of common Pb (f207) in titanite plotted relative to (a) date and (b) Sr concentration. ...
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.
... For example, based on existing experimental data on specific ferromagnesian minerals, we may be able to make some general assumptions regarding the energetics of mixing between Fe 2+ and Mg in all ferromagnesian minerals, and solve for other X Mg (Mg/(Mg + Fe 2+ )) of coexisting ferromagnesian minerals (Droop and Harte, 1995;Harte and Hudson, 1979). If the sequence of isograds or composition of coexisting phases predicted by calculations using the relevant a-X models is inconsistent with these observations, one or more may need to be adjusted (Cesare et al., 2003;Johnson et al., 2003a;Pattison et al., 2002). The a-X models, also known as solution models, are many and varied in their applicability and complexity (see for example: ...
... For example, in many crustal rocks, phosphorous, which currently cannot be modelled as a component, is mostly concentrated in apatite (Ca 5 (PO 4 ) 3 (OH, F, Cl)). In such cases, as each mole of P 2 O 5 in (end member) apatite is associated with 3.33 moles of CaO, this amount can be removed from the bulk composition before any phase equilibrium calculations (Johnson et al., 2003a). However, in many high-grade metapelitic rocks, most phosphorous resides in monazite, and P 2 O 5 can simply be ignored. ...
Partial melting is the fundamental process by which juvenile crust was produced from the mantle and subsequently reworked to become the stable, compositionally-differentiated continents on which we live and which host most of the elemental resources required by our modern technological society. Irreversible differentiation of the continental crust occurs principally through the production, segregation and migration of silica-rich (felsic) melts from deeper source rocks to shallower sinks where they erupt or, more commonly, crystallise as granite sensu lato. Here we provide for both novices and professionals a comprehensive, but accessible, account of the processes involved in crustal melting and suprasolidus phase equilibria from first principles to the forefront of modern research. To begin we introduce the tectono-metamorphic context for crustal melting before considering the evidence at outcrop and in thin section for partial melting, and briefly reviewing melt extraction from crustal rocks. As a prerequisite to understanding the physicochemical basis for crustal melting, we summarize the essential thermodynamics that underpin calculated phase equilibria, distinguish different types of melting reaction, and review the requirements for, methodology behind, and limitations of a phase equilibrium modelling approach based on equilibrium thermodynamics. We explain the various types of phase diagram used to investigate partial melting and assess open versus closed system processes, including internal and external buffering of H2O. Those crustal sources that partially melt to produce granite are considered in detail, namely basic rocks such as basalts and gabbros, clastic sedimentary rocks such as greywackes, siltstones and mudstones (pelites), and granites themselves. We concentrate mainly on intracrustal partial melting in convergent-margin settings, and on anatexis during exhumation of deeply-subducted continental crust from mantle depths. We discuss the behaviour of trace elements and accessory minerals during melting, and consider the implications for isotope geochemistry. To close we include a brief summary of some of the important points and offer a few suggestions for future lines of research on crustal melting.
... Р-Т условия стабильности хлоритоидсодержащих метапелитов варьируют от зеленосланцевой фации до средней части амфиболитовой фации ставролит-кианитовой зоны. В фации зеленых сланцев хлоритоид является обычным минералом, связанным с хлоритом и мусковитом в породах, содержащих железа и алюминия больше, чем необходимо для стабильности парагенезиса Chl-Ms (Turner, Verhoogen, 1960;Albee, 1983;Johnson et al., 2003). ...
... The area (Figure 1) is ideal for this type of study because the PT conditions, mineral and structural paragenesis are well documented by numerous studies (e.g. Guidotti, 1974Guidotti, , 1989Guidotti, , 2000Guidotti & Johnson, 2002;Henry & Guidotti, 2002;Johnson, Brown, & Solar, 2003;. ...
... The western Maine region, particularly around the Mooselookmeguntic pluton, represents one of the beststudied regions in the world for low-P high-T metamorphism with detailed mapping of mineral assemblages and metamorphic isograds. At least three metamorphic events have been described in detail (Guidotti, 1989;Guidotti & Holdaway, 1993;Guidotti & Johnson, 2002), and the overall metamorphic grade of the last metamorphism increases along strike towards the southwest from greenschist to upper amphibolite facies conditions, and widespread migmatization occurs south of the Mooselookmeguntic pluton (Johnson et al., 2003). ...
... Based on textural relationships between different porphyroblastic phases, a clockwise PT path was interpreted for the rocks of western Maine . However, Johnson et al. (2003), based on rocks within the anatectic domain, south of the area shown in Figure 1, proposed an anticlockwise PT path with the retrograde path occurring at a slightly higher pressure than the prograde path. ...
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.
... Most rocks that have experienced collisional metamorphism under amphibolite facies conditions (7-9 kbar/ 550-700°C) with subsequent retrograde decompression (4-5 kbar/450-500°C) would follow clockwise P-T-t paths (Fig. 4.2). However, there are several well-documented examples of metamorphic belts, which have counterclockwise P-T-t paths (Likhanov et al. 2013a, b;Reinhardt and Rubenach 1989;Bohlen 1991;Collins and Vernon 1991;Hand et al. 1992;Rubenach 1992;Johnson and Vernon 1995;Brown 2001;Johnson et al. 2003;Perchuk et al. 2006). The P-T-t paths inferred in these studies correlate with P-T trajectories predicted by thermomechanical models calculated for different crustal thickening and exhumation histories, accounting for different subsidence mechanisms and exhumation rates (Beaumont et al. 2001;Jamieson et al. 2002;Gerya 2014). ...
This chapter presents a generalized P-T-t diagram of the evolution of metamorphic complexes of different geodynamic nature that are characteristic of different types of metamorphism. This diagram was built using the most recent natural observations, which are characterized by the simultaneous presence of prograde and retrograde segments of a P-T path. This chapter discusses some of the ambiguous interpretations of P-T-t paths in areas with simultaneous manifestation of different metamorphic processes. Quantitative models for the analysis of different types of interaction of rocks undergoing metamorphism are exemplified using the following processes involved in the formation of distinctly expressed zoned reaction textures (coronites, kelyphites, symplectites, nodules, and segregations) and mineral transformations in texturally homogeneous rocks between matrix minerals and porphyroblasts. It was shown that mass transfer during metamorphic reactions occurs with the preservation of the material balance within very small local volumes of the rock, which increase from a few hundredths of a mm³ to a few cm³, depending on the duration of metamorphism, P-T parameters, strain intensity and a degree of fluid saturation of rocks. We also consider different kinetic parameters of diffusion-controlled metamorphic reactions, such as mineral reaction mechanisms and rates.
... However, the ingress of H 2 O into high-grade metamorphic rocks can promote additional melting (e.g. Brown, 2010, Yakymchuk & Brown, 2014 in circumstances involving shear zones (Johnson et al. 2001;Slagstad et al. 2005;Berger et al. 2008;Genier et al. 2008;Sawyer, 2010;Reichardt & Weinberg, 2012;Carvalho et al. 2016), extensional fracture systems (e.g. Ward et al. 2008) and contact aureoles (Pattison & Harte, 1988;Johnson et al. 2003Johnson et al. , 2011Droop & Brodie, 2012). Melting rocks may also obtain additional water from those adjacent rocks that have solidi of higher temperature and Modelling low pressure partial melting are undergoing sub-solidus dehydration (e.g. ...
Low-pressure regional aureoles with steep metamorphic field gradients are critical to understanding progressive metamorphism in high-temperature metasedimentary rocks. Delicately layered pelitic and psammitic metasedimentary rocks at Mt Stafford, central Australia, record a greenschist- to granulite-facies Palaeoproterozoic regional aureole, associated with S-type granite plutons, reflecting metamorphism in the range 500–800 °C and at ∼3 kbar. The rocks experienced minimal deformation during metamorphism and partial melting. Partial melting textures evolve progressively along the steep metamorphic field gradient from the incipient stages of melting marked by cuspate grains with low dihedral angles, to melt proportions sufficient to form diatexite with schollen. Phase equilibria modelling in the NCKFMASHTO system for pelitic, semi-pelitic and high- and low- ferromagnesian psammitic samples quantitatively illustrates the dependence of partial melting on rock composition and water volume. Pelitic compositions are more fertile than psammitic compositions when the water content in the rocks is low, especially during the early stages of melting. The whole-rock ferromagnesian component additionally influences melt fertility, with ferromagnesian-rich psammite being more fertile than psammite with a lower ferromagnesian component. Subtle variations in free water content can result in obvious changes in melt volume but limited variation in melt composition. Distinct melting histories of pelitic and psammitic rocks inferred from field relationships may be partially attributed to potential differences in water volume retained to super-solidus conditions. Melt composition is more dependent on the rock composition than the variation in water content.
... system, which adequately describes the composition of the studied metapelites, provides results similar to naturally developed mineral assemblages, and covers most of the spectrum of pelitic compositions in greenschist and amphibolite facies series (Cruz-Uribe, Hoisch, Wells, Vervoort, & Mazdab, 2015;Johnson, Brown, & Solar, 2003;Mahar et al., 1997;Tinkham, Zuluaga, & Stowell, 2001). Ti and Fe 3+ are not included in this model, and the biotite model assumes that the hydroxyl site is full. ...
The Araçuaí orogen is the Brazilian counterpart of the Araçuaí-West Congo orogenic system (AWCO), a component of the Ediacaran-Cambrian orogenic network formed during the amalgamation of West Gondwana. The northwestern portion of the Araçuaí orogen is dominated by a succession of metasedimentary rocks made up of Meso- to Neoproterozoic rift, passive margin and syn-orogenic sequences, locally intruded by post-collisional granites. These sequences are involved in three distinct tectonic units, which from west to east are: the southern Espinhaço fold-thrust system (SE-thrust system), the normal-sense Chapada Acauã shear zone (CASZ), and the Salinas synclinorium. Three deformation phases were documented in the region. The first two phases (D1 and D2) are characterized by contractional structures and represent the collisional development stage of the orogen. The third phase (D3) is extensional and currently viewed as a manifestation of orogenic collapse of the system. The distribution of the metamorphic mineral assemblages in the region characterizes two metamorphic domains. The M-Domain I on the west, encompassing the SE-thrust system and the CASZ, is marked by a syn-collisional (syn-D1) Barrovian-type metamorphism with P-T conditions increasing eastwards and reaching ~8.5 kbar at ~650°C between 575 and 565 Ma. The M-Domain II comprises the Salinas synclinorium in the hanging wall of the CASZ, and besides the greenschist facies syn-collisonal metamorphism, records mainly a Buchan-type metamporphic event, which took place under 3 to 5.5 kbar and up to 640°C around 530 Ma. The northwestern Araçuaí orogen exhibits thus a paired metamorphic pattern, in which the Barrovian and Buchan-type metamorphic domains are juxtaposed by a normal sense shear zone. Lithospheric thinning during the extensional collapse of the orogen promoted ascent of the geotherms and melt generation. A large volume of granites was emplaced in the high grade and anathetic core of the orogen during this stage, and heat advected from these intrusions caused the development of Buchan facies series over a relatively large area. Renewed granite plutonism, hydrothermal activities followed by progressive cooling affected the system between 530 and 490 Ma.
