Article

The Origin of Rapakivi Texture

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Abstract

The mantling of large ovoids of K-feldspar by a rim of plagioclase has been investigated in the rapakivi granites from the Mid-Proterozoic Wiborg batholith of SE Finland. The formation of rapakivi texture, in this the type area, was examined using a variety of techniques including isotopic analyses of mineral separates from specific textural sites. Cathodoluminescence combined with microprobe analysis points to the pulsed development of the mantles involving growth of successive plagioclases of composition An30, An25, and An3, the last being in optical continuity with perthitic plagioclase exsolved from the K-feldspar. Plagioclase mantles have high δ18O and 87Sr/86 signatures relative to K-feldspar, which indicate the presence of a late, low-temperature component thought to represent albite exsolved from the K-feldspar and redistributed onto the ovoid margin. Oligoclase components of the mantles are formed by a similar, although higher-temperature magmatic process. This involves the subsolvus re-equilibration of alkali feldspar compositions with evolving melt conditions. Redistribution of the exsolved plagioclase from the alkali feldspar phenocrysts is linked to high fluorine contents of rapakivi-type magmas, and this major reconstruction of the feldspar phenocrysts generates their distinctive ovoidal shape.

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... The rapakivi texture (sensu lato) observed in many granitoids is characterized by alkali feldspar megacrysts mantled by plagioclase, usually of oligoclase composition (Sederholm, 1891;Eskola, 1930;Vorma, 1971Vorma, , 1976Anderson, 1980;Bladh, 1980;Elliston, 1985;Nekvasil, 1991;Wark & Stimac, 1992;Dempster et al., 1994;Eklund & Shebanov, 1999;Haapala & Rä mö , 1999;Mü ller & Seltmann, 2002;Mü ller et al., 2008;Wang et al., 2011;Zhang et al., 2011;Vernon, 2016). The origin of the texture is enigmatic and numerous theories have been presented to explain the formation of such feldspars (see reviews by Emslie, 1991;Rä mö & Haapala, 1995;Vernon, 2016). ...
... Shkodzinsky, 1985;Nekvasil, 1991;Shebanov, 1993Shebanov, , 1994Shebanov & Eklund, 1997;Eklund & Shebanov, 1999). The subsolvus models for the formation of rapakivi feldspars involve high-temperature exsolution of plagioclase from ternary feldspars followed by its redistribution onto the margin of the phenocrysts in a fluorine-rich environment (Dempster et al., 1994). ...
... However, granitic bodies often preserve textural, mineralogical, and isotopic evidence for extensive deuteric/hydrothermal alteration and fluid-induced subsolidus re-equilibration (e.g. Parsons, 1978;Dempster et al., 1994;Zuddas et al., 1995;Parsons & Lee, 2000;Engvik et al., 2008;Plü mper & Putnis, 2009;Upadhyay & Pruseth, 2012). Oxygen isotope studies (Taylor, 1977;Taylor & Forester, 1979;Hoefs & Emmermann, 1983;Ferry, 1985;Simon, 1990;Gardien et al., 2015) show that many granitic complexes have undergone considerable water-rock interaction, involving both deuteric as well as external fluid circulation systems (Fyfe et al., 1978;Salvi & William-Jones, 1990Pollard et al., 1991;Yardley & Shmulovich, 1995;Zuddas et al., 1995;Zhao et al., 2011). ...
Article
The rapakivi texture found in many granitoids comprises alkali feldspar megacrysts mantled by plagioclase, usually of oligoclase composition. The existing models for their genesis involve magmatic or dry subsolidus processes. Here, we describe the occurrence of rapakivi feldspars in A-type granites from the Malani Igneous Suite in western India and use microtextural and geochemical evidence to argue that rapakivi textures can form by subsolidus deuteric alteration of feldspar megacrysts through a coupled dissolution-reprecipitation replacement process. The feldspars in granites from the Malani Igneous Suite crystallized at temperatures >720°C and subsequently underwent coherent exsolution, producing strain-controlled braid microperthite/antiperthite. At temperatures of 465-490°C, the feldspar megacrysts reacted with deuteric fluids, which led to the dissolution of the braid perthite/antiperthite along an inward-moving reaction interface and coupled precipitation of an oligoclase/albite mantle. As the rapakivi replacement front progressed inward, the fluids infiltrated into the interiors of the relict megacrysts along fractures and braid boundaries and reacted with the braid perthite via a dissolution-reprecipitation replacement process. This resulted in the formation of patch perthite/antiperthite. The replacement reactions were incomplete, preserving patches of the unreacted braid perthite. At temperatures of 253-283°C, the feldspars were partially albitized, whereby the oligoclase patches and the plagioclase mantle were partially pseudomorphically replaced by albite. Mass-balance constraints indicate that the replacement processes leading to the formation of the plagioclase mantle and the patch perthite/ antiperthite were not isochemical. The fluid composition was externally buffered for many of the elements, but internally controlled by feldspar dissolution-reprecipitation reactions for those elements that are normally incorporated in the feldspar structure. These results conclusively demonstrate for the first time that in addition to magmatic processes, rapakivi feldspars can form by subsolidus, fluid-induced, dissolution-reprecipitation replacement reactions. © The Author 2017. Published by Oxford University Press. All rights reserved.
... The process by which rapakivi feldspars formed has been hypothesized by many authors (e.g., Sederholm 1891;Whitney 1975;Stull 1979;Hibbard 1981;Bussy 1990;Nekvasil 1991;Dempster et al. 1994;Eklund and Shebanov 1999) and summarized by Rämö and Haapala (1995). Of these, three are more widely accepted and are described below. ...
... A third hypothesis has also been presented by Dempster et al. (1994). They propose that the rapakivi texture is formed by the subsolidus exsolution of plagioclase in the alkali feldspars. ...
... As noted in the introduction, the formation of rapakivi feldspars has been explained by crystallization during isothermal decompression of granitic magma (e.g., Nekvasil 1991;Eklund and Shebanov 1999), by magma mixing (e.g., Wark and Stimac 1992), and by subsolidus alteration (Dempster et al. 1994;Mondal et al. 2017). In the case of the Deer Isle pluton, we think that a subsolidus alteration model is not applicable for two main reasons. ...
Article
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Rapakivi and alkali feldspar phenocrysts from the Devonian Deer Isle Granite Complex, Maine were investigated to see if magma mixing or isothermal decompression was responsible for their formation. Pb-isotope and trace-element analyses of alkali feldspar ovoids and their plagioclase inclusions were obtained by LA-ICP-MS to determine the characteristics of the initial magma; these were compared with the plagioclase mantles. Rapakivi grains show a decrease in Pb-isotopic values and an increase in Ba, La, and Ce concentrations from the ovoids and inclusions to the innermost plagioclase mantles adjacent to the ovoids. These variations, along with CL imagery of quartz phenocrysts, indicate an open system with compositional changes in the magma chamber occurring during rapakivi feldspar growth. Repeated episodes of magma mixing/replenishment by Ba and LREE enriched magmas with lower Pb-isotopic ratios (along with hybridized variations with the host magma) created localized dissolution of alkali feldspar and quartz. Alkali feldspar phenocrysts proximal to these zones of thermal perturbation were first resorbed and then mantled by plagioclase that records the more primitive signature of the more mafic melt. Pb-isotopic values and Ba, La, and Ce concentrations trend back to the initial magma composition at the mantle rims as the effects of the mixing event dissipated. For grains that were somewhat more distal to these zones of mixing, resorption of the alkali feldspar crystals did not occur, yet the fresh supply of Ba resulted in Ba-rich alkali feldspar mantles over Ba-poor alkali feldspar cores. Other alkali feldspar crystals were too far from the site of mixing; hence, they lack any evidence of open system crystallization. As crystallization of the chamber continued along solidification fronts, batches of cooler crystal-rich magmas settled en masse to the floor. Disaggregation of these batches during settling, and subsequent accumulation on the reservoir floor, brought grains with disparate crystallization histories together. Filter pressing of the cumulate pile flushed highly evolved melts/fluids out from interstitial pores to the high silica, upper portions of the chamber. The crystallization, transportation, and juxtaposition of disparate types of feldspar phenocrysts suggest that a relatively large and active magma chamber, periodically recharged by localized batches of melt, must have existed. In this case, it is clear that rapakivi mantled feldspars are the result of magma mixing; such interpretations may apply to many other occurrences as well.
... Many ovoids show evidence of subsolvus and subsolidus changes, especially patchy and perthitic albite, commonly without myrmekite, the absence of which could be connected to the general absence of subsolidus deformation in rapakivi granitoids (Vernon, 1991b). These changes were linked to the formation of the ovoids and plagioclase rims by Dempster, Jenkin, and Rogers (1994). However, the origin of the ovoid shapes is independent of the origin of the rims, and the internal, relatively low-temperature changes would be unlikely to control the shapes of the megacrysts. ...
... p. 115) reported well-developed micrographic intergrowths of quartz and alkali feldspar in some ovoids. On the other hand, Terzaghi (1940) interpreted the quartz 'inclusions' as being due to replacement, and Dempster et al. (1994) suggested that they are formed by low-temperature filling of spaces left after exsolution of plagioclase from the alkali feldspar. Fenn (1986) showed experimentally that granophyric intergrowths are the result of simultaneous growth of alkali feldspar and quartz at conditions that favour development of planar feldspar surfaces. ...
... M€ uller et al. (2008, p. 1427) reported fine-scale, lowamplitude Ba oscillatory zoning, superimposed on broad-scale Ba variation, in mantled ovoid phenocrysts of alkali feldspar in the Mesoproterozoic Hammarudda porphyritic granite, which represents the initial stage of development of the A land rapakivi batholith, Finland. Dempster et al. (1994) observed evidence of compositional variation adjacent to inclusions in the alkali feldspar, but no regular growth zoning was reported. ...
Article
Mantling of alkali feldspar megacrysts by oligoclase (‘rapakivi texture’) generally can be interpreted as the result of magma mixing, although decompression is a viable interpretation, especially for high-level intrusions. Coexistence of mantled and unmantled crystals can be explained by transfer of mantled crystals (‘antecrysts’) from a mixed (hybrid) rock to a host granitoid devoid of mantled crystals, for example, by disintegration of microgranitoid enclaves. Processes capable of explaining multiple oligoclase shells include repeated increase and release of volatiles, and repeated replenishment by more mafic magma. The shells could be formed by transfer of megacrysts into and out of a magma-mixing zone during flow in dyke-like conduits or in the fronts of mafic flows moving across cumulate layers in plutons. Ovoid megacrysts, which occur especially in Proterozoic rapakivi granitoids, are difficult to interpret but are better explained by growth processes than by magmatic corrosion. The common presence of simple twinning, partial crystal faces, euhedral plagioclase inclusions and granophyre-like intergrowths with quartz favours normal magmatic growth. The common ovoid shapes with local facets could reflect incomplete development of crystal faces, owing to relatively rapid growth. Granophyre-like intergrowths in the ovoids, local granophyre occurring as megacryst rims and in the groundmass, and the common presence of miaroles suggest growth of the ovoids at relatively shallow depth, at conditions of delayed nucleation and consequent undercooling, resulting from accumulation and retention of fluid. Development of the ovoids is independent of plagioclase mantling.
... This can be interpreted as an initial formation of rapakivi texture, where Ab-rich plagioclase exolves from K-feldspar at lower temperatures in a cooling K-rich granitic magma body and indicate an interplay of fractional crystallization, magma convection, and cumulus processes (e.g. Dempster et al., 1994). However, the timing before the eruption was insufficient for the migration and accumulation of plagioclase at the crystal-rim. ...
... K-feldspar with inclusions of Ab-rich plagioclase, presumably related to the formation of rapakivi texture (Dempster et al., 1994), is subordinate in the Wurzen ignimbrite, but frequent in the WVS granite-and syenite-porphyries. Graphic quartz-feldspar overgrowth is quite rare and restricted to the evolved rhyolitic parts. ...
Article
The Lower Permian Wurzen caldera formed during a VEI 8 supereruption that tapped monotonous intermediate magmas in a continental rift. The well-preserved crystal-rich intracaldera ignimbrite (<58 vol%) and its related plutonic facies allow for compositional and textural studies on juvenile fragments and mineral phases, providing a unique opportunity to unravel the caldera's pre-eruptive evolution. The climactic Wurzen ignimbrite contains different fiamme of fine-grained mafic, fine-grained rhyolitic (71.8 to 76.6 wt% SiO2), porphyritic (trachy-) dacitic (64.5 to 67.6 wt% SiO2), and coarse porphyritic rhyodacitic composition (64.7 to 74.3 wt% SiO2), which point to a permanent interplay between rejuvenation, differentiation, and cooling in the Wurzen magma body. The similarity of their trace element concentrations suggests derivation from a shallow magma chamber (Nb and Ta at 22 and 1.6 ppm, respectively). The picture becomes more differentiated when considering mineral composition and thermobarometric estimations. Resorption textures in anorthitic plagioclase (An >45 mol%) are presumably formed by excess heat from underplating magmas. Coarse sieve textures in plagioclase indicate a rapid ascent of magma into the shallow magma body. In contrast, oscillatory zonation and rapakivi texture in quartz, albitic plagioclase, and sanidine indicate convection during crystal growth. The spatially discrete crystallization of augite and pigeonite indicates tapping of anhydrous Ca-saturated and -undersaturated magma. The presence of pigeonite indicates eruption from a superheated anhydrous (trachy-) dacitic magma batch at temperatures of 1010 °C, whereas the occurrence of annite implies simultaneous tapping of hydrous shallow magma chambers at low temperatures (~750 °C). By applied barometers on clinopyroxene and biotite, it suggests a deep-seated magma chamber at depths of 25 to 15 km and a shallow magma chamber at ca. 11 km, respectively. Mineral assemblage of the intracaldera Wurzen ignimbrite (quartz, sanidine, plagioclase, calcic and calcic-sodic clinopyroxene, pigeonite, annite) is more typical for crystal-poor, hot rhyolites like in the Snake River Plain (Idaho, USA) or Messum igneous complex (Namibia), than that of monotonous intermediates at active continental margins.
... The same type of magmatism ( Fig. 1) extends through southern Greenland (Dempster et al., 1991;Windley, 1991), Labrador Connelly and Ryan, 1999), and southern Laurentia (Emslie, 1978;Anderson et al., 1980;Anderson and Bender, 1989;Gower and Tucker, 1994;Karlstrom et al., 2001). Rapakivi granites are restricted to the Ketilidian orogen of South Greenland (Dempster et al., 1994;Garde et al., 2002). They develop at the southernmost area of Greenland, and are intrusive in either the psammite or pelite zones between 1755 and 1732 Ma (Brown et al., 2002). ...
... At present, few studies consider heating rates in experimental petrology. Rates control the time to reach equilibrium during melting, but they can also bring equilibrium to unexpected situations, as reflected in the development of rapakivi textures by displacing phase boundaries (Dempster et al., 1994). ...
Article
Rapakivi granites and the associated suite (anorthosite, mangerite and charnockite) that developed during the Mid-Proterozoic escape the global rules that control usual granitic magma formation. About 40 points that characterise the Proterozoic magmatism confirm the originality of the magmatism. Rapakivi granites developed without a global orogenic context within the supercontinent Columbia. The intrusions extend from the East European shield to the western US, through Fennoscandia, Greenland and Labrador. Other occurrences in the Amazonian shield, Australia or South China are less well documented. They show no definite trend in age or chemistry that would explain large-scale (mantle plume) effects. A mantle upwelling of material is contradictory with the re-assembly of a supercontinent because it occurred before this magmatic episode (1.9–1.8 Ga). At ∼1.7 Ga, the supercontinent Columbia amalgamated with the collision of the Yapavai and Mazatzal Provinces. A model is suggested that takes into account the supercontinent re-assembly, defining a downwelling flow in the mantle that anchors the continent above it. In contrast to a material flow, heat is still delivered to the base of the continental lithosphere, and is focused toward the juvenile suture zone. The base of the crust must reach 1200–1300 °C before producing anorthosite magmas. Under such a high temperature gradient lasting over a long time, magmas are transferred toward the upper crust, giving the thin (5 km) and square-shape (100 km) that the intrusions presently have. Heat delivery is essentially conductive, leading to long time-spans for intrusions. The presence of the supercontinent, immobile over a descending cell (poloidal mode of convection) developed a tangential force (toroidal mode of convection) that partly split the continent through strike–slip deformation due to plate rotation. It developed progressively between 1.57 and 1.3 Ga, starting from Fennoscandia, and then passing to Amazonia, western US and Labrador in a clockwise sense. The associated rotation induced sinistral shear manifested by small-scale shear zones and the orientation of late magmatic facies (topaz-bearing granites) in each province. The Proterozoic magmatism appears to be unique because it requires a supercontinent with a zone of juvenile crust surrounded by older cratons. The present Moho still shows remnants of this process, having bumpy undulations that may reach 22 km in amplitude over a distance of 200 km.
... The Parguaza batholith was chosen because it is one of the most thoroughly dated intrusive units in the Venezuelan Guyana Shield (Hurley et al., 1977;Olszewski et al., 1977;Gaudette et al., 1978;Barrios and Rivas, 1980;Barrios, 1983;Barrios et al., 1985;Gaudette and Olszewski, 1985). In addition, the geological and petrographic evidence for the metamorphism and alteration of these granites is almost nonexistent (Gaudette et al., 1978;Mendoza et al., 1977;Bangerter, 1985) which, besides the fact that rapakivi textures are thought to result from extremely slow cooling, with differences of ages as great as 200Ma within a single intrusion (Dempster et al., 1994), makes the Parguaza batholith an ideal location for applying paleomagnetism in studying the zoning pattern of these kinds of Precambrian plutons. ...
... In fact, it is a well known fact that rapakivi granites cool at very slow rates and at relatively low temperatures. These cooling conditions allow the exsolution of albite and oligoclase in K-feldspars (Dempster et al., 1994). Also in granitic bodies, hematite lamellae in primary magnetite grains or ilmenite exsolutions in hematite, have been widely observed (e.g. ...
Article
We report paleomagnetic and rock magnetic data from the rapakivi granites of the Parguaza batholith (Guyana Precambrian Shield, southwestern Venezuela). These results suggest that the pluton is inversely zoned with respect to the cooling ages. In order to explain such an age pattern, tentative structural settings are proposed placing the Parguaza intrusions in a plate tectonic context. Six sites were sampled along a 200 km transect that cuts through the northern lobe of the batholith. Thermomagnetic curves, X-ray diffraction and fluorescence, hysteresis loops, thermal and alternating field (AF) intensity plots, transmission (TEM) and scanning (SEM) electron microscope analyses and Königsberger ratios (Qn) values were used to identify the different magnetic mineralogies and their distribution of grain sizes. Magnetite, titanomagnetite near magnetite in composition and deuteric hematite are the three carriers of natural remanent magnetizations (NRMs) in these rocks. Magnetic granulometry indicators such as Königsberger ratios (Qn values) suggest the dominant presence of single domain magnetites with average grain sizes grading from finer to coarser away from the center of the transect. The paleomagnetic results reveal the existence of two primary thermoremanent and/or thermochemical magnetizations () for sites CSP-3, PI-2 and PI-4 (Decl. = 328°, Incl. = −21°, k = 15, α95 = 11.4°) and site CSP-2, PI-1 and PI-3 (Decl. = 16°, Incl. = 87°, k = 10, α95 = 13°), respectively. There is also a poorly defined G3R2 magnetization (Decl. = 284°, Incl. = −86°, k = 6, α95 = 27°) found in sites CSP-2, PI-1 and PI-3. The overlap of coercivities and the unblocking temperatures spectra, probably resulting from the coexistence of primary single-domain magnetic mineralogies with secondary exsolutions of single-domain-like Ti-poor (Fe-rich) regions (almost pure magnetite) in multidomain titanomagnetite grains, in most cases precludes the complete resolution of hybrid G1 (N + R) or G3 (N + R). The relative ages for these components were determined using a map of model age ‘chrontours’. G1 and G3 are the older and the younger , respectively, and were acquired at two discrete moments of the batholith's geological history. The final map resulting from integrating the paleomagnetic and data, shows an age pattern for this batholith, and the rest of the intrusions that belong to the Parguaza Igneous Complex, that could be explained either as the effect of inverse cooling of a single intrusive body, the sequential emplacement of magmas controlled by normal faulting or an internal tectonism resulting in a system of NE-trending horsts and grabens cutting through the pluton. Because of its feasibility and agreement with the most recent theories about the tectonic evolution of the Guyana Shield, we favor the latter hypothesis.
... Textural observations and relationships between alkali feldspar and plagioclase strongly suggest that postmagmatic processes operated within the RDG and HRG. The micro-rapakivi textures in the RDG likely relate to remobilization and redistribution of feldspar components following initial crystallization (Dempster et al. 1994). In addition, the recrystallization of alkali feldspar exsolution textures is suggestive of deuteric coarsening. ...
Conference Paper
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Feldspar replacement and re-equilibration reactions within the Roxby Downs Granite (RDG) and variably altered counterparts, host for the Olympic Dam IOCG deposit, are discussed. Feldspars in these rocks, and within a distinct quartz monzonite in the area (Horn Ridge Granite; HRG), preserve complex textural relationships. Alkali feldspars form intricate exsolution microtextures of microperthite and cryptoperthite which have recrystallized and subsequently coarsened to form patch perthite. Igneous plagioclase varies in composition; from oligoclase-andesine (~An11-39) in the RDG to oligoclase (~An24) in the HRG, and is pervasively replaced by albite ± sericite ± hematite. As hydrothermal alteration progresses, plagioclase and albite are almost completely removed, while sericite and red-stained K-feldspar are stable in altered RDG. Textural observations of both feldspar reactions are consistent with a replacement process governed by coupled dissolution-reprecipitation reaction. The evidence suggests a significant role for deuteric coarsening followed by albitization prior to mineralization in the RDG.
... In the haplogranitic system, their normative compositions yield a high orthoclase component and plot either in the plagioclase field if anorthite is taken into account, or in the orthoclase field if fluorite and calcite are added to the system (Vorma, 1971(Vorma, , 1976. Contrasting explanations were offered, ranging from magmatic to subsolidus models (Cherry and Trembath, 1978;Hibbard, 1981;Eklund et al., 1993;Dempster et al., 1994;Salonsaari, 1995;Eklund and Shebanov, 1999). The possibility that ovoid alkali feldspar and the first generation of quartz could represent xenocrysts, not phenocrysts, has not yet been fully explored. ...
Article
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Although A-type granites have long been recognized as a distinct group of granites, the term A-type was coined first less than thirty years ago. A-type suites occur in geodynamic contexts ranging from within-plate settings to plate boundaries, locations and times of emplacement are not random. Rare in the lower crust, as some charnockite suites, they are fairly common at shallower depths, especially at the subvolcanic level where they form ring complexes rooting caldera volcanoes. Characteristic features include hypersolvus to transsolvus to subsolvus alkali feldspar textures, iron-rich mafic mineralogy, bulk-rock compositions yielding ferroan, alkali-calcic to alkaline affinities, high LILE+HFSE abundances, and pronounced anomalies due to high degrees of mineral fractionation. Isotopic features evidence sources containing a large mantle input. Experimental data show that A-type magmas contain dissolved OH–F-bearing fluids, crystallised under reduced and oxidized conditions, and yield high-temperature liquidus, favouring early crystallisation of anhydrous iron minerals, such as fayalite. Though many petrogenetic models imply solely crustal derivation, no convincing A-type liquids were produced experimentally from crustal materials, nor have any leucosomes of A-type composition been detected within migmatitic terranes. As it occurs in association with mafic igneous rocks in continents as well as on the ocean floor, A-type granite is likely to come from mantle-derived transitional to alkaline mafic to intermediate magmas. Rare felsic materials found in the meteoritic and lunar record yield dominantly A-type features. Contrary to the more common types of granite, A-type granite is, therefore, not typical of Earth and was produced in planetary environments differing from those prevailing on Earth.
... Rb, Sr, Sm and Nd concentrations were determined by isotope dilution. Rb, Sr and the REE were separated using standard cation-exchange chromatography techniques (Dempster et al. 1994). Sm and Nd were isolated from Ba and the other rare earth elements (REEs) using three anion-exchange columns, similar to the procedure outlined by O'Nions et al. (1977). ...
Article
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The Sr-Nd isotopic data for selected granitoids of the Central Bohemian Pluton show a broad negative correlation with the total range of (87Sr/86Sr)330 = 0.7051–0.7129 and Nd 330 = +0.2 to –8.9. The older intrusions have more depleted Sr-Nd compositions and calc-alkaline geochemistry (Szava suite), whereas the younger intrusions shift towards K-rich calc-alkaline (Blatn suite) and shoshonitic rocks (any and ertovo bemeno suites) with more evolved isotopic signatures. The distribution of the data is interpreted as reflecting a diversity of sources and processes, rather than a single progressive crustal contamination trend. The Szava suite could have originated by partial melting of metabasites, or of a mantle source with an isotopic composition close to bulk earth, or by hybridization of crustally-derived tonalitic and mantle-derived magmas. Variation within the Blatn suite is modelled by mixing between a moderately enriched [(87Sr/86Sr)330 0.708, Nd 330 –3] mantle component with either an isotopically evolved metasedimentary component, or with more evolved magmas of the suite. The any suite was most probably produced by partial melting of peraluminous lithologies, possibly of the adjacent Moldanubian unit. The ertovo bemeno suite evolved from strongly enriched mantle-derived magmas [(87Sr/86Sr)3300.7128, Nd 330 –7], either through closed-system fractional crystallization or interaction with magma corresponding to leucogranites of the Central Bohemian Pluton.
... oclase, the fused and recrystallized domains, rounded and smudged growth zoning, and plagioclase mantles around alkali-feldspar phenocrysts that were related to changes in primary magma composition and environmental parameters (P, T, water content). This allowed plagioclase to nucleate on alkali-feldspar crystals (Hibbard 1981; Müller et al. 2005). Dempster et al. (1994) explained that the formation of a plagioclase mantle by redistribution of exsolved plagioclase from alkali-feldspar phenocrysts is linked to the high fluorine contents of magmas. This might well be the case of the Götemar Pluton, because it has a fluorine content of 0.35–0.5% (Alm and Sundblad 2002). Ascent of magma probably occurred vi ...
Article
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The emplacement of the Mesoproterozoic Götemar Pluton into Paleoproterozoic granitoid host rocks of the Transscandinavian Igneous Belt is re-examined by microfabric analysis, including cathodoluminescence microscopy. Field data on the pluton-host rock system are used to strengthen the model. The Götemar Pluton, situated on the Baltic Shield of SE Sweden, is a horizontally zoned tabular structure that was constructed by the intrusion of successive pulses of magma with different crystal/melt ratios, at an estimated crustal depth of 4–8 km. Initial pluton formation involved magma ascent along a vertical dike, which was arrested at a mechanical discontinuity within the granitoid host rocks; this led to the formation of an initial sill. Subsequent sill stacking and their constant inflation resulted in deformation and reheating of existing magma bodies, which also raised the pluton roof. This multi-stage emplacement scenario is indicated by complex dike relationships and the occurrence of several generations of quartz (Si-metasomatism). The sills were charged by different domains of a heterogeneous magma chamber with varying crystal/melt ratios. Ascent or emplacement of magma with a high crystal/melt ratio is indicated by syn-magmatic deformation of phenocrysts. Complex crystallization fabrics (e.g. oscillatory growth zoning caused by high crystal defect density, overgrowth and replacement features, resorbed and corroded crystal cores, rapakivi structure) are mostly related to processes within the main chamber, that is repeated magma mixing or water influx.
... K-feldspar (Or 91 -Or 95 ) is mesoperthitic, commonly displaying strip-like albite perthites ( Alkali feldspar crystals often display a rapakivi texture, which is common in A-type granites (e.g. Dempster et al., 1991Dempster et al., , 1994Vorma, 1976). Ovoids of perthitic orthoclase (Or 91 -Or 95 ) with lobate boundaries evidencing dissolution processes are mantled by a plagioclase rim (max. 2 mm), which consists of a series of discrete grains or of a symplectitic intergrowth of quartz and plagioclase (Fig. 2B). ...
Article
The uncommon association of cogenetic and nearly contemporaneous potassic K-feldspar A-type granites and sodic albite granites is observed within the 347 Ma-old bimodal Saint-Jean-du-Doigt (SJDD) intrusion, Brittany, France. A-type granites outcrop as small bodies (< 1 km2) of fine-grained, pinkish to yellowish rock or as meter-thick sills in-between mafic layers. They emplaced early within the thermally “cool” part of the SJDD pluton directly beneath the Precambrian host rock, forming the pluton roof. Albite granites are fine-grained hololeucocratic yellowish rocks emplaced slightly after the A-type granites in the thermally mature part of the pluton. They form meter-thick sills that mingle with adjacent mafic layers and represent ca. 1 vol.% of the outcropping part of the pluton.
... Lofgren (1974) showed experimentally that quench-grown feldspars will have a skeletal morphology, which is supported by the observations of Hibbard (1981). The reverse zoning patterns of cells and rims rules out an exsolution origin which would have the opposite zoning patterns (Dempster et al., 1994). The alternative model of Bussy (1990) proposes the partial dissolution of sodic plagioclase, with later infilling of dissolution channels and overgrowth by relatively more calcic plagioclase. ...
Article
The South-west Granite Complex of Jersey comprises four main facies of granite, namely Coarse Granite, Porphyritic Granite, Porphyritic Microgranite and Microgranite. The granites exhibit numerous microtextures characteristic of crystal-melt disequilibrium including rapakivi and cellular feldspars, ocelli, intergrowths, overgrowths, re-entrants, embayments, rapid growth textures and growth zone truncations involving several phases as well as modal and textural heterogenity on the centimetric scale. These features are interpreted as forming through open system behaviour involving mixing between coexisting grantoid magmas. While lobate and gradational boundries between units suggests that they were contemporaneous, undergoing limited mingling and mixing along contacts following emplacement, the internal coherence to each unit indicates their emplacement as separate batches. However, the presence of enclaves of one facies within another and the relatively homogeneous distribution of mineral phases exhibiting disequilibrium textures points to a more widespread mixing event prior to emplacement at currently exposed levels. Considering the field and petrographic evidence, which we interpret to show contemporaneity of the facies within the South-west Granite Complex, previous age data for these rocks should be viewed with caution.
... However, some researchers argue that these megacrysts grow up in a largely lock-up magma regime (Johnson and Glazner, 2010;Glazner and Johnson, 2013). The formation of Pl-mantled Kfs crystals (rapakivi sensu lato) is also controversial; possible mechanisms include magma mixing processes (e.g., Stimac and Wark, 1992;Gagnevin et al., 2008), the exsolution of Kfs (Dempster et al., 1994) and the sub-isothermal decompression of magma (Eklund and Shebanov, 1999). Thus, an integrated study combining multiple approaches in different scales is helpful to gain a full understanding of the formation of these fabrics. ...
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The porphyritic monzogranite of Laojunshan (eastern Qinling in central China) forms a W–E elongated pluton characterized by a varying abundance of cm-scale potassium-feldspar (Kfs) megacrysts (with or without plagioclase mantles), mafic microgranular enclaves (MMEs) and associated mafic dykes. The measurements of the shape preferred orientation of Kfs megacrysts and the tiling of megacrystic pairs throughout the pluton suggest that these megacrysts constitute magmatic foliations subparallel to the pluton's margin, possibly resulting from eastward non-coaxial magmatic flow. Felsic-mafic magma mixing and mingling were important during magma emplacement, as indicated by abundant MMEs, coeval mafic dykes and disequilibrium textures within Kfs megacrysts. Quantitative analyses of the crystal size distribution and the in-situ chemistry of Kfs megacrysts revealed that the temperature fluctuation in the deep magma chamber and the resultant textural coarsening are the most likely important mechanisms for the growth of Kfs megacrysts. The syn-tectonic and two-main-stage emplacement of the pluton in a sinistral transtensional regime is inferred by the mechanical coupling between magmatic–submagmatic–solid-state fabrics in the pluton and wall-rock deformations, as well as zircon U–Pb dating results, which produce eastward younging emplacement ages spanning from ∼122 Ma to ∼119 Ma. Combined with other studies, our work supports the idea that the Qinling-Dabie orogen was in a sinistral transtensional regime during the Early Cretaceous, which may correspond to the eastward extrusion tectonics in eastern China.
... 4.). It is a coarse-grained biotite-hornblende granite with ovoidal alkali feldspar megacrysts mantled by oligoclase-andesine rims (Lintala et al. 1991, Dempster et al. 1994. Simonen and Vorma distinguished darkcoloured wiborgite from typical wiborgite. ...
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Subjotnian: Rapakivi granites and related rocks in the surround­ ings o f the Gulf of Finland. Geological Survey o f Finland, Special Paper 21, 59-97, 11 figures. The southern Finnish mainland, Aland, the Gulf of Finland and parts of neighbouring Latvia, Estonia and Russia are classical rapakivi areas. A typical rapakivi (adopted Finnish word meaning ''rotten stone") is a coarse-grained granite with alkali feldspar ovoids either surrounded by plagioclase mantles (wiborgite) or not (pyterlite). Some other associated granite types (porphyritic with idiomorphic phenocrysts, equigranular and aplitic) are called also rapakivi. The granitic melt of the rapakivi plutons originated from partial melting of lower crust (a mixture of Archaean and Proterozoic lower crust in the Salmi and Ulyalegi plutons and Proterozoic in all the others) whereas the gabbro -anortho-sites and diabases represent partial melting of the subcontinental mantle. These conclusions are largely based on isotopic studies. The rapakivi granites o f the map area comprise three isotopic age groups: 1620-1650 M a (SE Finland and Estonia), 1540-1590 Ma (SW Finland and Latvia) and 1540-1560 Ma (Russian Karelia). Gabbros and anorthosites make up the southern part o f the Riga rapakivi pluton, the eastern part o f the Salmi pluton and the surroundings of the Ahve-nisto pluton. There are smaller bodies of mafic plutonic rocks in many other places, e.g ., the Abja pluton of southern Estonia and the gem-quality anorthosite (spectrolite) of Ylamaa, Vyborg batholith. There are several swarms of diabase dykes within the rapakivi areas; some swarms are more than 150 km long. Most of these diabases are slightly older than the rapakivi granites and their emplacement was probably connected to the upward movement of rapakivi granite magma in the crust. The trends of the dyke swarms show the directions of maximum stress during rapakivi intrusion.
... It has been suggested that the sodic plagioclase in the rims of rapakivi mantled feldspars represents recrystallisation of plagioclase exsolved from the perthitic alkali feldspars (e.g. Dempster et al. 1994). This is a possible origin here. ...
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In the Lachlan Orogen of south-eastern Australia, the high-level, postorogenic, 368-Ma, I-type Tynong pluton contains granitic to granodioritic rocks that crystallised from a variety of mainly crustally derived magmas emplaced in the shallow crust, in an extensional regime. The isotopic characteristics of the main plutonic rocks are relatively unevolved (87Sr/86Srt ~ 0.705–0.706 and εNdt ~ −0.4 to 0.6), suggesting source rocks not long separated from the mantle. We infer that arc mafic to intermediate rocks and associated immature greywackes formed the main crustal source rocks and that these are located in the largely unexposed Neoproterozoic–Cambrian Selwyn Block that forms the basement. As exposed near its southern margin, the pluton also contains minor, pillowed sheet-like intrusions of quartz dioritic rock that show mainly mingling structures with the enclosing granodiorites, as well as some hybrid pods and fairly abundant igneous microgranular enclaves that we infer to have been derived from the quartz dioritic sheets. Despite this evidence of direct mantle input into the Tynong magma system, the main granodioritic series do not appear to have been formed by magma mixing processes. Of any I-type granite in the region, the Tynong pluton has perhaps the most direct connection with mantle magmas. Nevertheless, the main mantle connection here is probably in the mantle-derived protolith for these crustal magmas and in the mantle thermal event that gave rise to melting of the deep crust in the Selwyn Block. This degree of mantle connectedness seems typical for I-type granitic rocks worldwide.
... At the microscopic scale, magma mingling and partial hybridisation is supported by a number of mineral instability features. Rapakivi and antirapakivi textures may arise from a number of processes (Dempster, Jenkin, & Rogers, 1994;Eklund & Shebanov, 1999), one of which is magma mixing (Hibbard, 1981;Wark & Stimac, 1992). When magmas of contrasting composition mix, quenching of the more mafic magma may result in the epitaxial growth of plagioclase onto K-feldspar crystals derived from the more felsic magma. ...
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The role of mafic–felsic magma mixing in the formation of granites is controversial. Field evidence in many granite plutons undoubtedly implies interaction of mafic (basaltic–intermediate) magma with (usually) much more abundant granitic magma, but the extent of such mixing and its effect on overall chemical features of the host intrusion are unclear. Late Devonian I-type granitoids of the Tynong Province in the western Lachlan Fold Belt, southeast Australia, show typical evidence for magma mingling and mixing, such as small dioritic stocks, hybrid zones with local host granite and ubiquitous microgranitoid enclaves. The latter commonly have irregular boundaries and show textural features characteristic of hybridisation, e.g. xenocrysts of granitic quartz and K-feldspars, rapakivi and antirapakivi textures, quartz and feldspar ocelli, and acicular apatite. Linear (well defined to diffuse) compositional trends for granites, hybrid zones and enclaves have been attributed to magma mixing but could also be explained by other mechanisms. Magmatic zircons of the Tynong and Toorongo granodiorites yield U–Pb zircon ages consistent with the known ca 370 Ma age of the province and preserve relatively unevolved ϵHf (averages for three samples are +6.9, +4.3 and +3.9). The range in zircon ϵHf in two of the three analysed samples (8.8 and 10.1 ϵHf units) exceeds that expected from a single homogeneous population (∼4 units) and suggests considerable Hf isotopic heterogeneity in the melt from which the zircon formed, consistent with syn-intrusion magma mixing. Correlated whole-rock Sr–Nd isotope data for the Tynong Province granitoids show a considerable range (0.7049–0.7074, ϵNd +1.2 to –4.7), which may map the hybridisation between a mafic magma and possibly multiple crustal magmas. Major-element variations for host granite, hybrid zones and enclaves in the large Tynong granodiorite show correlations with major-element compositions of the type expected from mixing of contrasting mafic and felsic magmas. However, chemical–isotopic correlations are poorly developed for the province as a whole, especially for 87Sr/86Sr. In a magma mixing model, such complexities could be explained in terms of a dynamic mixing/mingling environment, with multiple mixing events and subsequent interactions between hybrids and superimposed fractional crystallisation. The results indicate that features plausibly attributed to mafic–felsic magma mixing exist at all scales within this granite province and suggest a major role for magma mixing/mingling in the formation of I-type granites.
... Legends as for Fig. 7. for the formation of such unusual texture (Müller et al., 2008). Some researchers suggested that it may be formed by synneusis that the migration of plagioclase crystals or exsolution of albitic plagioclase from alkali feldspar (e.g., Dempster et al., 1994;Stull, 1979). Others have argued that it is related to marked release of pressure with small temperature change (e.g., Eklund and Shebanov, 1999;Nekvasil, 1991). ...
Article
Late Triassic granitoid intrusions are widespread in the South Qinling Belt (SQB), providing excellent subjects to understand the geodynamic evolution of the Qinling Orogenic Belt and the collision between the North China Craton (NCC) and Yangtze Craton (YZC). This study shows new obtained geological, geochemical and zircon U–Pb–Hf isotopic data of the Caoping and Shahewan intrusions, revealing that the Caoping intrusion consists of ~215 Ma fined-grained granites, and ~221–215 Ma porphyritic and coarse to medium-grained tonalites, granodiorites, and monzogranites, that assemble with coeval mafic magmatic enclaves (MMEs). The Shahewan intrusion is composed of ~215–210 Ma porphyritic granodiorites and monzogranites, which also assemble with coeval MMEs. The fine-grained granites from the Caoping intrusion are characterized by high SiO2, Rb and (La/Yb)N ratio, but low MgO, CaO, and Sc contents, with εHf(t) values of -8.6 to +4.3 and TDM2(Hf) ages of 883–1596 Ma, suggesting they are mainly derived from partial melting of the Meso- to Neoproterozoic metagreywackes. The porphyritic and coarse to medium-grained granitoid rocks from both Caoping and Shahewan intrusions are characterized by higher MgO, CaO, Sc, Mg# values, but low SiO2, Rb contents and (La/Yb)N ratio, with εHf(t) values of -0.7 to +2.8 and TDM2(Hf) values of 961–1158 Ma, suggesting they are mainly formed by magma mixing between melts that were derived from Meso- to Neoproterozoic basement rocks of the SQB and metasomatized lithospheric mantle. The MMEs from Caoping and Shahewan intrusions are characterized by low SiO2, Sr/Y ratio, high MgO, K2O, Rb, Sc, total REE contents, with εHf(t) values of +0.5 to +6.1 and TDM(Hf) values of 661–846 Ma, suggesting they are produced by partial melting of metasomatized lithospheric mantle. The rapakivi-like textures of the rocks from Shahewan intrusion may be caused by continues underplating and injection of mafic magma resulting in higher temperature (TZr=770–817℃, comparing with Caoping intrusion of TZr=727–773℃), together with the magma mixing and isothermal decompression processes. Integrated with previous regional data, the SQB shows that mantle-derived magmatic activations initiated at ~234 Ma near Wudang in the east, and westward systematically decreased to ~222–219 Ma near Caoping and to 212–208 Ma near Yangba. We attribute this temporal–spatial distribution of mantle-derived magmatism within the SQB to the progressive tear of the subducted oceanic slab. Combined with sedimentary studies on the foreland basins, paleomagnetic studies as well as numerical and seismic tomography models of slab break-off, we propose that the ~225–205 Ma magmatism in the SQB was resulted from a westward slab tear following the collision between the NCC and YZC.
... Decreases in pressure and/or temperature, or an increase in Ca and mafic components in the melt, are the preferred catalysts (e.g., Müller et al., 2008). Feldspar resorption textures in the RDG, interpreted to have occurred prior to deuteric coarsening, are supportive of latestage magma mixing/contamination and/or ascent (e.g., Emslie, 1991;Wark and Stimac, 1992;Dempster et al., 1994;Rämö and Haapala, 1995). Such processes would increase the degree of crustal melting and can also initiate magma convection, explaining the partial (rapakivi mantles) and complete resorption of some alkali feldspars observed in the RDG. ...
... Rifting was accompanied by high temperature gradients, slow migration of magma from subcrustal depths and deep faulting through the lithosphere. Rapakivi textures are thought to be the outcome of extremely slow cooling, with large age differences, within a single intrusion (Dempster et al., 1994). ...
Article
Free dowload link until 29 June https://authors.elsevier.com/c/1d2V014fdGwVac In order to improve our understanding of the NW-Amazonian Craton evolution, we present new petrographic, geochemical and geochronological analyses of 27 samples from the geotectonic Rio Negro-Juruena Province in eastern Colombia (Guainía and Vaupés departments). New LA-ICP-MS zircon U-Pb ages suggest that the oldest known rocks in Colombia are metamorphic rocks (migmatitic gneisses) with ages between ~ 1850 and ~ 1800 Ma, and gneisses and granitoids with ages between ~ 1800–1720 Ma which form part of the Mitú Complex, interpreted as the result of Statherian collisional and orogenic events (Querari Orogeny). Detrital zircons in low-grade meta-sedimentary sequences of the Tunuí Group (sandstones, conglomeratic sandstones and mudstones), that crop out over almost the entire basement, indicate older than ~ 1770 Ma source rocks. Intrusions of different suites of granitic rocks with syn- to post-collisional affinities suggest a termination of the collisional events between ~ 1600–1500 Ma which had affected the whole region, occasionally metasomatically overprinting parts of the Tunuí meta-sedimentary sequence. The recognizable metamorphic and magmatic-processes finish with ~ 1400–1340 Ma anorogenic granites without signs of tectonic deformation, resembling anorogenic granites in the Western Amazonian Craton in Brazil and the Parguaza Batholith in Venezuela. This study allows us to conclude that the basement records the collision of a continental arc (Rio Negro-Juruena Province) against the NW-Amazonian Craton (Ventuari-Tapajos Province) and its subsequent transition to anorogenic conditions in a continental rift setting long before the actual stable craton conditions.
... A few studies have attributed the origin of this texture to an abrupt decrease in pressure caused by a sudden loss of volatiles during the late stages of crystallization (Cherry and Trembath, 1978;Calzia and Ramo, 2005). Dempster (1994) proposed a subsolvus model in which high-temperature exsolution of plagioclase from ternary feldspars and its subsequent redistribution around the margins of phenocrysts in a fluorine-rich environment promoted the growth of rapakivi feldspars. Furthermore, Mondal et al. (2017) proposed that subsolidus fluid-induced dissolution of alkali feldspar phenocrysts followed by pseudomorphic replacement by oligoclase and albite may lead to the formation of rapakivi feldspars. ...
Article
The Ghansura Rhyolite Dome of the Bathani volcano-sedimentary sequence in eastern India originated from a subvolcanic felsic magma chamber that was intruded by volatile-rich basaltic magma during its evolution leading to the formation of a porphyritic andesite. The porphyritic andesite consists of rapakivi feldspars, which are characterized by phenocrysts of alkali feldspar mantled by plagioclase rims. Results presented in this work suggest that intimate mixing of the mafic and felsic magmas produced a homogeneous hybrid magma of intermediate composition. The mixing of the hot volatile-rich mafic magma with the relatively colder felsic magma halted undercooling in the subvolcanic felsic system and produced a hybrid magma rich in volatiles. Under such conditions, selective crystals in the hybrid magma underwent textural coarsening or Ostwald ripening. Rapid crystallization of anhydrous phases, like feldspars, increased the melt water content in the hybrid magma. Eventually, volatile saturation in the hybrid magma was reached that led to the sudden release of volatiles. The sudden release of volatiles or devolatilization event led to resorption of alkali feldspar phenocrysts and stabilizing plagioclase, some of which precipitated around the resorbed phenocrysts to produce rapakivi feldspars.
... Various petrogenetic models for the origin of rapakivi granites have been proposed, yet none reconcile with all observed data. The common occurrence of unmantled feldspars directly next to mantled feldspars casts doubt on mantle origins through sub-isothermal decompression (Nekvasil, 1991;Eklund & Shebanov, 1999), or via hydrothermal exsolution (Dempster et al., 1994). Magma mixing has also been suggested as a means of generating rapakivi mantles (Hibbard, 1981), and is supported by both experimental work (Wark & Stimac, 1992) and field studies (Vernon, 2016). ...
Article
The Orestes Melt Zone (OMZ) is a massive contact melt zone (∼20 m thick by several kilometers long), located in the McMurdo Dry Valleys of Antarctica. The OMZ formed at shallow crustal depths by melting of the A-type Orestes Granite owing to intrusion of the underlying, doleritic Basement Sill. The OMZ can be divided broadly into two melting facies. The upper melting facies is distal from the contact and formed by melting at low temperature and water-saturated, or near water-saturated, conditions. The lower melting facies is proximal to the contact and formed by melting at high temperature and water-undersaturated conditions. Separate melting reactions occurred in both of the melting facies, resulting in distinct textures and melt compositions. Melting in the distal facies generated melts with compositions that plot near a predicted eutectic composition. Melting in the proximal facies was accomplished in part by replacement reactions in restitic feldspars. These reactions resulted in the development of plagioclase mantles on both restitic plagioclase and K-feldspar, and melt compositions that diverged from predicted minimum melt along an unexpected path, towards enrichment in orthoclase component. Thermal modeling indicates that this melt zone was active for a minimum of ∼150 years, with a contact temperature of ∼900 °C. Upon cooling, recrystallization generated ocellar textures around restitic quartz, as well as faceted albite as a late-stage product. Observations of the OMZ, combined with thermal modeling, provide new insights into the origin of rapakivi and albite granites. This study has implications for the origin of these two associated granite types in other geological settings.
... The formation of the rapakivi tex ture has been studied since the late 19 th century (Sederholm 1891, Vorma 1971, Rämö & Haapala 2005, Vernon 2016). Several models have been proposed for the generation of the texture, includ ing subsolidus reorganization of feldspar compo nents (Dempster et al. 1991(Dempster et al. , 1994, magma mixing (Hibbard 1981) and subisothermal polybaric crystallization in a decompressing system (Nekvasil 1991, Eklund & Shebanov 1999, Elliott 2001 ...
Article
Located between the northern margin of Erlian-Hegenshan suture zone and the pericontinental accretion zone of the southeastern margin of the Siberia plate, the Bayinsukhtu granitic body is an intrusive complex composed of monzonitic granites emplaced at Carboniferous and Triassic-Jurassic respectively. Geochemical analyses suggest that both of the two periods of monzonitic granites are characterized by high contents of silica, Al 2O 3 and alkali, low contents of calcium and magnesium. They are enriched in LILE and depleted in HFSE, with moderately enrichment of LREE, weak fractionation of LREE from HREE and negative Eu anomalies (δEu = 0.35-0.66). Based on the petrologic and geochemical features, the Carboniferous monzonitic granite is classified as mainly high-K, calc-alkaline, metaluminous to weakly peraluminous syn-orogenic I-type granite, while the latter is A-type granite. Isotope geochemistry of the Carboniferous granite shows that values of ε Nd(t) and ( 87Sr/ 86Sr) i range from 0.9 to 1.5, and from 0.70062 to 0.70482, respectively, which indicates that the granitic magma was likely derived from a young mantle source or an accretionary island arc with some degree of crustal contamination. However, the Triassic-Jurassic granite has lower ε Nd(t) but higher ( 87Sr/ 86Sr) i values relative to the Carboniferous granite, ranging from -0.3 to 0.3 and from 0.70996 to 0.71019, respectively. This indicates the granite derived from the same source with relatively high degree of crustal contamination. The dating of zircons from the monzonitic granites determined by LA-ICP-MS yielded a weighted age of (296±3.5) Ma, indicating that in the late Carboniferous the studied area was a syn-collision pericontinental tectonic environment. According to geochemical analysis of Late Triassic-Early Jurassic granite, it is presumed that the syn-collisional magmatism lasted till Triassic-Jurassic, probably suggestive of a long duration of collision.
Article
Textural, chemical and Sr isotopic studies of feldspars from the subsolvus Shap granite, northern England, demonstrate that a number of magma mixing events have dominated the evolution of this pluton. K-feldspar megacrysts are phenocrysts formed in the magma chamber. They contain a number of Ba-rich zones that developed during periods of slight dissolution and regrowth linked to the hybridization of the granite by the intrusion of basic magmas. Diorite enclaves represent the relicts of these magmas and these also contain K-feldspar megacrysts, which show evidence of major dissolution. They are xenocrysts picked up from the host granite and incorporated in the basic magma. Increasing H2O contents during fractional crystallization caused a late switch from growth of megacrysts to finer-grained K-feldspars in the matrix. The chemically and isotopically zoned K-feldspar megacrysts preserve an exceptional record of the evolution of the magma, and the zones also had a significant influence on the development of exsolution microtextures during cooling.
Article
The rapakivi granites of South Greenland were emplaced into the Ketilidian orogenic belt at about 1.74 Ga during an episode of extensional tectonics. Petrographically, they can be subdivided into a roughly subequal black facies and white facies, distinguished by the colour (turbidity) of the alkali feldspars and by the associated mineralogy which is more hydrous (i.e., amphibole and biotite compared to olivine and pyroxene) in the white facies. The oxygen isotope composition of these rocks is notably homogeneous; from thirteen outcrops sampled over an area extending some 200 km×100 km all alkali feldspars have an average of 10.2±0.4‰. This lack of variation is consistent with the consensus petrogenetic model which invokes a very well mixed two-component protolith; the high abundance of (mean whole rock =9.4‰) argues that the major contribution (perhaps 90%), although originally mantle-derived, had undergone enrichment in a low temperature sedimentary cycle. There is no oxygen isotopic distinction between alkali feldspars in the black and white facies, which is argued to be a consequence of the relatively anhydrous nature of the melt. Hydrogen isotope compositions vary widely—by 30‰—but are coherent on a hand specimen scale, as demonstrated by approximately equal δD of biotite and alkali feldspar. It is not clear at which stage the D/H heterogeneity was introduced.
Article
Emplacement of 1.6 to 1.3 Ga Mesoproterozoic plutons in Baltica and Laurentia formed an immense belt of A-type granite batholiths that include (1) low-fO2, ilmenite-series granite intrusions from the Baltic region to Wyoming, (2) high-fO2, magnetite-series granite intrusions of the central to southwestern U.S., and (3) peraluminous, two-mica granite intrusions from Colorado to central Arizona. These mineralogic divisions are mirrored by substantial elemental and oxygen isotopic differences. The ilmenite-series granites, which often contain classic rapakivi textures, have the highest Fe/Mg ratios and are highest in LIL element enrichment. They also have the lowest whole-rock δ18O values at 5.7‰ to 7.7‰. The magnetite-series granites are less potassic, less LILE-enriched, and have higher whole-rock δ18O values, ranging from 7.6‰ to 10.8‰. Although they retain A-type characteristics, the peraluminous granites are the least LILE-enriched and have the lowest Fe/Mg ratios. They also have the highest whole-rock δ18O values ranging from 8.8‰ to 12.0‰. Feldspar, where strongly reddened, can exhibit elevated δ18O values, which is interpreted to indicate subsolidus exchange with surface-derived aqueous fluids. Quartz δ18O values are interpreted to generally retain their magmatic values. The transcontinental mineralogic, chemical, and oxygen isotopic variations are interpreted as indicative of broad changes in the composition of a lower crustal source, which is compatible with a reduced mantle-derived crustal source for the ilmenite-series granites and a more oxidized crustal source for the others, including a metasedimentary component in the source for the two-mica granite subprovince.
Article
 Microsampling of cm-scale feldspar crystals within an S-type granite from the Lachlan Fold Belt of southeastern Australia has revealed complex internal Sr and Nd isotopic variations. The observed isotopic zonations are in part interpreted as recording feldspar crystallisation in a dynamically mixing magma system, the isotopic composition of which was varying in response to the influx of more mafic and isotopically more mantle-like magmas, the latter stages of which are now represented in modified form by microgranular enclaves. Similar core to rim isotopic variations in feldspar megacrysts from a microgranular enclave and the adjacent host granite strongly suggest megacrysts in the enclave were transferred from the granitic magma during crystallisation. Feldspar rims have higher 87Sr/86Sri and lower ɛNd(i) than adjacent whole rock analyses, but match those of mineral separates from the surrounding enclave matrix. This suggests that the final stages of megacryst growth occurred in the presence of a component that had previously interacted with a high 87Sr/86Sr, low ɛNd(i) component such as metasedimentary wall rocks. Isotopic heterogeneities are also presererved within different mineral phases in the enclave matrix, suggesting that differing phases grew at differing stages of equilibration between the enclave magma and its host granitic magma. Our results reveal major isotopic heterogeneities on a single crystal and also inter-mineral scale in a pluton which shows well constrained evidence for magma mingling. These results indicate the suitability of feldspars as recorders of isotopic change in magmatic systems, even those which have cooled slowly in the plutonic environment and suggest that much heterogeneity in plutonic systems may be overlooked on a whole rock scale.
Article
Three sheet-like bodies of felsic gneiss containing abundant K-feldspar megacrysts (megacrystic felsic gneiss, augen gneiss or granite gneiss) surrounding the Broken Hill Line of Lode in western New South Wales, Australia, are inferred to be pre- to syn-D1 granitoids. We interpret the Feral gneiss to be a pre- to early syn-D1 intrusion, as it contains S1 as its earliest foliation. However, it has no magmatic flow foliation. The Alma Gneiss, and the megacrystic portions of the Rasp Ridge Gneiss, northwest of the Line of Lode, both contain S1 parallel to a magmatic flow foliation, and are interpreted as having been magmatic during D1. Therefore, the Alma and Rasp Ridge Gneisses may have been intruded during D1, probably just after the Feral gneiss, as the Alma Gneiss intrudes the Feral gneiss. S1 in the augen gneisses and the wall rocks is defined by biotite, sillimanite, garnet and ribbon quartz, and indicates that high-grade metamorphic conditions accompanied D1. Evidence suggesting that these rocks were originally granitoids includes: (i) the Alma Gneiss transecting and intricately intruding the Feral gneiss, the contacts being transected by S1; (ii) euhedral to subhedral K-feldspar porphyroclasts (former phenocrysts), especially those with concentrically arranged inclusions; (iii) microgranitoid enclaves, particularly where megacrystic and relatively large; (iv) aplite dykes (most common in plutonic rocks and therefore reliable indicators); (v) metasedimentary xenoliths; (vi) magmatic flow foliations overprinted by parallel tectonic foliations; and (vii) chemical affinities with undoubted Australian Proterozoic granitoids. Therefore, felsic gneisses at Broken Hill should not be used for stratigraphic correlation, unless they can be definitely determined to be of volcanic flow or tuffaceous origin. The inferred intrusion of granitoids early in the tectonic history of the Broken Hill Block suggests that they may have contributed to the metamorphic and/or hydrothermal heat, and may have helped concentrate metals to form orebodies.
Article
The Água Boa and Madeira igneous complexes at the Pitinga mine were emplaced into acid volcanic rocks of the Paleoproterozoic Iricoumé Group, and host major tin, rare-metal (Zr, Nb, Ta, Y, REE) and cryolite mineralization. The igneous complexes are elongate NE–SW and each is composed of three major facies that, in order of emplacement, include porphyritic and equigranular rapakivi granite and biotite granite in both igneous complexes, followed by topaz granite in the Água Boa igneous complex (ABIC) and albite granite in the Madeira igneous complex (MIC).
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The rapakivi texture (plagioclase-mantled K-feldspar ovoids, rounded quartz megacrysts and euhedral plagioclase megacrysts in a more fine-grained granitic matrix) has been studied in five Proterozoic (1.64–1.55Ga) anorogenic rapakivi granite batholiths in the Fennoscandian Shield with emphasis on mineral stability and inherited cores. K-feldspar ovoids and rounded quartz megacrysts with deep embayments consist principally of a core zone and an outer melted and recrystallized zone. When the K-feldspar ovoids are mantled by plagioclase, the outermost part of the mantle is homogeneous, but towards the K-feldspar, a skeletal texture develops. Intensive parameters obtained from different textural positions show that mineral inclusions in cores of the K-feldspar ovoids and quartz megacrysts were formed at low T (∼680–720°C) and high P (5–6kbar) conditions, while inclusions in the periphery of ovoids and the plagioclase mantles display high T (∼780°C) and intermediate to low P (3.5–1kbar). The lowest P is comparable to that during solidification of the matrix. The total water equivalent of the volatile content in the magma has been calculated as ∼2.5%. The amount of solids in the magma at ∼1kbar has been estimated as ∼40%. Theoretical calculations and experimental data for mineral stabilities in granitic systems suggest that the texture formed when a crystal-saturated (Kfsp+Qtz+Pl ∼60%) and volatile-undersaturated A-type granite magma was transported under approximately constant temperature (760–780°C) from the lower-middle crust (5–6kbar) to upper crustal levels. According to phase stabilities in the eutectoid granite system, quartz and K-feldspar were resorbed but plagioclase remained stable and precipitated during this sub-isothermal rise of magma. Textural (presence of disequilibrium textures) and mineralogical (presence of different mineral assemblages, including relics) evidence of a sub-isothermal rise of the rapakivi magmas is better preserved in subvolcanic and contact varieties of rapakivi granites than in the more deep-seated rapakivi granites formed by slow cooling.
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Quartz phenocrysts and xenocrysts are a common component of volcanic rocks but rarely contribute to our understanding of magmatic or eruptive processes due to an apparent internal chemical and structural homogeneity. Scanning electron microscopy cathodoluminescence imaging (SEM-CL) and secondary ion mass spectroscopy (ion microprobe) analysis (SIMS) are used here to demonstrate that quartz crystals in volcanic rocks show hitherto unrecognised internal structural and chemical zoning. Two contrasting crystal morphologies have been studied. Porphyritic euhedral bipyramidal quartz crystals in dacite from Bonne Nuit Bay, Jersey, UK show fine-scale growth zoning in SEM-CL. Zones are narrow (1–40 μm wide), planar, parallel to crystal faces, and overgrow more broadly zoned, rounded cores. These crystals are interpreted to have formed at small degrees of undercooling by diffusion-controlled growth. Quiescent magma conditions are indicated by the preservation of an Al-rich diffusive boundary layer which has not been removed by melt shearing during convection. In contrast, rounded and embayed porphyritic quartz grains in a quartz dacite from the Esperanza Mine, E1 Salvador, northern Chile show broad internal SEM-CL zones with irregular boundaries, some of which are truncated by grain margins indicating dissolution. Crystal rounding may occur by dissolution or melting due to thermal or thermal-chemical disequilibrium with surrounding melt, and embayments by gas bubble enhanced convective dissolution are consistent with the development of incipient ocelli textures (strings of amphibole inclusions adjacent to quartz rims). Several episodes of rounding can be recognised in some xenocrysts. Ion microprobe (SIMS) analysis of quartz from both samples shows that A1 contents are higher in areas of bright CL emission. Previous studies of CL in quartz have suggested that several factors control luminescence. Although A1 substitution for Si in SiO4 tetrahedra may be partly responsible for SEM-CL zoning recorded in volcanic quartz, additional mechanisms (including the effect of growth rate on defect density) probably also play a major role.
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A micro-drilling technique was used to detect and study the effect of entrainment of antecrystic zircon in rapakivi texture-forming megacrystic alkali feldspar ovoids in the ferroan granites of the Paleoproterozoic (c. 1630 Ma) Wiborg batholith of southeastern Finland. Texturally controlled analysis of major granite types (wiborgite, dark wiborgite, pyterlite, and porphyritic granite) across the batholith area revealed differences in the textural features, mineral inclusion paragenesis, secondary ion mass spectrometry U–Pb ages, and trace element compositions of inclusion and groundmass zircon populations. This suggests that the material that constitutes the Wiborg rapakivi granites crystallized from separate magmas in at least two stages at different P–T–X conditions. The first stage was characterized by crystallization of alkali feldspar megacrysts from compositionally relatively evolved granitic magmas, recorded by zircon inclusions within the megacrysts during a protracted period of time from c. 1635 to 1628 Ma. The second stage involved relatively rapid final emplacement at c. 1628 Ma, triggered by remobilization of the previously crystallized ovoid material and recorded by groundmass zircon. Differences in the Ti abundance patterns of the dark wiborgite granite type inclusion and groundmass zircon populations can be explained by involvement of coeval mafic (massif-type anorthositic) magmas during the remobilization stage, but the main wiborgitic granite type bears no evidence of mafic magma interaction. Zircon trace element compositions imply that at least in the Wiborg batholith the formation of rapakivi texture has not required mafic magma influence, as rock types without direct evidence of mixing also exhibit prominent rapakivi textures. This suggests that the formation process of the texture was dominated by changes in the intensive parameters (P–T) of the magmatic system. However, more precise quantification of the effects of these factors requires further independent evidence. Issue Section: Original Article
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Two granitoid bodies each from Palaeo-, Meso- and Neo-proterozoic periods have been studied for their chemical, petrological and source characteristics. The Proterozoic granitoids are megacrystic as well non-megacrystic with variable effect by tectonometamorphic events. The Palaeo-proterozoic granitoids are peraluminous, corundum normative with high proportion of biotite in their modal composition. They contain either orthoclase/sanidine or andesine as feldspars and iron rich biotite. The Meso-proterozoic granitoids have high silica with meta- to peraluminous composition. The feldspar chemistry indicates albite/ oligoclase composition whereas they exhibit high iron rich biotite and some being lepidomelane in composition. The Neo-proterozoic granitoids are invariably porphyritic having higher silica content (> 72 wt%), as compared to other granitoids and exhibit strong peraluminous affinity. The feldspar geothermometry indicate 1050°C for Palaeo-, 900-950°C for Meso- and 750°C for Neo-proterozoic granitoids. The Proterozoic granitoids are orogenic, the degree of deformation varies from pristine, non-foliated to gneissic and augen gneissic, mostly 'S' type indicating a sedimentary source material for their generation. The biotite discriminant diagrams also support 'S' type collisional tectonic setting. The garnet-biotite geothermometery for Palaeo-proterozoic granitoids where these minerals co-exist suggest equilibrium temperature of ∼ 540°C at 1 or 2 kb pressures. There is appreciable replacement in magnetites of Palaeo-proterozoic granitoids by K2O, SiO2, Al2O3 and negligible in Meso- and Neo-proterozoic rocks. There is gradual increase in SiO2 and DI from Paleo- to Neo-proterozoic granitoids. The basement complex, after attaining rigidity and cratonisation by the end Archaean igneous activity was rifted periodically during the Proterozoic times. Extensional events led to the formation of mafic dyke swarms, plutonic and volcanic activity and sedimentary platform covers. However, the aborted rifts were further effected by compressive tectonics giving rise to syn and late tectonic deformational signatures to the intrusive phases and their host rocks.
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The Qinling orogen between the North China and South China cratons was intruded at 211–217 Ma by calc-alkaline quartz monzonitic to monzogranitic plutons characterized by I- to A-type geochemistry and in many places contain plagioclase-mantled alkali feldspar megacrysts (rapakivi texture sensu lato). The felsic rocks contain mafic to intermediate magmatic enclaves suggestive of mingling and mixing of mafic and felsic magmas. The P–T conditions of crystallization have been determined for early mineral assemblages (inner parts of alkali feldspar megacrysts and their plagioclase, quartz, amphibole and biotite inclusions) and late assemblages (matrix minerals) of the rapakivi-textured granitoids. Al contents in amphibole from the early and late mineral assemblages yield pressures of 1.2–3.0 and 0.7–3.0 kbar, respectively, and indicate only minor pressure change between the crystallization of the early and late assemblages. Amphibole–plagioclase thermometry gives temperatures mainly of the order of 900 to 1000 °C for both the early and late assemblages indicating nearly isothermal conditions. Feldspar thermometers yield lower temperatures. Relative abundances of minerals and their chemical compositions indicate that the late mineral assemblages tend to be richer in MgO, Na2O and CaO than the early assemblages. Rapakivi texture is interpreted in this case mainly as a result of compositional changes related to the hybridization between granitic and more mafic magmas. Small release of pressure during crystallization of the magmas may have contributed to the origin of the mantled alkali feldspar megacrysts.
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The Devonian Deer Isle granite complex, Penobscot Bay, Maine, exhibits complex and highly diverse textures of minerals and rocks that are the result of thermal perturbations related to injections of new magma during the crystallization history of the pluton. The pluton-wide systematic spatial variation in the abundance of minerals, plagioclase-mantled alkali feldspar, schlieren, enclaves, and rock textures allows us to define four facies. Their spatial distribution across the Deer Isle granite complex reflects crystallization within a zoned magma-chamber. Lithological and textural variations were produced by a combination of the open-system behavior and crystal-melt fractionation during evolution of the magma chamber. The Flye Point and Oak Point facies are basal cumulates formed by crystal accumulation during emplacement and growth of the chamber. Both subsequently matured by compaction and filter pressing of residual melts. The transition from the relatively static cumulate-rich base into a dynamic magmatic flow regime is preserved by the texturally variable Settlement Quarry facies. The overlying leucocratic Crotch Island facies represents an evolved felsic cap that was modified by addition of low-density melts derived from the growing pile of cumulates. The decreasing abundance of mafic enclaves from the Flye Point facies toward the Crotch Island facies indicates that the importance of replenishments of mafic magma, in terms of adding heat and material to the chamber, waned during crystallization. As crystallization proceeded, convection ceased and texturally distinct granite domains were preserved.
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The Paleoproterozoic Svecofennian bedrock of southern Finland formed during a multiphase convergence and collision of lithospheric plates ∼ 1900 Ma ago. The next event to strongly reshape the Finnish bedrock was the emplacement of the rapakivi granites-∼1600 Ma ago, when the Svecofennian mountain chain had already been eroded down to its roots. At this time, changes occurred in the Earth's mantle within Fennoscandia, which led to rearrangements within the bedrock and the crystallization of rapakivi granites and accompanying mafic and intermediate rocks in the upper and middle parts of the continental crust. Sauna and rapakivi are the only Finnish words known in their original form in all civilized languages. The common people in Finland have applied the word rapakivi to describe the way certain rock outcrops weather into an easily crumbling rock or gravel.
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Granitoids varying in composition from biotite–amphibole granodiorite to monzogranite with rapakivi textures have recently been recognized in the Qinling orogen (central China). The granitoids are magnesian, calc-alkalic and metaluminous (A/CNK=0.71–0.95). They contain many mafic to intermediate microgranular enclaves. The enclaves are quenched mafic melts in granitic host magma as evidenced by fine-grained sinuous margins against or transitional contacts with the host granitoids as well as disequilibrium textures and mineral assemblages. U–Pb zircon dating of the granitoids yields: 209±2Ma and 212±2Ma and the enclaves 210±2Ma and 211±2Ma, respectively. The host granitoids and enclaves show obvious silica gap but similar REE and trace element patterns. The whole-rock 87Sr/86Sr(t), εNd(t) and zircon εHf(t) values for the granitoids vary from 0.70516 to 0.70634, −2.3 to −4.4 and 0.7 to −7.6, and for the enclaves from 0.70467 to 0.70600, −1.1 to −4.0, and 1.2 to −8.8, respectively, showing overlapped εNd(t) and zircon εHf(t) values. All these suggest that the two magmatic systems represented by the host rock and enclave are probably derived from distinct sources with sufficient interaction, or common origin but underwent different degrees of crustal contamination. Mineralogical and geochemical data point to enclave dissolution in silicic magma as a major process accounting for the origin of the rapakivi texture. The possible bimodal magmatism suggested by the coeval granitoids and lamprophyre dykes, combined with the structural pattern, geochemical features and regional tectonics, suggest a post-collision setting for the Qinling rapakivi-textured granitoid plutons.
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The compositional zoning of plagioclase-mantled K-feldspar, defining the rapakivi texture, and of the associated quartz phenocrysts from the Paleozoic Land’s End (U.K.) and Altenberg–Frauenstein (Germany) granites, and the Proterozoic Hammarudda (Finland) granite porphyry, has been investigated by laser-ablation – inductively coupled plasma – mass spectrometry, electron-probe micro-analyses, cathodoluminescence and thermal ionization mass spectrometry in order to investigate the formation of the rapakivi texture in two different eons. Recent analytical developments and the Ti-in-quartz geothermometer lead to interpretations of the trace-element zoning in quartz phenocrysts coexisting with rapakivi feldspars. There is an approximate coincidence with Ba-rich growth zones in plagioclase-mantled K-feldspar and Ti-rich zones in coexisting quartz phenocrysts. Both types of zoning indicate increasing temperatures of crystallization. The formation of the plagioclase mantles seems to be related to quartz-resorption events. The inferred temperature of crystallization increased after marginal resorption of quartz phenocrysts by about 82°C in the Altenberg–Frauenstein magma and 44°C in the Hammarudda magma, on the basis of the Ti-inquartz geothermometer. The temperature increase is correlated positively with the crystallization of plagioclase mantles on the K-feldspar. The quartz phenocryst in the Land’s End granite shows normal core-to-rim zoning of Ti (decreasing concentrations), indicating a gradual decrease in magma temperature. We contend that the increase in the quartz-crystallization temperature of >25°C after a resorption event is indicative for the interaction with mafic magma. Therefore, the interaction of a crystal-saturated granitic magma and a mafic magma is the driving force causing nucleation and crystallization of plagioclase on K-feldspar phenocrysts, even though the Pb isotope, Ba, Sr, and Rb zoning of the mantled K-feldspar phenocryst have not clearly recorded an interaction between granitic and mafic magmas. The frequency of rapakivi feldspars in the rock correlates with the portion of mafic magma involved in the mingling and mixing process. Isothermal decompression during adiabatic magma ascent may have contributed to the plagioclase mantle formation in the case of the Altenberg–Frauenstein and Hammarudda granites. The rare rapakivi feldspars in the Land’s End granite developed during an early stage of magmatic evolution; as a result, tracing the conditions of formation of the rapakivi texture is speculative in that case.
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Five, Neoproterozoic, poorly studied felsic intrusives from NW Saudi Arabia have been subjected to a detailed geological, geochemical and mineralogical study to identify their characteristics and to understand their processes of formation. The results have indicated that the individual plutons in the region are each subtly different. This is mainly displayed by the existence of different types of granites, based on chemistry and petrography (peralkaline, alkaline and calc-alkaline) and mineralogy (particularly the presence of different types of amphibole, both sodic and calcic). The granites were most likely derived by low-pressure, partial melting of crustal rocks with possible crustal contamination, followed by fractional crystallization and later sub-solidus alteration by fluids. The geochemical differences between the granites could be related to their formation during different stages of the region’s evolution. The granites contain relatively high contents of rare earth elements and rare metals, mostly hosted by phosphates (monazite-Ce and xenotime), Nb oxide (fergusonite-Y) and possible rare earth carbonates (synchysite). Further detailed geochemical study would determine the economic significance of the studied granitoids and allow understanding how the plutons were fit into tectonic setting of the region.
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Three of the largest granite porphyries of northern Portugal were studied to improve current knowledge on the regional felsic vein hypabyssal magmatism. All porphyries exhibit microcrys-talline groundmasses of variable granularity (composed of quartz, K-feldspar, and muscovite), quartz, K-feldspar, plagioclase, biotite, and cordierite phenocrysts, antirapakivi and rapakivi feldspars, em-bayments in quartz, and enrichments in rare metal incompatible elements. The veins were emplaced along fractures generated during the last phase of the Variscan orogeny. Textural features were presumably conditioned by fast cooling, volatile loss, subsolidus annealing, and the magnitude of thermal contrasts at the time of emplacement. All veins were altered by subsolidus hydrothermal fluids, as suggested by several petrographic and geochemical evidence. The generation of mantled feldspars is probably related to isothermal decompression and magma mixing, which is compatible with the εNdi signatures (−3.76 to −4.40). Based on this research, both processes have contributed to the petrogen-esis of the studied porphyries.
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The Acapulco intrusion is a composite pluton that belongs to the coastal batholithic belt of southern Mexico, intruding the Xolapa metamorphic complex and cropping out in the neighboring area of Acapulco city. The Acapulco intrusion has been considered as an anomaly based on its age, which contrasts with the surrounding plutons and the general age trend from the coastal batholithic belt and corresponds to an Eocene-Oligocene age. It ranges in composition from granite (sensu stricto) to syenite and diorite. The most distinctive characteristic of the Acapulco intrusion is the rapakivi texture developed in the granites, which are characterized by biotite, amphibole, allanite, and fluorite as distinctive minerals, plus titanite, zircon, and apatite as accessory phases. Geochemically, the Acapulco intrusion varies from metaluminous to peraluminous, and displays the distinctive signatures of arc-related magmas. The studied rocks show strong negative Sr, Ba, and Eu anomalies, coupled with incompatible element enrichments and high Ga/Al ratios, which are typical characteristics of A-type granites that underwent strong plagioclase fractionation from a formerly metaluminous magma. Initial isotopic ratios (87Sr/86Sr from 0.7035 to 0.7100, and Nd from +5.50 to +1.78) indicate a range from depleted mantle compositions to compositions consistent with crustal contamination by continental crust, particularly from the surrounding Xolapa Complex. U-Pb geochronology in zircons by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) established crystallization ages of 49.40 ± 0.40 Ma, 50.20 ± 1.0 Ma, 50.42 ± 0.39 Ma, and 50.56 ± 0.39 Ma for different lithologies of the Acapulco intrusion. These geochronological data, together with previous published works, confi rm that post-Laramide plutonism between 50 and 60 Ma is widespread in the southern continental margin of Mexico as a major magmatic event. Finally, new thermobarometric determinations established emplacement conditions of ~700 °C at 8-10 km depth (2.08-2.8 kbar), indicating an exhumation rate of ~0.21 km/m.y. between 50 and 20 Ma, which is slower than the previous estimated rate of 0.44 km/m.y. These results call for a review on models suggesting fast and/or slow exhumation of the southern Mexico coastal batholitic belt.
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The enclaves from rapakivi-textured granite plutons in the Qinling orogenic belt are mainly dioritic magmatic enclaves, having low content in SiO2 (50%-62%), high in K2O + Na2O (7.01%-9.4%), δ(5-9), F, REE, Cr, Ni, V and Ga. The major oxide contents of the enclaves and their host rocks plots on the same linear trends. The pattern of REE, trace element and isotopic signatures of the enclaves are similar to those of the host granites. Exchanges of active elements, HLE and LREE occurred between the enclaves and the host rocks and almost reached equilibrium. Those characteristics were resulted from the nature of magma mingling/mixing, i. e. the formation of rapakivi-textured granite plutons at least involved two kinds of magmas hybrid. Low (87Sr/ 86Sr)i(0.70514-0.70624), high εNd(t) (-0.95 - -3.3), Cr, Ni and V of enclaves suggest that their magma of the enclave may have originated from mantle and may have been a basaltic magma in nature before its mixing with the silisic magma. The relationship between the enclaves and host rocks suggests that the way of the magma mingling/mixing is the basic magma injecting into the acidic magma. This study provides new evidence for the genesis of the rapakivi-textured granites. That is, the granites crystallized from a hybrid magma formed by mingling of the crust-derived acidic magma and the mantle- derived basic magma.
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A combined gravimetric and geochronological interpretation of the Parguaza intrusion in the Guayana shield, southwestern Venezuela, indicates that there is a pattern of inverse cooling within this pluton. Based on a positive correlation between Rb/Sr whole rock apparent ages and residual Bouguer anomaly, density contrasts over the Parguaza intrusion are linked to temperature and crystallization of mineral phases. This hypothesis is supported by density measurements on hand samples and by independent geochemical evidence. A simple 2D gravity model is constrained by surface geology, Rb/Sr apparent ages (whole rock) and residual Bouguer anomalies. Gravimetric modeling implies a model of horsts and grabens that accounts for inverse zoning of the intrusion as a result of geological and age contrasts across the faults.
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The potential use of cathodoluminescence still remains relatively unknown to many who could use this technique for a fuller examination of geological material. This text aims to act as an introduction to the technique and to describe the CL properties of commoner minerals. An introduction to luminescence phenomena and a history of CL observations is followed by descriptions of the CL properties of native elements; sulphides; oxides and hydroxides; halides; carbonates, nitrates and borates; sulphates, chromates, molybdates and tungstates; phosphates, arsenates and vanadates; and silicates. Chapters then deal in greater detail with the CL properties and their geological applications of feldspars, quartz and carbonates. The final two chapters deal with some of the lesser known applications of the technique. The text is illustrated with colour plates collected over twenty years and includes a bibliography of relevant literature and subject and author indexes. -A.W.Hall
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This chapter considers the magmatic part of the history of an igneous rock from the point of view of isotope geochemistry. First, we briefly discuss the general range of isotopic compositions in the principal igneous rock types. Second, the most important processes leading to D/H and **1**8O/**1**6O changes during the generation, evolution, and crystallization of magma are discussed, including some applications of these ideas to actual occurrences.
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Both F and B, in addition to H2O, may have a very great effect on phase relationships in the hydrous silica-saturated residua system Q-Ab-Or, giving much lower melting T. Although F and B minerals are refractory under dry conditions, when H2O is present, F-bearing micas and amphiboles are relatively unstable and their breakdown products include melt. In most high-grade metamorphic rocks, minor but significant amounts of F occur and can affect anatectic melt generation.-R.A.H.
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The wide compositional range of rapakivi-bearing rocks strongly suggests that process, not composition, governs the formation of rapakivi texture. A computational investigation was undertaken to determine whether decompression can induce the formation of the common features of rapakivi. Crystallization path calculations were conducted for seven rapakivi-bearing magma compositions at pressures in the range of 1-8 kbar to determine the effect of decompression on the phase stabilities of plagioclase, quartz, and alkali feldspar. These calculations indicate that decompression can produce parageneses that can lead to all of the characteristic features of rapakivi under certain conditions. -from Author
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Alkali feldspars with water at 5 kb begin to melt at an isobaric eutectic: 703±2 °C, Or 28.5 (wt. per cent) recalculated anhydrous. The liquidus of albite-H2O at 5 kb is 758±3 °C. The K feldspar-rich, water-saturated liquidus is essentially that of Yoder, Stewart, & Smith (1957), who furthermore found orthoclase-H2O to melt at 876 °C. The alkali feldspar solvus, determined by synthesis from glass and a few reversals using crystalline material, has a calculated critical temperature of 730 °C and a critical composition of Or31 (wt. per cent) using parametric equations (Thompson & Waldbaum, 1969). The Margules equation yields Tc = 738 °C. The feldspars produced are structural equivalents of low sanidine on the basis of their 060 and 04 powder X-ray diffraction peaks. The critical line intersects the beginning of melting curve for the system Ab-Or-H2O at 4.2 kb, 715 °C, the minimum conditions for the coexistence of two feldspars and liquid in this system. The slope of the critical line, using Orville's (1963) 2 kb results, is 18.3 °C/kb, leading to an intersection with the kyanite-sillimanite curve of Richardson, Bell, & Gilbert (1968) at about 10.6 kb, 834 °C. The intersection with the andalusite-sillimanite curve of these authors (1969) falls at about 3.5 kb, 703 °C. Such intersections may be used to estimate limits of P and T for a variety of crustal rocks. Because isobaric crystallization of feldspars in the water-deficient region must enrich liquids in H2O to the saturation point, haplosyenitic liquids with a finite initial water content must, at 5 kb total pressure, eventually crystallize two feldspars.
Article
The compositions of feldspar pairs crystallized from stoichiometric alkali feldspar gels with water at P H 2 O ≈ 1 kbar in runs of up to 3000 hours' duration vary with Na:K in the starting material. This is because, for most bulk compositions, rapid nucleation in the gel leads to growth of one feldspar phase outside the solvus. The composition of this initial feldspar phase can be thought of as reflecting the configuration of the liquidus-solidus curves in the system Ab-Or. In general, the attainment of the stable equilibrium solvus (binodal) requires that one phase approach the binodal from outside; for compositions with < 30 mole % Or this phase becomes more potassic with time, with > 30 mole % Or, more sodic. Homogeneous gels of bulk composition Ab 70 Or 30 initially crystallize feldspar pairs inside the binodal, which rapidly (< 17 hours) unmix to define a solvus-like curve (probably the spinodal, with an apparent critical point at 602 °C and 31 mole % Or) and then unmix slowly, giving a solvus after 3000 hours with a critical point at 657 °C and 36 mole % Or. Because this curve has been approached from two directions it is the best approximation to the binodal (metastable only with respect to Al-Si order) obtained. It is similar to the 2 kbar ‘peralkaline’ solvus of Orville, 1963, provided an adjustment for pressure of 16 °C/kbar is made. Differences between solvi determined by Luth and Tuttle (1966) for ‘non-stoichiometric’ bulk compositions may reflect differences in initial crystallization behaviour, as may the breaks in the solvus limbs suggested by Luth et al. (1974).
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Textural and compositional similarities between rapakivi texture in extrusive and intrusive rocks suggest that they have a common origin. Moreover, modification of early-formed textures due to protracted crystallization, subsolidus reactions, and metasomatic-hydrothermal alteration in the plutonic environment accounts for most of the observed differences. The association of coeval mafic rocks and evidence for mafic-felsic magma interaction in many plutons containing rapakivi texture confirm that magma mixing plays a key role in its formation in some intrusive examples. Evidence for a compositional control and multiple episodes of mantle formation are also compatible with repeated cycles of recharge and mixing. Textural evidence from volcanic rocks and experiments indicates that development of rapakivi texture is controlled by cation diffusion in a dissolution boundary layer developed on K-feldspar. Mantles appear to form in a two-stage process that involves initial epitaxial nucleation of plagioclase on sanidine, followed by simultaneous dissolution of sanidine and inward growth of plagioclase. -Authors
Article
The Precambrian complex of the Gold Butte-Bonelli Peak area is about 25 by 35 km and consists of gray porphyritic perthite-quartz-biotite granites and quartz monzonites (adamellites) that intrude Precambrian garnet-cordierite-sillimanite and hornblende gneisses, migmatites, and older granites, pyroxenites, and hornblendites. These younger granites strikingly resemble the Finnish rapakivi granites geologically, petrographically, and chemically. They are exposed over an area approximately 20 by 30 km and represent the roof of a batholith with numerous stocks, dikes, roof pendants, inclusions, and plutonic breccias. They have sharp contacts with their host rocks and have affected these rocks only slightly. No orientation of feldspar phenocrysts can be observed in most of the granite body. The crystallization of this granite has apparently been undisturbed by orogenic movements. It is a postkinematic disharmonious granite. Potassium feldspar and quartz started to crystallize first in this granite. The main constituents are perthite, quartz, oligoclase-andesine, albite, and biotite. Accessory minerals are sphene, hornblende, zircon, apatite, allanite, and fluorite. Typical rapakivi texture with some ovoid perthite phenocrysts surrounded by oligoclase-albite mantles occurs in many places but seems to be more common near the contacts. Quartz appears in two generations, the first of which is commonly subhedral to euhedral (hexagonal bipyramids). Small irregularly shaped miarolitic and mineralogically complex pegmatite bodies occur in the immediate vicinity of the contacts, partly in the granite and partly in the metamorphic rocks. They contain much allanite; some contain concentrations of samarskite, columbite, zircon, and other rare minerals. They commonly show well-developed zoning. No larger pegmatites associated with the granites could be observed. Gold-bearing fluorite quartz veins are numerous. These granites weather in some areas into a sharp-edged coarse gravel, best described by the Finnish term moro. The feldspar phenocrysts and other mineral grains are partly separated by this weathering process, more so than during the weathering of regular granites. It is remarkable that the moro develops in this dry, hot desert climate in the same way as in the relatively humid and cold climate of Finland.
Article
SOLID-STATE diffusion plays an important part in controlling the chemical and physical nature of minerals and rocks, and the mechanism and kinetics of diffusion are in turn affected by the physical and chemical environment. We have investigated the effects of fluorine on alkali interdiffusion in feldspars because it is known that fluorine strongly affects the structure and viscosity of melts1, fluid-melt partition coefficients2, mineral melting temperatures3,4and, most significantly, the rate of cation diffusion in melts5. We report the results of diffusion-couple experiments run at 600-800 °C and 200 MPa for 2-24 days. At relatively low fluorine fugacities the alkali interdiffusion rate was increased by about four orders of magnitude compared with the fluorine-absent system, and the activation energy was decreased. The magnitude of the effect is great enough to explain unexpectedly large exsolution lamellae in some rhyolites, and also calls into question the effects of fluorine on other solid-state reactions involving diffusion. Because fluorine is a common volatile component in igneous and metamorphic rocks, the implications of our results may extend far beyond this simple example.
Article
Enchanted Rock batholith is one of several moderate-sized plutons that intruded a framework of tightly folded Precambrian metamorphic rocks in Middle Precambrian time. Tectonic features within the mass and structural attitude of wall rocks indicate the northern third is phacolithic. The remainder is both discordant and concordant batholithic, although the schists show a tendency to box the compass around the batholith. The phacolithic part occupies a synclinal trough plunging 35°-40° SE. The batholith consists of four concentric zones: (1) outer zone, medium to coarse leucogranite and granite; (2) intermediate zone, medium to coarse granite and quartz monzonite; (3) intermediate central zone, coarse quartz monzonite and alkalic granodiorite; and (4) the core, fine to medium leuco-quartz monzonite and leucogranite. Rapakivi are most abundant in zone 3. Seriate porphyritic texture prevails in all zones except the core where it is increasingly hiatal. Chilled border rocks are 10-20 feet wide and consist of leuco-quartz monzonite and quartz monzonite. Apophyses of the batholith in metamorphic wall rocks are leuco-quartz monzonite and leucogranite. Hiatal porphyritic texture prevails in chilled border and apophyses rocks. Internal structures of the batholith and its relation to country-rock structures indicate intrusion during the late, most severe stages of regional compression. Tensile stresses along axial planes of folds exerted structural control and guided the rise of the magma, as indicated by the pear-shaped outline of the batholith. The most probable mechanism of magma generation is selective fusion (melting) of random rock material. This occurred at depth in down-buckled zones below the present position of the batholith. Concentration of the proportions of normative constituents of individual rock zones is strikingly grouped about a composition having a 1:1:1 ratio of albite: orthoclase: quartz. Mineralogically there are systematic gradations in proportions of plagioclase, microcline, and quartz between rock zones suggesting that unmixing of sodic plagioclase from microcline was a dominant process. Outline shape of rock zones, textural relations between plagioclase and microcline crystals, and maxima-minima concentrations of unmixed sodic plagioclase suggest migration of part of the unmixed material within the batholith during the later stages of its structural and intrusive history. Field and chemical evidence indicate that probably not more than 5 per cent of the granites of the batholith are of replacement origin. Failure of roof rocks with partial escape of volatiles, cessation of regional tectonism, and depletion of the magma reservoir in the late stages of intrusion probably are responsible for finer grain of the core. Age determinations by the "Larsen Method" give an average age for the batholith of 815 million years.
Article
Experiments designed to simulate the dissolution of alkali feldspar during magma mixing produced plagioclase mantles that are texturally and compositionally similar to those in some hybrid volcanic rocks. In hydrous dacite melt (69% SiO2) at 0.8 GPa, 850C, orthoclase (Or93) and sanidine (Or30) partially dissolved and were mantled by sodic plagioclase (An25–30). Although plagioclase nucleated epitaxially as a thin shell on the alkali feldspar surface near the time of initial resorption, plagioclase subsequently grew inward —mostly in the form of parallel blades — toaard the receding dissolution surface. Orthoclase dissolved at a rate approximately proportional to the square root of run duration, indicating diffusional control. Plagioclase grew inward within a static boundary zone of melt that formed between the original crystal-dacite interface and the dissolution surface. During orthoclase dissolution, this boundary zone rapidly and simultancously gained Na (by diffusion from dacite) and lost K (by diffusion into dacite); Ca diffused more slowly into this zone, from which non-feldspar species were mostly excluded. Plagioclase was stable where sufficient Ca had diffused in that the boundary zone melt intersected the plagioclase-saturation liquidus. Plagioclase subsequently grew toward the receding dissolution surface as the Ca compositional gradient (and hence the site of plagioclase saturation) stepped inward. Crystallization of plagioclase in the form of parallel blades allowed continued diffusive exchange of melt components between the dissolution surface and the host melt. Bladed growth also served to maintain (at blade tips) proximity of plagioclase to the dissolution surface, thereby apparently preserving (locally) a thin zone of low-variance melt. In natural systems, mantling of alkali feldspar by plagioclase will occur in a similar manner when (a) P, T, or X are changed to induce alkali feldspar dissolution, (b) sufficient Ca is available in the host melt to drive (by diffusion) boundary zone melt compositions to plagioclase saturation, and (c) temperatures are low enough to stabilize sodic plagioclase and to maintain a coherent boundary zone. These reqjirements are satisfied in volcanic systems when alkali feldspar is juxtaposed during mixing with hybrid melts of dacitic composition. Mantled feldspars in some intrusive systems (i.e., rapakivi granites) may form by a similar dissolution- and diffusion-controlled mechanism. Textural evidence of a similar origin may be obscurred in intrusive rocks, however, by products of late-stage magmatic and subsolidus processes.
Article
The key to mantled feldspar genesis is epitaxial nucleation of plagioclase on K-feldspar or K-feldspar on plagioclase. Once this nucleation takes place there is a relatively straightforward process of crystal growth yielding rapakivi and antirapikivi textures. The most common mantling is plagioclase on K-feldspar which occurs in both volcanic and plutonic environments. In the volcanic environment the morphology of the plagioclase overgrowth typically is dendritic, though in subvolcanic and shallow plutonic environments dendritic growth is followed by a more or less continuous non-cellular shell of plagioclase. In the plutonic environment, early stages of plagioclase overgrowth also tend to be dendritic, although with coarser-grained characteristics. Dendritic morphology is thus a common denominator in rapakivi genesis. Since growth of dendritic plagioclase is clearly related to marked undercooling in silicate melt systems its occurrence in many volcanic rocks is to be expected. Equivalent quenching in the plutonic environment requires a cooling mechanism independent of conductive heat transfer to wallrock and also independent of effective cooling related to sudden loss of volatile phases that could only occur late in the crystallization of most magmas and therefore after much dendritic plagioclase had already formed. Internal quenching of portions of magma systems must occur if mafic magma is abruptly mixed with felsic magma. Such magma mixing yields a heterogeneous system at first, one that is in a drastic state of disequilibrium and tending to force nucleation of one feldspar type on the surface of another resulting in epitaxial crystallization of dendritic plagioclase on K-feldspar. Mantling of one feldspar type by another during magma mixing is paralleled by dendritic growth zones in coexisting plagioclase crystals.Mantling textures occur in hybrid rocks of magma mixing origin. Some of the hybrid rocks are fine-grained, mafic-rich, and may contain phenocrysts of quartz, plagioclase, and K-feldspar. They occur as rounded inclusions in calc-alkaline granites and granodiorites. The host plutons themselves commonly have mantled feldspars or at least plagioclase with the unusual zoning characteristics commonly accompanying rapakivi texture. Magma-mixing tends to occur in batches so that hybrid crystal-melt systems, the calc-alkaline granitic plutons, become intrusive into earlier hybrid crystal-melt systems, represented by the mafic-rich inclusions.
Article
Isotopic and major and trace element analysis of nine samples of eruptive products spanning the history of the Mt. St. Helens volcano suggest three different episodes; (1) 40,000–2500 years ago: eruptions of dacite with εNd = +5, εSr = −10, variable δ18O,206Pb/204Pb ∼ 18.76, Ca/Sr ∼ 60, Rb/Ba ∼ 0.1, La/Yb ∼ 18, (2) 2500-1000 years ago: eruptions of basalt, andesite and dacite with εNd = +4 to +8, εSr = −7 to −22, variable δ18O (thought to represent melting of differing mantle-crust reservoirs), 206Pb/204Pb= 18.81−18.87, variable Ca/Sr, Rb/Ba, La/Yb and high Zr, (3) 1000 years ago to present day: eruptions of andesite and dacite with εNd = +6, εSr = −13, δ18O∼6‰, variable206Pb/204Pb, Ca/Sr ∼ 77, Rb/Ba= 0.1, La/Yb ∼ 11. None of the products exhibit Eu anomalies and all are LREE enriched. There is a strong correlation between87Sr/86Sr and differentiation indices. These data are interpreted in terms of a mantle heat source melting young crust bearing zircon and garnet, but not feldspar, followed by intrusion of this crustal reservoir by mantle-derived magma which caused further crustal melting and contaminated the crustal magma system with mafic components. Since 1000 years ago all the eruptions have been from the same reservoir which has displayed a much more gradual re-equilibration of Pb isotopic compositions than other components suggesting that Pb is being transported via a fluid phase. The Nd and Sr isotopic compositions lie along the mantle array and suggest that the mantle underneath Mt. St. Helens is not as depleted as MORB sources. There is no indication of seawater involvement in the source region.
Article
New U-Pb data on zircons, monazites, and baddeleyite suggest that the Wiborg rapakivi batholith and associated mafic rocks in southeastern Finland were emplaced mainly between 1650 and 1625 Ma. The earliest anorogenic magmatism related to the intrusion of the rapakivi granites was the emplacement of some diabase dykes about 1665 Ma ago, while the youngest porphyries intruded the rapakivi granites at 1615 Ma. The process involved three peaks of diabase activity at 1665, 1645, and 1635 Ma and two major granite events at 1640±5 and 1630±5 Ma, the former of which comprises also the intrusion of minor gabbroic-anorthosite bodies. On the whole, the result was the emplacement of at least 105 km3 of rock material over a period of 50 Ma. Within southern Finland, the rapakivi magmatism continued until 1540 Ma by emplacement of the West Finnish intrusions, which combined are as extensive as the Wiborg area. Globally, the Proterozoic rapakivi event probably represents the largest pulse of intracratonic magmatism which occurred during geological history and may be a consequence of rapid growth of continental masses in the Early Proterozoic.
Article
Thesis (doctoral)--Vrije Universiteit te Amsterdam, 1988. Includes bibliographical references (p. 401-416). Summary in Dutch.
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