Anatexis and chemical evolution of pelitic rocks during metamorphism and migmatization in the Hidaka metamorphic belt, Hokkaido.
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Abstract:
The southernmost Hidaka metamorphic belt consists mainly of cordierite tonalite intrusions and pelitic metamorphic rocks ranging from the greenschist to the granulite facies. Anatectic migmatites are common in the higher amphibolite and granulite facies zones. Compositional changes in major, rare earth elements and some other trace metals are so small that they are undetectable among the pelitic metamorphic rocks of zones A + B + C and D, but they are large enough to be detected in the higher amphibolite (zone D) to the granulite facies rocks (zone E). The enrichment of Fe, Mg, Na, Eu, and Sc, and the depletion of K, P, La, Ce, Nd, Cs and Rb are statistically significant in pelitic granulites, while heavy REEs are very variable. The chemical variation of pelitic granulite was derived from the accumulation of plagioclase + garnet. This suggests that more than 50-60% of the total volume of pelitic granulite was melted to produce a large amount of tonalitic magma, leaving pelitic granulite as a restite. Migmatites of the higher amphibolite facies are anatexites, and their K, P, Cs, Rb and light REE content is the same as that of lower grade metamorphic rocks. Migmatites of the higher amphibolite facies melted incipiently to segregate only a small amount of melt, and could not produce a large magmatic mass such as the cordierite tonalites. Cordierite tonalites are S-type granites, and their major elements, Cs, Rb and light REE concentrations are similar to those of lower grade metamorphic rocks. The chemical variation of cordierite tonalites is explained by the extraction of plagioclase + garnet from a tonalitic magma and the variation of original sedimentary rocks. The small chemical difference between the cordierite tonalites and the lower grade metamorphic rocks suggests that the former was derived from a massive melting of metapelites or that much of the restite is retained. The material migration among higher amphibolite facies rocks, pelitic granulites, migmatites and cordierite tonalites took place through mineral/melt interaction in the lower crust.Keywords:
Migmatite
Anatexis
Pelite
Cordierite
Greenschist
Sillimanite
Higher Himalayan Crystalline(HHC) complex of the Sikkim Himalaya predominantly consists of high-grade pelitic migmatites.In this study,reaction textures,mineral/bulk rare earth elements (REE),trace element partition coefficients and trace element zoning profiles in garnet are used to demonstrate a complex petrogenetic process during crustal anatexis.With the help of equilibrium REE and trace element partitioning model,it is shown that strong enrichment of Effective Bulk Composition(EBC) is responsible for the zoning in garnet in these rocks.The data strongly support disequilibrium element partitioning and suggest that the anatectic melts associated with mafic selvedges are likely produced by disequilibrium melting because of fast melt segregation process.
Migmatite
Pelite
Disequilibrium
Anatexis
Trace element
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Anatexis
Migmatite
Dalradian
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Anatexis
Migmatite
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Summary Banded gneisses and migmatites in the Champira Dome contain assemblages of the biotite-cordierite-almandine subfacies of the granulite facies. The banded gneisses represent a sequence of arkosic sediments metamorphosed to sillimanite-cordierite gneisses. The migmatites were derived from more argillaceous and potassic sediments, with lower oxidation ratios and lower Ba/Rb ratios. They consist of a quartzo-feldspathic leucosome, representing anatectic melt, with schlieren of the refractory minerals sillimanite, garnet, biotite, and oxide minerals. Cordierite developed from garnet and biotite, except in rocks of high FeO/(FeO + MgO) ratio. Both rock types contain assemblages of magnetite + hercynite + corundum + ilmenite + hematite, formed by unmixing of high-temperature solid solutions. Rb-Sr studies of the banded gneisses gave an age of 2327 ± 25 Ma, which is interpreted as the date of metamorphism, and an initial 87 Sr/ 86 Sr ratio of 0.7064 ± 0.0003. The migmatite samples plot close to this 2327 Ma regression line, but the strontium isotopes were considerably disturbed, though not homogenized on the scale of sampling, 962 ± 34 Ma ago. It is considered that the anatexis in the migmatites was contemporaneous with the metamorphism of the banded gneisses and that the 962 ± 34 Ma event may be correlated with recrystallization of the migmatites accompanying the growth of cordierite.
Sillimanite
Migmatite
Anatexis
Cordierite
Recrystallization (geology)
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ABSTRACT Experimentally constrained calibrations of the incorporation of H 2 O and CO 2 into cordierite as functions of P–T- a H 2 O - a CO 2 are integrated with KFMASH grids which define mineral-melt equilibria in pelites. This is used to explore the impact of the volatile content and composition of cordierite on anatexis and melt-related processes in high-temperature (HT) and ultra-high-temperature (UHT) metamorphism. The strongly temperature-sensitive H 2 O content of cordierite coexisting with dehydration melts (0·4–1·6 wt.%) causes a 10–25% relative decrease in the amount of melt produced from pelites compared with models which treat cordierite as anhydrous. KFMASH melting grids quantified for a H 2 O demonstrate consistency between the measured H 2 O contents in cordierite from granulite-migmatite terrains and mineral equilibria. These indicate anatexis with a H 2 O in the range 0·26–0·16 at 6–8 kbar and 870–930°C. The pressure-stability of cordierite+garnet with respect to orthopyroxene+sillimanite+quartz in KFMASH is strongly influenced by cordierite H 2 O content, which decreases from 1·1 to 0·5 wt.% along the melting reaction Grt+Crd H +Kfs=Opx+Sil+Qz+L. The lower-T invariant point involving biotite (8·8 kbar/900°C) that terminates this reaction has a H 2 O of 0·16±0·03, whereas the higher-T terminating invariant point involving osumilite (7·9 kbar/940°C) occurs at a H 2 O 0·08±0·02. Osumilite-bearing assemblages in UHT terrains imply a H 2 O of <0·08, and at 950–1000°C and 8–9 kbar calculated a H 2 O is only 0·04–0·02. Cordierites stable in osumilite-bearing assemblages or with sapphirine+quartz have maximum predicted H 2 O contents of ca. 0·2 wt.%, consistent with H 2 O measured in cordierites from two sapphirine-bearing UHT samples from the Napier Complex. The addition of CO 2 to the H 2 O-undersaturated (dehydration-melting) system marginally decreases the temperature of melting because of the stabilisation of cordierite, the solid product of the peritectic melting reactions. The preferential incorporation of CO 2 enhances the stability of cordierite, even at fixed a H 2 O , and causes the stability fields of Grt+Crd+Sil+Kfs+Qz+L and Grt+Opx+Crd+Kfs+Qz+L to expand to higher pressure, and to both higher and lower temperatures. The minimum solubility of H 2 O in granitic melt is independent of the CO 2 content of cordierite, and the distribution of H 2 O between melt and cordierite is similar at a given melt H 2 O-content to the H 2 O-only system. This enhanced stability of CO 2 -bearing cordierite leads to a reduced stability range for osumilite-bearing assemblages to temperatures of ca. 950–975°C or greater. Cordierites in the Napier Complex UHT gneisses contain 0·5 and 1·05 wt.% CO 2 , consistent with a role for CO 2 in stabilising cordierite with respect to osumilite in these unusual sapphirine-bearing granul
Cordierite
Anatexis
Sillimanite
Migmatite
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Anatexis
Migmatite
Cordierite
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Migmatite
Anatexis
Dalradian
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Migmatite
Anatexis
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Anatexis
Migmatite
Leucogranite
Fractional crystallization (geology)
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