The effects of solid and vapour phase compositional variations on the amphibolite facies to granulite facies transition in the Agto metadolerites
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Abstract:
Metadolerites occurring in the vicinity of Agto (within the Isortoq Complex of central West Greenland) have been metamorphosed in upper amphibolite to garnet granulite facies. Detailed sampling and petrographic study of these demonstrates that the entire sequence of assemblages may occur in individual dykes, with the amphibolite assemblages normally restricted to dyke margins, garnet granulite facies rocks occurring in the dyke cores, and pyroxene granulite facies rocks intervening between these two. Bulk rock chemical analyses demonstrate that the dykes are chernically homogeneous with respect to all oxides except Al2O3 and Na2O. Nevertheless, variation in the latter two oxides is small, and demonstrably is not a parameter which significantly controls assemblage development.Keywords:
Pyroxene
Abstract This work presents the results of a fluid inclusion study of an amphibolite‐granulite facies transition in West Uusimaa, S.W. Finland. Early fluid‐inclusions in the granulite facies area are characteristically carbonic (CO 2 ), in contrast to predominantly aqueous early inclusions in the amphibolite facies area. These early inclusions can be related to peak metamorphic conditions (750‐820°C and 3‐5 kbar for peak granulite facies metamorphism). Relatively young CO 2 inclusions with low densities (<0.8g/cm 3 ) indicate that the first part of the cooling history of the rocks was characterized by a near isothermal uplift. N 2 ‐CH 4 inclusions, with compositions ranging between pure CH 4 and pure N 2 (Raman spectral analysis), were found in the whole area. They are probably syn‐ or even pre‐early inclusions. Only nearly critical homogenizing inclusions have been found (low density). Pressure estimates, based on densities of early fluid inclusions, show that the rapid transition of amphibolite towards granulite facies metamorphism is virtually isobaric. Granulite facies metamorphism in West Uusimaa is a thermal event, probably induced by the influx of hot, CO 2 ‐bearing fluids.
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A Thesis Submitted to the Faculty of Science
University of the Witwatersrand, Johannesburg
for the Degree of Doctor of Philosophy.
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Hornblende
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Summary This paper presents wet chemical analyses of forty-one pyroxenes (thirty-nine of them new) from pyroxene granulites and a few neighbouring rocks from Swat Kohistan and the adjoining Indus Valley. The granulites, considered to be derived from plutonic norites of an island arc tholeiitic nature, constitute one of the most extensive belts of its kind in the world. The pyroxenes are not unusual in any respect but they bear a closer resemblance to metamorphic than to igneous pyroxenes crystallized from deep-seated tholeiitic magmas. The distribution of Mg, Fe, and Mn (average K D Mg−Fe =0.57) and the tie-line intersections on the Wo-En join (generally from Wo 80.5 to Wo 76.5 ) between the coexisting eighteen pyroxene pairs from the granulites are discussed. Based on eight different methods of geothermometry and other considerations, it is concluded that the pyroxene granulites were metamorphosed at around 800 ° C and 7 to 8 kbar.
Pyroxene
Charnockite
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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.
Migmatite
Anatexis
Pelite
Cordierite
Greenschist
Sillimanite
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Pyroxene
Almandine
Glaucophane
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Trace element
Rare-earth element
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Abstract The enthalpy of reaction of plagioclase and pyroxene to produce garnet and quartz has been a major source of error in granulite geobarometry because of relatively uncertain enthalpy values available from high‐temperature solution calorimetry and compiled indirectly from experimental phase equilibria. Recent, improved calorimetric measurements of Δ H R are shown to yield palaeopressures which are internally consistent between orthopyroxene and clinopyroxene calibrations for many South Indian granulites from the Archaean high‐grade terranes of southern Karnataka and northern Tamil Nadu. This represents a considerable improvement over previous calibrations, which gave disparate results for the two independent barometers involving orthopyroxene and clinopyroxene, requiring a 2‐kbar ‘empirical adjustment’to force agreement. Palaeopressures thus calculated for 30 well‐documented two‐pyroxene garnet granulites from South India give internally consistent pressures with a mean of 8.1°1.1 kbar at 750°C, consistent with the presence of both kyanite and sillimanite in many areas. Those samples for which garnet–pyroxene exchange thermometers give plausible granulite‐range temperatures and whose minerals are minimally zoned give the best agreement of the two barometers. Samples which yield low palaeotemperatures and different rim and core compositions of minerals yield pressures for the orthopyroxene assemblage as much as 2 kbar lower than for the assemblage with clinopyroxene. This disparity probably represents post‐metamorphic‐peak re‐equilibration. We conclude that considerable confidence may be placed in geobarometry of two‐pyroxene granulites where apparent palaeotemperatures are in the granulite facies range (>700°C) and where mineral zonation is minimal. Of the several possible sets of activity–composition relations in use, those constructed from analysis of phase equilibria give slightly higher palaeopressures and appear more consistent with analytical data from the Nilgiri Hills uplift, where kyanite is the only aluminium silicate reported to be stable in peak‐metamorphic assemblages. The present results support a palaeopressure gradient, increasing generally from south to north, across the Nilgiri Hills as inferred by previous geobarometry.
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Sillimanite
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