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Progressive Metamorphism of Mafic Rocks from Greenschist to Granulite Facies in the Dharwar Craton of South India

P. Raase, M. Raith, D. Ackermand and R. K. Lal
The Journal of Geology
Vol. 94, No. 2 (Mar., 1986), pp. 261-282
Stable URL: http://www.jstor.org/stable/30062150
Page Count: 22
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Progressive Metamorphism of Mafic Rocks from Greenschist to Granulite Facies in the Dharwar Craton of South India
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Abstract

In the Archean Dharwar craton of southern India N-S trending belts of metabasic rocks are exposed which underwent regional metamorphism at about 2.5 Ga ago. The progressive changes in the assemblages and mineral chemistry of metabasites was studied in a N-S traverse covering the Chitradurga and Nagaman-gala belts, the Sargur area, and the Nilgiri Hills. Towards the south with increasing metamorphic grade greenschists ($chl + act + ab + ep \pm carb$, qtz) give way to amphibolites ($hbl + plag \pm qtz$, gar, cumm) and mafic granulites ($pyx + plag + gar \pm hbl$, qtz). The amphibole composition changes from actinolite in the greenschist zone to tschermakitic hornblende in the amphibolite zone and pargasitic-hastingsitic hornblende in the granulite zone. The Ti content of the amphiboles systematically increases in this direction. The plagioclase composition changes from albite in the greenschist zone to oligoclase/andesine in the amphibolite zone and andesine/labradorite in the granulite zone. Almandine-rich garnets first occur in the high-grade amphibolite zone whereas garnets in the granulite zone are higher in pyrope and grossularite components. Pyroxenes, which occur only in the granulite zone, are hypersthenes and salites with distinctly lower tschermak components in the pyroxene-granulite zone than in the hornblende-granulite zone, suggesting higher pressures of metamorphism in the latter zone. Geothermobarometry, mineral stability data, and mineral isograds demonstrate that P-T conditions along the N-S traverse increase continuously from about 500°C/4-5 kbars in the greenschist zone to about 600°C/5-7 kbars in the amphibolite zone. Further south there is a more pronounced temperature increase to about 700-750°C/8 kbars in the hornblende-granulite facies zone. This transition virtually coincides with the change from Dharwar to Sargur metabasites. South of the Moyar shear zone, which is characterized by higher pressures (9.5 kbars), metamorphic conditions of 700-750°C/6-7 kbars have been determined in the pyroxene-granulite zone of the Nilgiri Hills. The increase of metamorphic grade toward the south is also documented by the systematic partitioning of Na between plagioclase and the A-site of coexisting amphibole. The progressive development of the metabasites not only reflects increasing temperature and pressure of metamorphism but also a change in the fluid regime from hydrous in the greenschist and amphibolite zones to water-deficient in the granulite zone.

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