Orthopyroxene-bearing granulites (with or without cordierite) occur as patches within garnetiferous quartzofeldspathic gneisses (leptynites) near Punalur area in the Kerala Khondalite Belt (KKB), South India. The small-scale transition zones (from three quarry exposures) investigated in this study represent local transformation of foliated, light grey garnet-biotite gneiss into a dark, greenish brown, coarse-grained charnockite. Textural criteria and compositional characteristics of the different phases document several mineral reactions: garnet + biotite + quartz = orthopyroxene + alkali feldspar + plagioclase + ilmenite + H2O biotite + quartz = orthopyroxene + alkali feldspar + v biotite + garnet + quartz = orthopyroxene + cordierite + alkali feldspar + v biotite + quartz = cordierite + orthopyroxene + alkali feldspar + H2O garnet + quartz = cordierite + orthopyroxene Fluid-inclusion data in conjunction with the mineral reactions, show that the granulites of the present study area followed an isothermal decompression path with fluid entrapment conditions at ca. 600-700°C and 3-4 kbar. Mineralogical and textural evidence for this path includes reaction intergrowth of skeletal biotite and garnet with neoformed orthopyroxene and plagioclase, cordierite coronal rims around garnet and spinel, symplectitic intergrowth of cordierite-quartz and cordierite-orthopyroxene, and bluish pleochroic haloes of cordierite around tiny zircon crystals. Fluid-inclusion studies on minerals associated with alkaline pegmatites occurring in the same area revealed the presence of high density CO2-rich fluids. The wide occurrence of similar pegmatite dykes adjacent to the incipient charnockites in many localities in KKB, suggest that the fluids expelled from the alkaline magmas upon their freezing were channeled along faults/shears effecting dehydration along the fluid pathway and generating incipient charnockites through CO2-induced metasomatism. These pegmatites possibly represent part of an alkaline magmatic phase with abundant volatile activity, correctable with the Pan-African alkaline magmatism widely recognized in the southwestern Indian shield. We propose that incipient charnockite formation in KKB is intimately related to magmatic processes, in that the heat and volatiles were derived from CO2- and volatile-enriched magmas.
|Number of pages||28|
|Journal||Journal of Geosciences|
|Publication status||Published - Dec 1 1997|
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