K-feldspar metasomatism in granulite-facies rocks of Palghat region, Kerala: Evidence and implications of brines in charnockite-forming metamorphism

Formation of granulites requires low H2O activity and fluids play a major r ole in their evolution. A wide variety of granulite-forming processes has b een discussed in the literature. Among the models proposed, the presence of fluid inclusions, alkali metasomatism and loss of large ion lithophile (LI L) elements support the fluid-driven granulite-forming process. In this pap er we document abundant petrographic evidence in the Palghat region for the alkali metasomatism and evidence for metamorphic fluid of low H2O in rocks like banded charnockite, hornblende-biotite gneiss and incipient charnocki te. Evidence of potash metasomatism is noted as (a) rims of K-feldspar alon g orthopyrosene, opaque, plagioclase and K-feldspar grains, (b) veins of se condary K-feldspars traversing plagioclase and hornblende and (c) in the fo rm of blebs in plagioclase grains and as exsolution. Excellent evidence of K-feldspar veins growing along the borders of orthopyroxene grains indicate s that the vein network represents deposition of migrating XH2O halide-rich fluids. These low H2O brines may have created a conducive environment for localized breakdown of biotite and development of arrested charnockite.