The evolution of the Palim granite in the Bastar tin province, Central India

The Palim granite, hosted by the metasedimentary country rocks in the Basta r tin province, is a heterogeneous pluton that comprises hornblende granite , biotite granite and two-mica granite. Spherical inhomogeneous surmicaceou s enclaves occur within the granites with coarse grained cores of muscovite mantled by finer muscovite-quartz-biotite ( sillimanite) rims. Geochemical features imply that the granites are highly evolved and geochemically dist inct. Petrographic and geochemical considerations point towards a transitio n from metaluminous I-type hornblende-bearing granite in the south to peral uminous volatile-enriched S-type like lithologies (biotite and two-mica gra nites) towards north. Modeling of highly incompatible elements such as Nb a nd Cs, implies 31 to 33% assimilated fractional crystallization of a melt w ith an initial composition close to that of the hornblende granite to form the two-mica granite. Hornblende geobarometry, plagioclase-hornblende therm ometry (in hornblende granite) and phengite barometry (in two-mica granite) , yield P-T estimates of 5-7kb/725 degrees -760 degreesC, and 6 kb/700 degr eesC, respectively. The study further implies that a genetic link exists be tween granite magmatism and the formation of tin pegmatites in the region. The preponderance of peripheral pegmatites to the north-east of the Palim g ranite is regarded a result of outward crystal-melt fractionation and tecto nic tilting of the pluton.