SYMPLECTITES IN HIGH-GRADE PELITIC GNEISSES OF USILAMPATTI, TAMIL-NADU - P-T CONDITIONS AND GEOCHEMISTRY

Occurrence of fine-grained, fingerprint-like symplectic intergrowths o f cordierite, K-feldspar and quartz formed due to the retrogressive, g arnet breakdown reactions following decompression during uplift, is re ported here from metapelites of Usilampatti area. These metapelites co nsisting of cordierite, garnet, hypersthene, biotite, hercynitic spine l, sillimanite, K-feldspar assemblage were metamorphosed at 6.5-7.5 Kb and 750-800 degrees C. The symplectites were formed at 600-670 degree s C and 5 Kh pressure. The textural and mineralogical evolutions in th ese metapelites indicate a rapid decompressional P-T evolution. Petrol ogical and geochemical data suggest that these metasediments were deri ved mostly from an evolved basement source of granodiorite-granite com position with a minor basic component.