Mineralogy and petrology of the Sittampundi complex, Salem district, Madras State, India

Abstract

The mineralogy and petrology of a thoroughly metamorphosed and reconstituted layered complex of anorthositic rocks with associated gabbroic and chromitic facies from Peninsular India are described in detail. This is one of the few known occurrences of highly metamorphosed anorthositic rocks, and the mineral associations are unique. New chemical analyses and optical studies are presented for two plagioclases, three amphiboles, three garnets, and one each of clinozoisite, cordierite, and chromite together with eight chemical analyses of the rocks. Of special interest are the very calcic plagioclases (An_80-100) in the anorthosites. Chemical analyses in duplicate of a calcic plagioclase, found to be An₁₀₀ by optical methods, indicated it to be An₉₈, perhaps the nearest approach in nature to synthetic anorthite. The complex consists of a layered sequence of meta-anorthositic gneisses and eclogite-gabbros, with the former containing layers of chromitite and perknite; by analogy with other described occurrences of stratiform sheets, this is interpreted as a metamorphosed gravity-stratified sheet. A plot of available chemical analyses of garnetiferous metagabbros and eclogites and their associated garnets is given, and the possibility of a compositional field for most true eclogites as distinguished from garnetiferous metagabbros is pointed out. Deformation and mineral reconstitution during two periods of Archean orogeny, following primary crystallization from a basic magma, modified the paragenesis and mineralogy of these rocks with the development of new minerals such as hornblende, anthophyllite, pyralmandite, epidote-clinozoisite, grossularite garnet, porphyroblastic corundum (in part with calcite rims), and chromite with unmixed rutile. It is proposed to call this stratiform sheet "Sittampundi complex" after the village near its type exposure.