Petrography and geochemistry of the massif-type anorthosite complex from the Bavali Shear Zone, Southern Granulite Terrane, south India

Abstract

Bavali Shear Zone (BSZ), the western extension of the Moyar Shear Zone (MSZ) separating the Southern Granulite Terrane from the Dharwar Craton, is situated between the Coorg Block in the north and the Nilgiri Block in the south. Massif-type anorthosites arepresent along the western part of the BSZ are intruded by ferrodiorite dykes, veins and pods. Other associated rocks with anorthositesare ferrodiorites, diorites, gabbros, leucogabbros and charnockites. Anorthosites composed of > 90% plagioclase,at some places quartzand k-feldspar are associated with plagioclase. The clinopyroxenes, occasionally coexist with plagioclase,are retrogressed to biotite. Ferrrodiorites are composed of medium to coarse subhedral grains of pyroxene, hornblende, plagioclase feldspar and opaque minerals with k-feldspar, quartz. They show hypidiomorphic granular texture. Leucogabbros composed of olivine, pyroxene and plagioclase exhibit sub-ophitic texture whereas the charnockites composed of orthopyroxene, plagioclase, quartzand orthoclase exhibit antiperthitic texture. Anorthosites and associated rocks are calc-alkaline to alkaline in nature. In all the rocks major element oxides show a negative relation with SiO2 except in anorthosite samples, K2O shows a positive correlation. The anorthosites and associated rocks displayed a moderately depleted light rare earth elements (LREE) and flat heavy rare earth elements (HREE) pattern may suggest partial melting of metasomatized mantle in a subduction zone in an island arc environment. The rocks did not show any noticeable Eu or Ce anomalies except for one leucogabbro, which shows a positive Eu anomaly referring to plagioclase accumulation. The ferrodiorites show high concentrations of LREE (La 2.44-18.112 ppm, Ce 5.088-39.12 ppm) than HREE (Eu 0.56-1.648ppm, Lu 0.16-0.352ppm). Primitive mantle normalized multi-element patterns in these rocks show considerable depletion of high field strength elements (HFSE) (Zr 1.071-27.492ppm, Ta 0.01-0.14ppm, Nb 0.39-7.15ppm) but relative enrichment of large-ion lithophile elements (LILE) (Ba 135.44-335.44ppm, Sr 436-2928.6ppm) are indicative of late stage residual melts that evolved through anorthosite crystallization. Plagioclase subhedral crystals in anorthosite provided evidence that the minerals developed at depth in a rather calm environment. The low K2O content in anorthosites, cumulate plagioclase, and mafics support a deep environment of origin. Although the Bavali Shear Zone may have just served to facilitate the passage of magma, the rock types appear to have formed independently from distinct magma sources and are chemically unrelated