Mafic and ultramafic rocks in parts of the Bhavani complex, Tamil Nadu, Southern India: Geochemistry constraints

Abstract

Present paper deals with the petrogenesis of gabbros and pyroxenites of Bhavani complex, Tamil Nadu, Southern India. Studied gabbros are mainly composed of pyroxenes with minor plagioclase (An 10-30 ) and amphibole minerals. Pyroxenites are composed of coarse-grained clinopyroxenes, orthopyroxenes and medium-grained hornblende minerals. Geochemically, in the total alkali vs. SiO 2 diagram studied samples are broadly classified as gabbros and their magma type is tholeiitic in nature, however, based on the TiO 2 , MnO, P 2 O 5 diagram most of the studied samples fall within the calc-alkaline basaltic (CAB) field. In chondrite-normalized rare earth element (REEs) patterns, pyroxenites show negative Eu anomaly with slight enrichment of heavy rare earth elements (HREEs). The negative Eu anomaly in these samples indicates fractionation of plagioclase. The flat chondrite-normalized REEs pattern of gabbros in association with low CaO, Al 2 O 3 , Sr content and absence of Eu anomaly suggest removal of plagioclase component from basic parental magma. In primitive mantle (PM) and mid-ocean ridge basalt (MORB) normalized incompatible trace element patterns studied samples have enrichment of large ion lithophile elements (LILEs) and light rare earth elements (LREEs). Tectonic setting discrimination diagrams, in addition to their geochemical characteristics such as Nb-Ta, Zr-Hf negative anomalies and low values of (La/Yb)cn and (La/Sm)cn, suggest volcanic arc tectonic setting.