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 (An10-30) and amphibole minerals. Pyroxenites are composed of coarse-grained clinopyroxenes, orthopyroxenes and medium-grained hornblende minerals. Geochemically, in the total alkali vs. SiO2 diagram studied samples are broadly classified as gabbros and their magma type is tholeiitic in nature, however, based on the TiO2, MnO, P2O5 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, Al2O3, 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. Key words: Bhavani, layered complex, Geochemistry, light rare earth elements (LREEs), large ion lithophile elements (LILEs), petrogenesis.
Highlights
The Bhavani ultramafic complex and Sittampundi ultrabasic complex is considered to be age of 2898 ± 50Ma years arrived at by the Sm-Nd systematics (BhaskaraRao et al, 1996)
A group of ultrabasic rocks ranging in composition from dunite, peridotite, garnetiferous gabbro, gabbroic anorthosite to anorthosite are traced along the Bhavani river valley in the Coimbatore and the
It could be inferred that the tectonic environment of eruption is volcanic arc environment and the studied samples fall within the Calc-alkaline basaltic field
Summary
The Bhavani ultramafic complex and Sittampundi ultrabasic complex is considered to be age of 2898 ± 50. The detailed field studies of two hillocks (Odhimalai and Thenkalmalai) lead to the recognition of an array of rock types including metagabbro, meta-anorthositic gabbros, pyroxenites, and peridotite, occurring as layers in close association (Rao et al, 1996). Pyroxenites in thin section are texturally characterized by distinct adcumulate texture They are quite mafic and probably represent pyroxene rich and plagioclase poor portion of cumulate. These pyroxenites are composed of coarse-grained clinopyroxenes, orthopyroxenes and medium grained hornblende crystals. The coarse grains of clinopyroxenes in pyroxenites display exsolution texture defined by very thin orthopyroxenes with parallel extinction They are regularly spaced and uniformly distributed throughout the grain, reflecting a compositional gradient across the pyroxene during exsolution. These regularly spaced exsolution lamellae and uniform distribution support the mechanism of exsolution by a homogeneous nucleation and growth in a slowly cooling magmatic system (Buseck et al, 1980) rather than heterogeneous nucleation and growth in which irregular thicker pyroxene exsolution lamellae are developed
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