PGE Geochemistry of Komatiites from Neoarchean Sigegudda Greenstone Terrane, Western Dharwar Craton, India

Abstract

The Neoarchaean Sigegudda greenstone terrane is a small (ca. 30 km 2 ) greenstone belt, situated to the north of the Holenarsipur belt in the western Dharwar craton. Two compositionally distinct units of ultramafic rocks are recognized in the Sigegudda greenstone belt and have been interpreted as metamorphosed komatiites and komatiitic basalts. These komatiites have been analyzed for Platinum Group Elements (PGE) concentrations which provide significant petrogentic implications on the mantle melting conditions and sulphide saturation history of the parent magma. The PGE abundances of Sigegudda komatiites are characterized by 0.2-3.6 ppb Os, 0.02-1.2 ppb Ir, 0.4-1 ppb Ru among IPGE; and 0.4-4.8 ppb Pd, 0.4-7.6 ppb Pt, 0.2-1.2 ppb Rh among PPGE. These komatiites have lower concentrations of SPGE in the range of 1.02-15ppb. The chondrite normalized PGE patterns show variable PGE fractionation. The overall PPGE and IPGE variations are marked by both lower ΣPPGE/ΣIPGE ( 1); Pd/Pt ratios lower (0.3-0.5) and higher (0.6-2.0) than the primitive mantle average of 0.55; Pd/Ir > Pt/Pd and Pt/Pd > Pd/Ir. Pd/Ir vs. ΣPGE and Pd vs. Cu variations suggest a sulphur saturated to undersaturated character of the parent magma which indicates variable degrees of mantle melting. PGE distributions of Sigegudda komatiites are consistent with olivine and sulphide fractionation trends of parent magma. Relative enrichment of PPGE over IPGE and pronounced enhancements of Pd relative to Pt in some komatiite samples reflect a multi-stage petrogenetic process marked by melt percolation and contributions from sub-continental lithospheric mantle (SCLM).