Chromitites from an Archean layered intrusion in the Western Dharwar Craton, southern India

Abstract

Chromite deposits hosted in a layered ultramafic–mafic intrusion in the Archean Chithradurga greenstone belt are part of a suture that divides the Western and Central Dharwar craton blocks in southern India. Serpentinised ultramafic rocks including dunite, peridotite, and phlogopite-rich clinopyroxenite occur with massive chromitite. The olivine compositions show two distinct groups, one with Fo ~91–92 olivines in the massive chromitite, and another group with olivine Fo values of ~95–96, in the serpentinised dunite. The chromite occurs as massive chromitite and as an accessory phase in dunite. The former is characterized by high Cr# (Cr/(Cr + Al) × 100 = 67–70) and moderate to high Mg# (Mg/(Mg + Fe) = 52–66); The latter shows a larger variation in Cr which is likely related to sub-solidus re-equilibration of the chromite during the serpentinization event. In the massive chromitites, sub-solidus re-equilibration of chromite is less significant or absent. Bulk-rock major and trace element analyses of the peridotites reveal a composition less magnesian than depleted mantle rocks, with low TiO2, Na2O, CaO and high Cr, Ni content, a general depletion in Sr and enrichment in Th, U. The chromitites contain variable PGE abundances (ΣPGE = 8-244 ppb) with enrichments of Os, Ir and Ru, relative to Pt and Pd . These features suggest that the chromitites are cumulates formed from a highly magnesian parental magma with komatiitic affinity, derived by large degrees of partial melting, , which underwent crustal contamination. The ReOs isotope data demonstrate a minimum Re-depletion model age (TRD) of 2895 ± 24 Ma for the serpentinized dunite (MY11/3), and a more robust age of 3120 ± 12 Ma for two chromitite samples, which we interpret as the best estimate of the magmatic age of this ultramafic intrusion. Abundant zircon grains also occur within these ultramafic rocks with UPb ages spanning from ca. 2.9 Ga to ca. 2.4 Ga, with a dominant age population in the range 2.8–2.6 Ga. We interpret the upper intercept age at ca. 2.9 Ga to represent the initial crystallization of zircon in the ultramafic rocks, potentially as a result of an early stage of metamorphism and hydrothermal alteration. The 2.5–2.4 Ga zircon ages reflect a late-stage high-grade metamorphic overprint.