Petrogenesis and geochronological constraints on the timing of Carbonatite-syenite magmatism at the Archean-Proterozoic boundary in the Western Dharwar Craton, southern India.

Abstract

An integrated study involving field, geochronology, whole rock geochemistry and multi-isotope (Sr-Nd-Pb-Os-C-O) systematics for a newly reported carbonatite-alkaline syenite intrusive system is presented from the Gundlupete area, Western Dharwar Craton, India. The Gundlupete carbonatite (GC) is a coarse-grained calcio-carbonatite, predominantly constituting calcite, apatite, magnetite, monazite and other silicate phases. Field evidence shows its intrusive relationship with spatially associated syenite, occurring close to an E-W to ENE-WSW trending shear zone at the purported tectonic contact between the Western Dharwar Craton and the Southern Granulite Terrain, southern India. The carbonatite shows elevated LILE-LREEs content with (La/Yb)CN between 103-202 but is significantly impoverished in HFSEs. The syenite shows an ultrapotassic to shoshonitic composition (K2O/Na2O: 0.90-2.42) and a broadly similar geochemical pattern. Our in-situ U-Pb titanite and monazite dating yielded crystallisation ages of 2590 ± 42 Ma and 2470 ± 27 Ma for the syenite and carbonatite respectively, implying two discrete magmatic events. The carbonatite shows a narrow Sr [87Sr/86Sri: 0.70307-0.70321], Nd [εNdi: -3.7 to -2.1], Pb [206Pb/204Pb: 13.42-14.04, 207Pb/204Pb:14.69-14.78,208Pb/204Pb:26.60-35.20] isotopic range, whereas the syenite, although shows restricted Nd [εNdi: -1.4 to 1] isotope range but large spread in Sr (87Sr/86Sri: 0.66834-0.70210), and Pb [206Pb/204Pb: 6.60-24.38, 207Pb/204Pb:13.47-16.34, 208Pb/204Pb:13.56-164.37] isotopes limits its further usage. C-O isotopic compositions of GC exhibit -10.2 to -9.4‰ and 7.7-10.3‰, respectively, and cluster slightly below the ‘PIC’ field. The present study suggests that the GC evolved from primary carbonatite melt through processes like magmatic differentiation/fractional crystallisation and assimilation en route to the surface without involving liquid immiscibility and/or fractional crystallisation from a common parental carbonated silicate magma. The combined initial Sr-Nd-Pb isotope systematics suggest that the carbonatite magma tapped a mantle source with marginally higher time-integrated Rb/Sr, Nd/Sm, and U-Th/Pb ratios with respect to Bulk Earth and CHUR. We propose that immediately after the terminal Neoarchean collision between the Dharwar Craton and the Southern Granulite Terrain, the carbonatites and syenite were emplaced utilising similar conduits during the transition from a collisional to a post-collisional extensional regime across the Archaean-Proterozoic boundaryKeywords: Carbonatite, syenite, Archean, Proterozoic, Dharwar Craton, Southern Granulite Terrain