Early crystallizing magmatic olivines as probes into mantle sources, magma crystallization and lithospheric thickness: Insights from the 2.37 Ga and 2.21 Ga mafic dyke swarms of the Dharwar Craton, India

This study presents first report on composition of olivines and associated minerals in eleven distinct olivine-bearing mafic dykes of the Dharwar Craton, representing the 2.37 Ga and 2.21 Ga large igneous provinces (LIPs). These dykes comprise of olivine, clinopyroxene (augite and pigeonite), plagioclase (labradorite to bytownite) and minor orthopyroxene (enstatite) in association with oxides (titano-magnetite, ilmenite, spinel) and sulphide (chalcopyrite). The olivine grains vary from being unzoned (Ol-I, Mg# 84–63) to zoned (Ol-II, Mg# 78–38), and represent primary and secondary growth during magmatic crystallization. Continuously zoned grains show decrease in Ni, Cr, Al, Ca contents and an increase in Mn, Co and Ti concentrations, from core to rim. Increase in the Ni/(Mg/Fe)/1000 ratios (0.7 to 3.0) and a decrease in 100Mn/Fe ratios (1.5 to 1.0) in olivines from the 2.37 Ga dykes to the 2.21 Ga dykes may be interpreted as a transition of the subcontinental lithospheric mantle (SCLM) from a peridotite-dominated composition with lower proportions of recycled oceanic crust (X_ROC=0.2–0.3) at 2.37 Ga, to pyroxenite composition having higher proportions of recycled oceanic crust (X_ROC>0.3), at 2.21 Ga. Fractional crystallization modelling suggests that the olivines crystallized first, followed by clinopyroxenes and later by plagioclases within both the dyke swarms however, the two swarms exhibits diverse magmatic processes: 2.37 Ga dykes essentially reflect fractional crystallization and magma mixing processes while, the 2.21 Ga dykes, reflect multiple episodes of magma fractionation and recharge. Calculated NiO/MnO ratios in Fo₈₉ olivines of the dykes show gradual increase from the eastern (1.16–1.36) through central (1.69–1.81), to the western parts (1.88–2.92) of the craton irrespective of their ages, indicating a thicker lithosphere for the Western Dharwar Craton. Al-in-olivine geothermometer registers high olivine crystallization temperatures of 1294–1466°C (±25°), compatible with the olivine crystallization temperatures of several worldwide LIPs (NAIP, Emeishan, Karoo) and suggests involvement of mantle plume. However, olivine compositions along with negative Nb-Ta anomalies, fractionated LREE with flat HREE patterns, variable (Dy/Yb)_N values (0.94–1.69), and largely negative ԐNd(t) values independent of SiO₂, indicate essential role of SCLM in the genesis and suggests generation of the 2.37 Ga and 2.21 Ga mafic dyke swarms through plume-lithosphere interaction.