Origin of the Paleoproterozoic basaltic dikes from the central and eastern Dharwar Craton and sills and volcanics from the adjoining Cuddapah Basin, southern India

Abstract

Reverse fractionation modeling considering energy-constrained assimilation-fractional crystallization is performed to estimate primary magma compositions, degree of crustal contamination, pressure–temperature of equilibrium with mantle, and potential temperatures for the origin of the Paleoproterozoic (~ 2.37–1.88 Ga) basaltic dikes in central and eastern Dharwar Craton and sills and volcanics in the adjoining Cuddapah Basin, southern India. Mineral thermobarometry indicates that the dikes crystallized at upper crustal conditions (~ 1–6 kbar/ ~ 1120–1210 °C). Hence, the reverse fractionation calculations are performed at low pressures by adding olivine + plagioclase + clinopyroxene, olivine + plagioclase and only olivine in equilibrium with melt, and simultaneously subtracting an upper crustal partial melt in small steps until the melt is multiply saturated with lherzolite at a high pressure. The results indicate that the basalts are 5–30% contaminated, and their enriched light rare earth element (REE) patterns can be attributed to upper crustal assimilation. The upper crust was pre-heated to 665–808 °C during dike emplacement. The primary magmas of all basalts were last equilibrated with spinel lherzolite at 10–16.5 kbar/1291–1366 °C, and they resemble pooled polybaric incremental melts generated along a ~ 1450 °C adiabat. The estimated mantle potential temperatures (1293–1515 °C) are similar to Paleoproterozoic ambient mantle temperatures. All basalts and their primary magmas show lower chondrite-normalized DyN/YbN ratios than the plume-derived mid-Proterozoic Mackenzie dikes of Canadian Shield, and the primary magmas show flat REE patterns indicating spinel lherzolite melting. The low estimated potential temperatures, low DyN/YbN ratios, and a spinel-bearing mantle source are at odds with an origin of the basalts from mantle plumes.