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

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 (similar to 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 (similar to 1-6 kbar/ similar to 1120-1210 degrees 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 degrees C during dike emplacement. The primary magmas of all basalts were last equilibrated with spinel lherzolite at 10-16.5 kbar/1291-1366 degrees C, and they resemble pooled polybaric incremental melts generated along a similar to 1450 degrees C adiabat. The estimated mantle potential temperatures (1293-1515 degrees C) are similar to Paleoproterozoic ambient mantle temperatures. All basalts and their primary magmas show lower chondrite-normalized Dy-N/Yb-N 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 Dy-N/Yb-N ratios, and a spinel-bearing mantle source are at odds with an origin of the basalts from mantle plumes.

Automated summary

Reverse fractionation modeling is used to estimate the primary magma compositions, degree of crustal contamination, pressure-temperature equilibrium with mantle, and potential temperatures for the basaltic dikes and sills/volcanics in southern India. The results suggest that the basalts are contaminated and have enriched light rare earth element patterns due to upper crustal assimilation. The estimated mantle potential temperatures are similar to Paleoproterozoic ambient mantle temperatures. The low Dy-N/Yb-N ratios and a spinel-bearing mantle source contradict an origin of the basalts from mantle plumes.