Raichur schist belt, eastern Dharwar Craton, India: A geochemical analogue of western Aleutian arc

Abstract

Oblique convergence of the Pacific plate beneath the North American plate produced the western Aleutian arc. The western Aleutian basalts, and to a large extent, the Mg-andesites concur with partial melting of a mantle source infused with subducted slab-derived melts. By analogy, MORB-like depleted rare earth element patterns (La_N/Yb_N ≤ 1) and arc-like incompatible trace element enrichments relative to high field strength elements Ba/Nb > 25 in the Raichur basalts are consistent with their genesis in a setting, similar to the modern-day intraoceanic arcs. Geochemical attributes of the Aleutian arc lavas preclude melting of pelagic sediment during the course of subduction. By analogy, the radiogenic initial εNd_{(t =2.7 Ga)} = +2.05 to +3.15 isotopic compositions in the Raichur basalt-andesite suite preclude crustal contamination. The rare earth element patterns and Ba-La-Yb systematics between the Aleutian arc lavas and the Raichur belt are identical, and concur with the absence of subducted sediment in their mantle source. A hypothetical slab-melt + mantle wedge interaction model, discounting sediment melt, can explain the genesis of Raichur Mg-andesites, which is identical to that proposed for the western Aleutians. Apparently, the geochemical process of arc magma genesis recorded in the Neoarchean greenstone belts of the Dharwar Craton resembles Phanerozoic-style magmatism in the Cenozoic oceanic arcs. Consequently, the mechanism of continental crust formation has neither changed significantly over time nor across the Earth.