Boninites from the Neoarchaean Gadwal Greenstone belt, Eastern Dharwar Craton, India: implications for Archaean subduction processes

Abstract

Boninites are rare in Archaean arcs all over the world whilst there are some reports from the Phanerozoic convergent margins. In this paper, boninites are reported for the first time from an Archaean greenstone belt of India, which have not been reported from any other Indian greenstone belt as yet. The greenstone belts of western Dharwar Craton are characterized by high-Mg basalts and komatiites whereas in the greenstone belts of eastern Dharwar Craton boninites represent the high-Mg lavas. These boninites are characterized by (1) high MgO (12–24 wt.%), (2) enriched in transition elements such as Ni (423–1184 ppm), Cr (943–2765 ppm), (3) low TiO2 (0.22–0.36 wt.%), (4) high Al2O3/TiO2 ratio (29–50), (5) Gd/Ybn less than one, (6) slight enrichment in LREE, depletion in MREE compared to LREE and HREE resulting in U-shaped REE patterns, (7) negative Nb, Ta, Zr, Hf, Ti and V anomalies despite the depletion in MREE. These characteristics differentiate them from deep mantle plume-related magmas. These boninites are compositionally similar to those found in modern and fossil island arcs from intraoceanic subduction settings and provide evidence to propose that two-stage melting of the Archaean mantle wedge generated this type of high-Mg magma. The other high-Mg type magma (komatiitic suite) found in the greenstone belts of western Dharwar Craton has an enriched deep mantle plume source and its dry melting whereas boninites are generated by wet melting of the refractory depleted mantle wedge under high temperature and fluid pressure generated by dehydration of the slab. The geological setting, HFSE and REE data show that possibly the 2.7-Ga peak of geological activity had transitional characteristics between those of Paleoarchaean and Phanerozoic. High thermal and fluid flux is inferred to generate these Mg-enriched and MREE-, HFSE-, LFSE-depleted suite. This discovery of boninites from Gadwal greenstone belt indicate that even in Archaean, subduction of small, young and hot oceanic plates produced magmatic assemblage resembling to those of some of the Cenozoic subduction complexes.