Geochemistry of Khandadharpahar-Thakuranipahar metabasites from the western Singhbhum Craton, eastern India: implications for subduction-zone tectonics and mantle-wedge metasomatism
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The identification of new rock types in the volcano-sedimentary sequences of the Singhbhum Craton has attracted much attention in recent years. The present study deals on newly identified Nb-Enriched Basalts (NEB) from the Khandadharpahar-Kadakala-Thakuranipahar (KKT) section, western Singhbhum Craton, which is comparable in composition to basalts-basaltic andesites and calk-alkaline in character. These metabasites have a porphyritic texture with phenocrysts of pyroxene and plagioclase, as well as a groundmass that has metamorphosed to the greenschist facies. High Nb contents (7.5-22.8ppm) combined with high (Nb/Th)PM (0.28-0.59), (Nb/La)PM (0.40-0.69) and Nb/U (11.7-34.4) ratios, compared to arc basalts ((Nb/Th)PM= 0.10-1.19; (Nb/La)PMn 0.17-0.99, Nb/U<10), characterized them as NEB. Negative Nb, Zr, Hf and Ti anomalies, and Nb/Th vs La/Nb and Th/Nb vs. La/Sm relationships, collectively indicate typical arc volcanics. The available geochemical parameters suggest a genesis of KKT metabasites through i) slab melt migration from the downgoing oceanic crust, ii) low-degree melting of the garnet-bearing peridotite in the mantle wedge metasomatized by the slab melts, iii) slab melt - peridotite interaction triggering increasing Nb concentrations and iv) NEB generation in an arc-related environment. The discovery of KKT NEB sheds new information on Paleoproterozoic subduction-zone processes and crustal growth in the Singhbhum craton.
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