Episodes 2022; 45(1): 5-27
Published online March 1, 2022
Copyright © International Union of Geological Sciences.
by Salim Akhtar, Ashima Saikia*, Priyanka Negi, and Bikas Jyoti Kalita
Department of Geology, University of Delhi, Delhi -110007, India
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Petrological, mineral chemical and geochemical characteristics are reported for the basalts of South Andaman ophiolite suite (India) of Cretaceous age to infer its petrogenesis, magma source characteristics and tectonic setting. In the field, both massive basalts and basalts with wellpreserved pillow morphology are present. The basalts consist of plagioclase, clinopyroxene, Fe-Ti oxide and Cr-spinel. They are tholeiitic in composition with low concentrations of Na2O + K2O (3 - 5 wt.%). The representative samples of these basalts have a nearly flat chondrite-normalized REE pattern [(La/Yb)N = 0.29 - 2.27)] with minor Eu anomaly (Eu/Eu* = 0.90 - 1). Our findings suggest that the arc-like massive basalts were generated before the back-arc rifting initiated in an earlier stage of back-arc extension as a result of eastward subduction of the Neotethyan oceanic slab. The MORB-like pillow basalts were erupted due to further back-arc spreading in response to the roll-back of the Neotethyan oceanic lithospheric slab. The pillow and massive basalts assemblage in the South Andaman ophiolite records the evolution of a back-arc basin from initial rifting to subsequent spreading, documenting how basaltic magmatism evolves during the formation of the back-arc basin in a suprasubduction zone setting.