Re-Os isotope constraints on the offshore extent of supra subduction zone along the NW continental margin of India

The north-western continental margin of India (NWCMI) is juxtaposed with onshore supra-subduction zone (SSZ) type ophiolitic complexes (e.g. the Muslim Bagh and Bela) linked with protracted convergence between Indian and Eurasian plates. Prior studies envisaged that these N-S trending SSZ complexes, likely extend further south into the Arabian Sea. However, paucity of offshore basement sampling has impeded in-depth investigations into such propositions. In view of the recent findings from the scientific ocean drilling and coring in the Laxmi Basin (LB) of the northern Arabian Sea, here we assess offshore continuation of the onshore SSZ ophiolite complexes. We employ new geochemical and first report of Re-Os isotopic signatures of the LB basement from Site U1457 of the IODP Expedition 355. We report that the geochemical characteristics of the LB volcanics show unambiguous signatures of SSZ crust. These include low TiO₂ (0.63–1 ppm), low Zr (<50 ppm), low Ti/V ratios (10−20) and uniform patterns of progressive depletion in high field strength elements, with typical negative anomalies of Nb and Zr, coupled with progressive enrichments in large ion lithophile elements (K, Rb, Sr and Ba). Using robust classification diagrams, the LB volcanics are categorised as forearc basalts (FAB) and boninitic FAB (B-FAB). The novel Re-Os isotope data from the Site U1457 show Re concentrations varying between 1.37 and 1.97 (ppb) and very low Os concentrations (0.06–0.18 ppb). The positive linear trend between suprachondritic ¹⁸⁷Os/¹⁸⁸Os ratios (0.1683–0.2484) with subduction derived fluid elements implies modification of original Os during subduction initiation process. Further, comparisons of ophiolites from NWCMI and Laxmi Basin allow us to infer that the onshore SSZ ophiolite belt continues into the Arabian Sea, at least till the LB. Our new findings would have significant implications towards paleo plate reconstructions of the final Gondwana dispersal.