A Cretaceous forearc ophiolite in the Shyok suture zone, Ladakh, NW India: Implications for the tectonic evolution of the Northwest Himalaya

The northwestern Himalaya in India contains critical evidence of the convergent margin and collision tectonics between the Ladakh arc and the Karakoram block. Here we present new petrochemical and geochronological data from a forearc ophiolite at the Shyok-Nubra river confluence. Whole-rock geochemical data show relatively high large-ion lithophile elements (ULE), light rare earth elements (LREE), and Ba, U and Pb anomalies, and depletions in La, Ce and Zr, particularly, the high-field strength elements (e.g. Nb, Ta): these geochemical characteristics are similar to those in modern ophiolites that formed in arc-related environments. Meta-volcanic greenschists that contain spinel layers have high MgO. Ni. Co. and Cr contents. In contrast. they have low contents of TiO2, very low Nb and Zr that are diagnostic of high-Ca boninitic magmas in modem forearc settings, as in Izu-Bonin-Mariana (IBM) and Tonga. The spinels have high Cr# [Cr/ (Cr + Al)] and Mg#[Mg/ (Mg + Fe2+)], which are characteristic of spinels in forearc boninite-type of melts. The spinel-free meta-volcanic samples have incompatible trace element abundance patterns similar to those of MORB. However, their enrichments in Cs, Rb, Pb, U and depletion in HFSE may reflect an input of subduction fluids that are different from MORB; these MORB-like basalts are suggestive of a forearc complex that erupted prior to the formation of boninitic rocks. Our data from the Shyok ophiolite indicate the existence of supra-subduction rocks on the southern Karakoram margin. Albite porphyroblasts in greenschists yield a K-Ar age of 104.4 +/- 5.6 Ma that represents the time of early exhumation of the greenschists. The boninite-type melts formed prior to 104 Ma, thereby indicating that the time of initiation of subduction on the southern margin of the Karakoram block was before 104 Ma. These geochemical signatures together with the spatial and temporal distribution of the arc rocks on the southern margin of Karakoram block suggest northward subduction of NeoTethys in the Early Cretaceous. The subsequent collision between the Ladakh arc and the Karakoram block thrusted/obducted the forearc ophiolite onto the southern edge of the Karakoram block probably between 74 Ma and 97 Ma. (C) 2012 Elsevier B.V. All rights reserved.