Tectonic setting, origin, and obduction history of the spontang ophiolite,Ladakh Himalaya, NW India

The Spontang ophiolite forms the highest tectonic thrust slice above the Me sozoic-Early Tertiary continental margin of the north Indian plate in the L adakh-Zanskar Himalaya. Detailed field mapping, combined with geochemical a nalysis, has defined two major units: a full ophiolite sequence (Spontang o phiolite) overlain by an upper unit consisting of >500-m-thick basalt-andes ite volcanic and volcano-sedimentary rocks of island arc affinity (Spong ar c). The Spontang ophiolite comprises a harzburgitic mantle sequence, gabbro ic and ultramafic cumulates, isotropic gabbros, and highly tectonized sheet ed dikes feeding pillow lavas, with a few uncommon highly fractionated plag iogranites. A separate lherzolitic peridotite unit, affected by possible tr ansform-related shearing, was thrust over the harzburgites in the west, pos sibly during the early stages of subduction initiation beneath the ophiolit e. Whole-rock geochemistry and U-Pb geochronology show that the ophiolite f ormed at a normal mid-ocean ridge spreading center during the mid-Jurassic and that the intraoceanic island arc sequence was erupted on top of the oce anic basement during the Campanian. The Spong arc is interpreted to have fo rmed above a northward-dipping subduction zone that was responsible for the obduction of the Spontang ophiolite during the Late Cretaceous-Early Paleo cene. Although the Spong arc shows many similarities to the andesitic Dras arc within the Indus suture zone, structural, tectonic, and palaeomagnetic constraints indicate that the Spong arc was a separate intraoceanic island arc. This interpretation requires three northward-dipping Tethyan subductio n zones during the Late Cretaceous, beneath the Spong arc, Dras-Kohistan in traoceanic arcs, and the southern margin of Asia, similar to the western Pa cific region today.