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 Mesozoic-Early Tertiary continental margin of the north Indian plate in the Ladakh-Zanskar Himalaya. Detailed field mapping, combined with geochemical analysis, has defined two major units: a full ophiolite sequence (Spontang ophiolite) overlain by an upper unit consisting of >500-m-thick basalt-andesite volcanic and volcano-sedimentary rocks of island arc affinity (Spong arc). The Spontang ophiolite comprises a harzburgitic mantle sequence, gabbroic and ultramafic cumulates, isotropic gabbros, and highly tectonized sheeted dikes feeding pillow lavas, with a few uncommon highly fractionated plagiogranites. A separate lherzolitic peridotite unit, affected by possible transform-related shearing, was thrust over the harzburgites in the west, possibly during the early stages of subduction initiation beneath the ophiolite. Whole-rock geochemistry and U-Pb geochronology show that the ophiolite formed 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 oceanic basement during the Campanian. The Spong arc is interpreted to have formed above a northward-dipping subduction zone that was responsible for the obduction of the Spontang ophiolite during the Late Cretaceous-Early Paleocene. 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 subduction zones during the Late Cretaceous, beneath the Spong arc, Dras-Kohistan intraoceanic arcs, and the southern margin of Asia, similar to the western Pacific region today.