Tectonic Setting, Origin, and Obduction History of the Spontang Ophiolite, Ladakh Himalaya, NW India

Abstract

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.