DISCOVERY OF LOWER CRETACEOUS RADIOLARIANS FROM THE SPONTANG OPHIOLITE, LADAKH, NW INDIA: IMPLICATIONS FOR NEO-TETHYAN EVOLUTION

The Spontang Ophiolite is one of numerous fragments of supra-subduction zone (SSZ) ophiolitic rocks and associated arc volcanics that formerly comprised parts of a Late Mesozoic intra-oceanic arc system within Neotethys (Aitchison et al., 2000). As Neotethyan oceanic lithosphere was consumed beneath this arc system it converged upon India and rocks of the SSZ were emplaced southwards as nappes onto the northern margin of Greater India. The Spontang massif covers approximately 300 km² and is located in the Photoksar region south of the Zanskar Valley. It is divided into two main units: a >500m thick succession of arc-related volcanics and volcano-sediments overlying a continuous ophiolite sequence. Pedersen et al. (2001) reported an age of 177±1 Ma (U-Pb zircon) for a diorite intruded into high-level gabbros in the ophiolite and suggested that this represents a minimum age of generation of oceanic material. They also reported an age of 88±5 Ma for an andesitic sample, from the overlying Spong Arc. With the exception of these dates, no other well-constrained radiometric or biostratigraphic data are available from this area. Six red ribbon-bedded chert samples were collected from west of Photoksar on the northern flank of Photang Kangri. We present the first detailed description of these assemblages, which provide new constraints on the timing of development of the Spontang ophiolite and Spong arc. The ~177 - 88 Ma age range indicates that the intra-Tethyan oceanic island arc was long-lived. Given that the first unequivocal indication in the sedimentary record of collision between this arc and the northern margin of India is the abrupt appearance of serpentinite and other ophiolite-derived detrital clasts in the Paleocene Chogdo Formation (Searle et al., 1990) in India and at Sandanlin in southern Tibet (Fig 6(c); Aitchison et al., 2007) suggests that intra-oceanic subduction is likely to have continued until ~55 Ma.