METAMORPHOSED TETHYAN SEQUENCE ROCKS AND STRUCTURE OF THE XIAO GURLA RANGE, INDUS-YARLUNG SUTURE ZONE SOUTHWEST TIBET

Xiao Gurla Range is located within the Indus-Yarlung suture zone between Gangdese batholith rocks of the Lhasa terrane to the north and the south Tibetan detachment system of the high Himalaya to the south near the southeastern end of the right-lateral strike-slip Karakoram fault. Metamorphic rocks of the Range are composed largely of metasandstone, phyllite, schist (± garnet, ± sillimanite), calc-gneiss and marble (± pyroxene), paragneiss (± pyroxene), quartzite, metagranite, leucogranite (± garnet). These units are exposed structurally beneath Neogene–Quarternary basin fill and serpentinized ultramafic ophiolitic rocks which can be correlated to the Kiogar-Jungbwa ophiolite. Measurements of ductile and brittle kinematic indicators suggest top-to-the-west sense of motion along the fault similar to the Gurla Mandhata Fault System ~60 km to the southwest. Foliation measurements and shallow dipping topography on the western side of the ranges indicate this fault is at a low angle. We suggest that this fault is a continuation of the Gurla Mandhata Fault System and that slip along this fault has been fed into the Karakoram fault during arc-parallel extension. U-Pb geochronology was conducted on five detrital zircons samples of schist, phyllite, and quartzite. These samples yield maximum depositional ages which range from 644–363 Ma and lack significant populations of Early–Middle Proterozoic age zircon, indicative of Lesser Himalayan sequence rocks, but have significant populations of ages at ~975–900 and 550–500 Ma and have age probability distributions that more similar to Tethyan sequence rocks than Greater Himalayan sequence rocks. Leucogranite bodies intruding the metasedimentary footwall rocks yield two distinct U-Pb zircon ages at ~ 23 Ma and ~15 Ma. Together these observations suggest the Tethyan Sequence rocks may be more extensively metamorphosed and intruded by early–middle Miocene leucogranites than previously thought.