The Sumdo Eclogite Belt in the Lhasa Block, Tibet: A New Discovery and Its Tectonic Implications

The Sumdo eclogite belt was discovered recently in the eastern part of the Lhasa Block, Tibet. The eclogite belt extends in EW for at least 60 km long with a width of 2-3 km. The eclogites occur as tectonic slices in garnet-bearing, mica-quartz schist. They are generally fresh and form thick, massive layers that consist chiefly of garnet (Grt)+ omphacite (Omp) + phengite (Phe) + rutile (Rut) + quartz (Qtz). P-T calculations based on the Grt-Omp-Phe mineral assemblage yielded peak metamorphic conditions of 2.7 GPa and 730°C, close to the phase boundary between coesite and quartz and thus the eclogites can be regarded as part of a very high pressure metamorphic belt. Petrochemical data suggest that the eclogite protoliths were typical MORB basalts, derived from depleted mantle. SHRIMP U-Pb dating of zircons from the eclogite yielded metamorphic ages ranging from 242±15 Ma to 292±13 Ma, with an average value of 262±5 Ma. The MORB eclogites are interpreted to be remnants of Paleo-Tethyan oceanic lithosphere. The eclogites, along with Permian island arc volcanic rocks to the north, are believed to mark a Carboniferous-Permian suture zone dividing the Lhasa Block into a northern and southern segment. This newly identified suture zone suggests that the border of the Paleo-Tethyan Ocean jumped southward from north of the Bangong-Nujiang suture to within what is now the Lhasa Block