Closure Of A Paleo-Tethys Back-arc Basin In The Lhasa Terrane

The Sumdo high-pressure metamorphic belt in the eastern Lhasa Terrane of the Tibetan Plateau is regarded as a Permian-Triassic tectonic boundary between the central and southern Lhasa blocks.The Sumdo eclogites have chemical signatures of mid-ocean basalts,experienced the peak eclogite facies metamorphism at 262–227 Ma and amphibolite facies retrograde metamorphism at 230–200 Ma.As country rocks of the Sumdo eclogites,the Leilongku and Mabuku Formations of the Sumdo Group mainly consists of quartz schists and quartzites.Their formation ages are still not clear.Here we carried out field mapping,microstructral analysis and geochronology to explore formation mechanisms and exhumation history of the Sumdo metamorphic belt.The results will provide new insights into tectonic evolution of the Lhasa Terrane and the Paleo-Tethys Ocean.We found a nearly east-west striking,steeply dipping ductile shear zone along the Niyang River,which is?1 km thick and extends at least 10 km with nearly east-west trending subhorizontal stretching lineation.However,in the core of the Niyang River shear zone,an early steeply dipping lineration is preserved.The Niyang River shear zone is composed of strongly deformed eclogite boudins and sheath folds,quartzites,quartz schists and migmatitic gneisses.Quartz fabrics of foliated quartz schists and quartzites were analyzed using electron backscatter diffraction technique.Most samples show the low-temperature basal<a>slip,whereas samples from the core of the Niyang River shear zone develop the high-temperature prism<c>slip.This indicates that the Niyang River shear zone was formed under high temperature?>650o?and overprinted by low temperature??350o?defomation.Using the Ti-in-muscovite thermomter,metamorphic temperatures of quartz schists and quartzites are between 481–704°C,which is consistent with deformation temperature from quartz farbics.The outcrop observations and asymmetric quartz fabrics confirm that the earlier top-to-the-south thrusting was followed by the strike slip with a top-to-the-north thrust component under transpression.The ⁴⁰Ar/³⁹Ar cooling ages of muscovite from deformed quartzites yield 226.86±2.15 Ma,coeval with amphibolite facies retrograde metamorphism of eclogites.Hence exhumation of the Sumdo eclogites was controlled by activation of the Niyang River shear zone.According to petrology and distribution of eclogites,three E-W striking eclogite-bearing Niyang River,Xindaduo and Bailang ductile shear zones are recongizned in the Sumdo metamorphic belt.U-Pb ages of detrital zircons from quartzites and quartz schists in the Leilongku and Mabuku Formations were determined by LA-ICP-MS analysis.The results reveal sedimentation in a back-arc basin and a similar age probability with Paleozoic metasedimentary rocks from the Lhasa Terrane.The youngest detrital zircon U-Pb ages from different samples vary from 536 to 321 Ma,yielding the possible maximum depositional ages in the Late Carboniferous.Combined with plate reconstruction models,we propose that subduction of the Paleo-Tethys Ocean beneath the Lhasa Terrane resulted in opening of the Sumdo Sea back-arc basin,which began to subduct at least in the Late Permian and formed eclogites.Due to oblique convergence,eclogites and meta-supracrustal rocks were exhumed rapidly in inclined transpression zones during 230–220 Ma and formed the Sumdo high-pressure metamorphic belt.