Early Mesozoic Metamorphism Of The Eastern Segment Of The Lhasa Terrane In South Tibet And Its Tectonic Significance

The Lhasa terrane in the southern Tibetan Plateau is located between the Bangong-Nujiang suture and the Indus-Yarlung Tsangpo suture zones,which has experienced the multi-stages of orogenesis related to the Tethys subduction and continent-continent collision.Therefore,the Lhasa terrane is critical to reveal the formation and evolution of the Tibetan Plateau.A series of high-grade metamorphic rocks outcrop in the eastern Lhasa terrane.They have a complex rock association,including gneiss,schist,quartzite,marble and amphibolite.The Dongjiu and Basongco metamorphic complexes,located in the east end of the Lhasa terrane,are included in the Paleozoic sedimentary.They are mainly composed of schist,paragneiss,orthogneiss and a few marble.This dissertation investigates in detail the petrology,geochemistry,U-Pb zircon geochronology and phase equilibria modelling of these two metamorphic complexes.The results suggest that the protoliths of these high-grade metamorphic rocks are Ordovician sedimentary,and then experience metamorphism during the early Mesozoic.On the basis of petrography,mineral chemistry and phase equilibria modelling,this study suggests that the Dongjiu metamorphic complex has experienced a kyanite-grade metamorphic evolution,which is characterized by a decompressional vector with slight cooling from a peak of 4.9kbar and 665°C to medium-pressure amphibolite-facies metamorphic overprinting at 5~6kbar and 600~630?C.LA-ICP-MS in situ U-Pb analyses on metamorphic zircons yielded concordant 206Pb/238 U ages of 194~182Ma,suggesting that the Dongjiu metamorphic rocks were formed during the early Jurassic.The metamorphic complex in the Basongco area has experienced medium-pressure amphibolite-facies metamorphism under the conditions of T=670°C and P=5.2kbar.This P-T condition is located in the apparent warm thermal gradients of 20-40°C/km,which is typical of Barrovian-type regional metamorphism.The protolith of this metamorphic complex is a series of sedimentary,including pelite,greywacke,limestone and volcaniclastic rocks.The inherited detrital zircon of the metasedimentary rocks yielded widely variable206Pb/238 U ages ranging from 2520 Ma to 511 Ma,with three main age populations at 1499 Ma 1161Ma and557 Ma.It indicates the sources of the metasedimentary has experienced multi-stage magmatism and deposit less than 511 Ma.The metamorphic zircons from all rocks yielded the consistent metamorphic ages of 201~190Ma,which reflects that the high-grade metamorphic rocks from the Basongco complex were formed at early Jurassic.The Basongco-Dongjiu metamorphic belt,together with the western and coeval Sumdo and Nyainqentanglha metamorphic belts,constitutes a nearly continuous suture zone with an E-W extension of at least 500 km between the northern and southern Lhasa terranes.The metamorphic ages of these belts,ranging from 230 Ma to 182 Ma,show a younger trend from west to east,indicating that the central segment of the Lhasa terrane experienced an eastward asynchronous collisional orogeny during the late Triassic to early Jurassic.Therefore,the present studies provide new sights into the formation and evolution of Tibetan Plateau from Late Paleozoic to early Mesozoic,and key constraints on the compound orogenic dynamics.