Origin And Evolution Of The Lhasa Terrane, Tibetan Plateau: Constraints From The Petrological And Geochronological Studies Of The High-grade Metamorphic Rocks

The Lhasa terrane, the southern Tibetan Plateau, is located between the Bangong-Nujiang suture and the Indus-Yarlung Tsangpo suture zones and experienced the multi-stages of orogenies related to the Tethys subduction and continent collision. Therefore, the Lhasa terrane is key that reveal the formation and evolution of Tibetan Plateau.A series of high-grade metamorphic rocks outcrop in the central and southern Lhasa terrane. They have a complex rock association, including gneiss, schist, quartzite, amphibolite, marble, granulite and eclogite. Previous studies show that some metamorphic rocks are the Precambrian metamorphic basement of the Lhasa terrane, named as the Nyainqentanglha rock group, Bomi rock group, Nyingchi rock group or Gangdise rock group. In present thesis, petrology, geochemistry and U-Pb zircon geochronology of metamorphic rocks from the southeastern segment of Lhasa terrane are systematically studied. This work shows that the protoliths of the high-grade metamorphic rocks consist of Paleozoic, Mesozoic to Cenozoic sedimentary rocks and intrusion rocks, and experienced the multi stages of tectonic-thermal events during Paleozoic to Cenozoic, instead of the Precambrian metamorphic basement of the Lhasa terrane as considered traditionally.Most meta-sedimentary rocks of the southern Lhasa terrane were formed during Cambrian to Carboniferous, consisting of gneiss, amphibolite, schist, quartzite and marble. The inherited detrital zircons of meta-sedimentary rocks yield high variable U-Pb ages ranging from 1900 to 300 Ma, with some populations at about 1840 Ma,1550 Ma,1140 Ma,980 Ma,580 Ma and 340 Ma. These, together with the regional comparisons, show that the meta-sedimentary rocks have the similar material provenance with the Tethyan Himalayan sequence, recording the tectonic-thermal events related to the Grenville (about 1000 Ma) and Pan-African orogenies (about 550 Ma).The present thesis shows that the oldest metamorphic intrusion rock is the Cambrian granite (496 Ma). Moreover, the Late Devonian (367 Ma) meta-granitoids are common in the southern margin of the Lhasa terrane. The Gangdise batholith formed during Mesozoic and Cenozoic is also the main component of high-grade metamorphic rocks of the southern Lhasa terrane.The southern Lhasa terrane understood the multi-stages of metamorphism, including the Late Paleozoic to Early Mesozoic (260 to 210 Ma) high-pressure (HP) eclogite-facies metamorphism, the Late Mesozoic (98 to 83 Ma) medium-pressure (MP) granulite-facies to amphibolite-facies metamorphism as well as the Cenozoic (55 to 49 Ma and 33 to 27 Ma) two stages of MP amphibolite-facies metamorphism. The Permian to Triassic HP metamorphic belt occurs in the central segment of the Lhasa terrane and is characterized by the Sumdo eclogite. These rocks, together with their country rocks, underwent the retrograde metamorphism under amphibolite-facies condition during 220 to 210 Ma. The Late Cretaceous MP granulite-facies to amphibolite-facies metamorphic belt occurs in Mainling area. The granulite-facies peak-metamorphic rocks consist of two pyroxene granulite and garnet-bearing two pyroxene granulite, with mineral paragenesis of orthopyroxene + clinopyroxene + plagioclase + quartz+Ti-rich amphibole ± garnet. The country rocks of granulite include gneiss, amphibolite and marble, with mineral paragneisses of garnet +oligoclase+muscovite+quartz ± biotite± amphibole ± epidote, garnet+oligoclase+muscovite+biotite +hornblende+quartz and calcite+quartz, indicating an amphibolite-facies metamorphism formed at 98 to 83 Ma. The temperature and pressure conditions of peak-metamorphism of granulite-facies are 0.9 to 1.3 GPa and 830 to 900 ℃, respectively. The Eocene (55 to 49 Ma) MP amphibolite-facies metamorphic belt occurs in Bayi area, consists of gneiss, schist, quartzite and leptynite, with the mineral paragenesis of garnet+andesine+biotite+quartz ± muscovite, andesine+biotite+quartz ± amphibole ± muscovite ± garnet, garnet+amphibole+andesine+biotite+quartz, garnet+anorthite+diopside+quartz, with metamorphic conditions of 0.4 to 0.55 GPa and 625 to 679 ℃. The Oligocene amphibolite-facies metamorphic belt located at Bujiu area, consists of gneiss, schist and quartzite, with the mineral paragenesis of andesine+biotite+quartz ± garnet ± sillimanite ± muscovite, garnet+andesine+biotite+ muscovite+quartz ± sillimanite ± kyanite, garnet+plagioclase+sillimanite+biotite+muscovite+quartz. This belt was metamorphosed at 33 to 27 Ma, has temperature and pressure conditions of 0.5 to 0.80 GPa and 615 to 663 ℃. Therefore, there are a Triassic metamorphic belt and a Mesozoic to Cenozoic compound metamorphic belt in the central and southern segments of the Lhasa terrane, respectively.Based on research results of the sedimentation, magmatism and metamorphism of the high-grade metamorphic rocks from the southern Lhasa terrane, a tectonic-evolution model of South Lhasa terrane is reconstructed. Firstly, at Early Paleozoic, the South Lhasa terrane, together with the Tethyan Himalaya sequence, were formed as the sedimentary cover of northern margin of Gondwana supercontinent, and sequentially experienced an Andean-type orogeny caused by the subduction of oceanic crust beneath the Gondwana supercontinent and was intruded by the Cambrian granite. Secondly, at Middle Paleozoic, the South Lhasa terrane was separated from the northern margin of Australia Gondwana and underwent Late Devonian magmatism during the process of continuous subduction of ocean basin. Thirdly, from Neopaleozoic to early Mesozoic, along with subduction and close of oceanic crust, the South Lhasa terrane has collided with the North Lhasa terrane, and resulted in the Triassic orogeny in the central segment of Lhasa terrane. Fourthly, at Late Mesozoic, the northward subduction of Neo-Tethyan oceanic crust beneath Lhasa terrane has derived the strong Andean-type orogeny, resulting in the Gangdise magmatism and coeval high-temperature (HT) granulite-facies metamorphism. The HT metamorphic belt and the coeval HP metamorphic belt formed at the subduction oceanic crust comprise a paired metamorphic belt during the convergent orogeny between ocean and continent. Fifthly, at early Cenozoic, the collision-subduction between the India continent and Lhasa terrane has generated the wide syn-collision magmatism and MP amphibolite-facies metamorphism. Finally, at Late Paleogene, the continuous subduction of the Indian continent beneath the Lhasa terrane has resulted in the HP granulite-facies metamorphism of the deeply subducted High Himalayan complex and the MP amphibolite-facies metamorphism of the hanging wall of subduction zone. The two metamorphic belts comprise a paired metamorphic belt during continent-continent orogeny.The high-grade metamorphic rocks located on the southern Lhasa terrane almost comprise all rock types of the South Lhasa terrane, not only record multi-stages of metamorphism and magmatism formed during convergent orogenies from Mesozoic ocean-continent to Cenozoic continent-continent, but also reveal the multi-stages of pre-Mesozoic tectonic-thermal events experienced by the Lhasa terrane. Therefore, the present studies provide new sights into the formation and evolution of Tibetan Plateau during Phanerozoic, and key constraints on the compound orogenic dynamics.