The Early Mesozoic Magmatism,Metamorphism And Tectonic Implication In Southeast Tibet(Changdu-Chayu Area)

As the main dominant of the Tibetan orogen,the southeast Tibet(Chang–Chayu area)plays an important role in the tectonic evolution of the Tibet.With many suture zones and many terranes in a small area,the southeast Tibet attracts rare attention to the geological study,especially the early Mesozoic magmatism,metamorphism and the tectonic implication.In this paper,we present new petrological,geochemical,and geochronological data for the metasediments and granitoids from the southeast Tibet,which place new constrains on the petrogenesis,tectonic framework and evolution history during early Mesozoic.The southeast Tibet witnessed three stages of granitic magmatism during early Mesozoic: early Triassic(248-243 Ma),late Triassic(213-208 Ma)and early Jurassic(ca.187 Ma).The early Triassic granitoids,including granite,granodiorite and diorite,are collected in the Leiwuqi area of the North Qiangtang terrane.They have zircon εHf(t)values from-3.4 to +5.3 with TDM2 model ages of 0.94-1.50 Ga,indicating a Mesoproterozoic crustal source.Based on the whole-rock geochemical data,they are high-potassium and calc-alkaline to alkaline rocks,including slightly peraluminous I-type granites and strongly peraluminous S-type granites,which were related to the northward subduction of the Longmu Tso-Shuanghu-Dingqing Paleo-Tethys Ocean(LPTO).The late Triassic granitoids,consisting of granite and granodiorite,are collected in the Leiwuqi area of the North Qiangtang terrane and the Chayu area of the South Lhasa terrane.With negative zircon εHf(t)values of-5.5 ~-16.2 and TDM2 model ages of 1.43-1.99 Ga,they have a Paleo-Mesoproterozoic crustal source.They are peraluminous and high-K calc-alkaline to alkali-calcic rocks and are considered as S-type granites.The granitoids from Leiwuqi area formed in the tectonic setting of the collision between the North Qiangtang terrane and Tongka block resulted from the closure of the LPTO,and the granitoids from Chayu area formed in the tectonic setting of the collision between the North Lhasa and South Lhasa terranes resulted from the closure of the Songduo Paleo-Tethys Ocean(SPTO).The early Jurassic granitoids are composed of granite and granodiorite and collected in the Tongka area of the Tongka and Jiayuqiao blocks.With negative zircon εHf(t)values of-5.7 ~-16.7 and TDM2 model ages of 1.42-2.02 Ga,they have a Paleo-Mesoproterozoic crustal source.They are peraluminous and high-K calci to calc-alkaline rocks and included both I-and S-type granites,which were probably generated in an arc-related setting during the northward subduction of the Bangong-Nujiang Tethys Ocean(BNTO).The southeast Tibet witnessed two stages of early Mesozoic metamorphism: late Triassic(209–198 Ma)and early Jurassic(178–176 Ma).The late Triassic metasediments from South Lhasa terrane include gneisses collecting in the Bomi area and schists collecting from Chayu area,whereas the schist from North Lhasa terrane are collected in Chayu area.The metapelitic rocks from South Lhasa terrane experienced similar peak-stage MP high amphibolite-facies metamorphism and partial melting with geothermal gradient of 25–45°C/km,the clockwise P–T paths are defined that evolve from the peak conditions along a decompression cooling path.These indicate that the studied rocks resulted from the crustal thickening related to the collision between North and South Lhasa terranes,following with the close-up of the SPTO.The early Jurassic schists collected from Tongka block underwent at least three stages of metamorphism with a peak metamorphism of HP granulite-facies under P–T conditions of 10.4–12.3 kbar and 770–810 °C,and records a clockwise P–T path with a prograde process of both temperature and pressure increase and subsequent retrogression of isothermal decompression followed by isobaric cooling.The geothermal gradient is < 17°C/km.These indicates Tongka block northward subducted beneath the North Qiangtang terrane and exhumated during early Jurassic.Therefore,we suggest a tectonic evolution model that attributes the early Mesozoic tectonic-magmatic-metamorphic event to the southeast Tibet.The studied area is divided into five terranes,from north to south,they are North Qiangtang terrane,Tongka block,Jiayuqiao block,North Lhasa terrane and South Lhasa terrane,which are separated by the Dingqing,Tongka,Basu and Bomi ophiolitic zones.In the early Triassic,the LPTO between the North Qiangtang terrane and the Tongka block northward subducted beneath the North Qiangtang terrane,whereas the SPTO between the North Lhasa terrane and the South Lhasa terrane northward subducted beneath the North Lhasa terrane.Then the Tongka block northward collided with the North Qiangtang terrane and the South Lhasa terrane northward collided with the North Lhasa terrane following with the close-up of the LPTO and SPTO respectivly during late Triassic.The Tongka block northward subducted beneath the North Qiangtang terrane and exhumated during the early Jurassic,then the Jiayuqiao block collided with the Tongka block and the BNTO northward subducted beneath the Tongka and Jiayuqiao blocks.