Cenozoic Multistage Adakitic Magmatism And Tectonic Significance Of The Southeastern Segment Of The Gangdese Belt,Tibetan Plateau

The Gangdese magmatic belt, in the north side of Indian-Eurasia orogen, consists of a large number of Mesozoic to Cenozoic magmatic rocks, which are formed during the Andean-type orogeny derived from the Neo-Tethyan subduction and the Himalayan orogeny resulted from the Indian-Eurasia collision. These magmatic rocks are mainly calc-alkaline, showing typical characteristics of island arc magmatic rocks, but Late Cretaceous (90～80Ma) and Miocene (26～10Ma), magmatic rocks display the adakitic geochemical characteristics. The Late Cretaceous adakite was formed during the flattening of the Neo-Tethyan subduction or Neo-Tethyan mid-ocean ridge subduction; whereas the Miocene adakite was developed under the post-collision extensional environment.This study reveals that the Gangdese belt include four stages of adakitic granitoids during the Cenozoic time, with Zircon U-Pb ages of62Ma,53Ma,38Ma and29Ma, respectively. The Early Paleocene adakite has been metamorphosed into the gneiss, consisting of plagioclase, quartz, hornblende, biotite and garnet. The zircon magmatic cores yielded a crystalline age of62Ma, the zircon metamorphic rims yielded the metamorphic age of29Ma. The Early Eocene adakite shows heterogeneity deformation, and composed of plagioclase, feldspar, quartz and biotite. The Early Eocene adakitic granite veins mainly composed of plagioclase, K-feldspar, quartz, biotite, with minor epidote and muscovite. The Late Eocene adakite contains abundant mafic enclaves, consist of plagioclase, K-feldspar, quartz, biotite and hornblende. The Oligocene adakite is mainly composed of plagioclase, K-feldspar, quartz, biotite, hornblende, epidote.The studied adakitic granites have Sr contents of383～1214ppm, Yb of0.3～1.17ppm and Y contents of3.73～14.4ppm, with Sr/Y of52.3～163and (La/Yb)N of18.4～136, showing typical adakitic geochemical characteristics. In addition, the granites have high SiO2(63.28%～72.64%), low MgO (0.72%to2.73%), Mg#(39～54), Cr (6.29～29.9ppm) and Ni (3.1～19.6ppm), indicating that these rocks originated from the partial melting of thickened lower crust. Most of the granites show depleted and large range of zircon Hf isotopic composition, reflecting juvenile crust source. The Early Eocene adakite has relatively low εHf (t) values (-5.3to1.2), as well as the old TDMC model ages (1.1to1.5Ga), showing addition of old terrigenous material.The presence of multistage adakitic granites shows the southeastern segament of the Gangdese belt experiencing a continuous magmatic activity in the Paleocene-late Oligocene. Early Paleocene (62Ma) adakitic magmatism, as well as contemporaneous anatexis and metamorphism, indicate the southern Lhasa terrane crust had thickened to>40km in the early Paleocene, followed by further crust thickening response to India-Eurasia collision process as the (La/Yb) N of the adakitic granites is obvious higher since early Eocene.