Early Paleozoic Multi-stage Granitic Magmatism And The Geological Significance In The South Altyn Tagh HP-UHP Metamorphic Belt

The Altyn Tagh, orogen is located at the northern margin of the Qinghai-Tibet Plateau with the adjacent Qilian orogen to the east and Kunlun orogen to the west. It lies between the Tarim block to the north and the Qaidam block to the south. The studies of orogenesis and timing of the Altyn Tagh orogen are significant in understanding the geological and tectonic evolution, and connections among the adjacent geological units in western China. On the basis of investigations of geology, petrology, geochronology and geochemistry of the granitoids in the South Altyn Tagh region, this thesis established the spatial distribution, magmatic paragenesis and petrogenesis of the early Paleozoic granitoid in the study area. Combined with the latest research results of the regional ophiolite and high pressure-ultra high pressure (HP-UHP) metamorphism, the geodynamic setting of granitic rocks are discussed.This study provided evidence of magmatism in understanding of the early Paleozoic tectonic evolution of the South Altyn Tagh.The main results of this thesis are as follows:(1) In-situ LA-ICPMS zircon U-Pb dating distinguished four episodes of granitic magmatism from the early Paleozoic HP-UHP metamorphic belt southern Altyn Tagh orogen:1) Episode1(517Ma), which is only restricted to the area of the Huangtuquan pluton;2) Episode2(501Ma-496Ma), distributing in a relatively wider area;3) Episode3(460Ma-451Ma), the most widely distributed magmatic activity in the region;4) Episode4(426Ma-385Ma), the distribution of this magmatic activity is limited. The ages Episode1and2magmatisms are consistent with the ages of the peak metamorphism and granulite facies retrogressive metamorphism in the region, respectively. These magmatic activities are responses to the deep subduction and exhumation of the continental crust.(2) geochemical studies show that:the Episode1magmatism produced adakitic rocks with low contents of SiO2, K2O, high contents of Na2O, MgO and high Mg#values, which marked the early Paleozoic subduction of oceanic crust in the South Altyn Tagh region; the Episode2magmatism generated a series of peraluminous K calc-alkaline adakitic rocks with high contents of Al2O3and Sr content, high ratios of (La/Yb)N and Sr/Y and extremely low contents of Yb and Y, which are products of partial melting of the thickened continental crust induced by continental subduction; and the Episode3magmatism mainly formed peraluminous high-K calc-alkaline S-type granites which were sourced from the metasedimentary rocks of the middle-upper crust in a post-collision tectonic setting, demonstrating a switch of tectonic regime from compression to extension in the South Altyn Tagh; the Episode4magmatism is featured by high-K calc-alkaline A-type granites which are products of partial melting of the lower crust in an extensional tectonic setting after continental collision.(3) The comparison of the four episodes of early Paleozoic magmatism between the South Altyn Tagh and the North Qaidam suggest that, the similar tectonic settings happened in the South Altyn Tagh were all approximately50Ma earlier than the North Qaidam. Thus, the South Altyn Tagh and the North Qaidam were not belong to a single tectonic belt. This result has significant implications to understanding of the early Paleozoic tectonic regime.(4) Regional geological and geochronological studies suggest that the subduction of oceanic slab of the South Altyn Tagh should be northward.These observations can applied for a more reasonably interpretation of the geodynamic relationship of the early Paleozoic granite, ophiolite and HP-UHP metamorphic rocks which are spatially closely occurred in the South Altyn Tagh.(5) This research combined with the published geochronological data of ophiolite and HP-UHP metamorphism subdivided the evolution of tectonic settings in the South Altyn Tagh into five stages:Stage1, oceanic extension happened during622Ma～510Ma, which is represented by a suit of deep-water sediments and the Apa-Mangai ophiolite belt; Stage2, subduction of the oceanic crust which happened during≥517Ma～500Ma, the Huangtuquan O-type adakite was formed during this period; Stage3, deep subduction of continental crust during504Ma～487Ma. This stage produced the widespread HP-UHP metamorphic rocks and a small proportion of C-type adakite; Stage4, the broke-off and exhumation of the deeply subducted continental crust during460Ma～451Ma or slightly earlier. The switch of tectonic setting not just resulted in the granulite facies retrogressive metamorphism of the HP-UHP metamorphic rocks, but also induced a regional crustal partial melting, triggered extensive peraluminous S-type granites and rift-ralated mafic-ultramafic rocks (similar to bimodal volcanic rocks), and formed of Changshagou V-Ti-Fe oxide deposits; Stage5, in a post-collisional extensional setting, the progressive exhumation of the subducted continental crust during426Ma～385Ma resulted in the overlapping of amphibolite facies retrogressive metamorphism on the HP-UHP metamorphic rocks and generation of A-type granitic magmatism.