149 / 2019-05-31 18:53:25

Origin and tectonic implication of Paleozoic granitoids in the eastern segment of the South Altun block, NW China: constraints from zircon U-Pb dating, elemental geochemistry and Lu-Hf isotope

Tectonic setting,South Altun block,Paleozoic granitoids,Lu-Hf isotope

Abstract: The South Altun block (SAB) sited in the northern margin of Qaidam Basin, consisting of Precambrian blocks and two ophiolitic melange belts, which records tectono-thermal events since Proterozoic. Accurate delineation of granitoid magmas has further implication for deducing geotectonic settings, but few research contributions have focused on the granitoids of the eastern segment of SAB. The detailed processes about subduction and collision in this region remain controversial, and the diverse magmatic episodes related to SAB tectonic setting are still not clear. Therefore, this study presents zircon U-Pb dating, elemental geochemistry and Lu-Hf isotope analysis to focus on granitoids from the the eastern segment of South Altun block, with the aim of constraining the petrogenesis and tectonic setting of this block during Paleozoic. The zircon U-Pb study in this paper suggests that Paleozoic magmatic events can be subdivided into four stages: Late Cambrian (~502 Ma), Late Ordovician (~444 Ma), Early Devonian (418 Ma) and Middle Permian (~271 Ma). Petrology and elemental geochemistry imply that three types of granitoids have developed, including granite, diorite and monzonite. Paleozoic peraluminous granitoids in this region were negative Eu and Sr anomalies, mainly concentrating high K alkali series. The majority of granitoid source rocks are dominated by Paleaoproterozoic ancient crustal components, while partly melting from Neoproterozoic juvenile crust. Late Cambrian granitoids were mainly produced from magmas derived from partial melting of a thickened crust. S- and I-type granitoids mark the crustal thickening with collision orogenic events, while A-type granitoids suggest the post-collisional extension tectonic setting. Late Ordovician igneous rocks were probably originated by partial melting of an accreted crust, and A-type granitoids typically associated with magmatism in post-collisional extensional setting. Early Devonian S-type granitoids were probably formed from magmas that derived from melting of the ancient crust of the SAB, and emplaced under the compressional tectonic setting related to the Proto-Tethys ocean subduction. Middle Permian magmatism indicates the partial melting of ancient crust, while S-type granitoids imply the final closure of the Paleo-Tethys ocean.