Petrogenesis and tectonic implication of the Permian-Triassic syenogranites from the eastern segment of the East Kunlun Orogen, China

Abstract

Permian-Triassic granitoids are widespread in the East Kunlun Orogen (EKO) and are influential in the re-construction of Paleo-Tethyan tectono-magmatic activities. In this contribution, zircon U–Pb dating and Lu–Hf isotopic as well as whole rock major and trace elements analysis have been conducted on a series of syenogranite plutons from the eastern segment of the EKO to constrain their petrogenesis and tectonic implication. The syenogranites are characterized by dominant mineral assemblages of K-feldspars and minor plagioclase and biotite, in contrast to the coexisting granodiorites that are rich in amphibole and plagioclase. Zircon U–Pb dating reveals that the studied syenogranites represent a two-stage magmatism from Middle Permian to Late Triassic (ca. 265 Ma and 235–232 Ma). Geochemical and mineralogical evidences show that the syenogranites are classified as high-K calc-alkaline, weakly peraluminous, highly fractionated (DI = 93–95) and moderately fractionated (DI = 82–91) I-type granites. They yield negative ɛHf (t) values (−13.25 to −0.11), which, together with their high Th/Nb (0.73–4.88), Th/La (0.29–2.65) and La/Nb (1.07–4.48) ratios and moderate pressure conditions (7–10 kbar) imply magma generation through partial melting of the normal-thickened lower crust. The magma underwent extensive plagioclase and subordinate K-feldspar fractionation during evolution. Given the old two-stage Hf model ages (TDM2 = 1270–2100 Ma), we interpret the ancient lower crust as their dominant source rocks with limited contribution from the juvenile lower crust. Our new data, in tandem with published data show that the Middle-Permian (ca. 265 Ma) Jialuhe syenogranites were formed during a period of local extension concomitant with the northward subduction of the Paleo-Tethyan ocean. In contrast, the Late-Triassic Qingshuihe (235 Ma), Keri (234 Ma), Wutuo (232 Ma) and Balong syenogranite plutons were generated in response to asthenospheric upwelling, lower crust delamination and subsequent continental rifting in a post-collisional extension stage along the convergent plate margins.