Petrogenesis of Permian to Triassic granitoids from the East Kunlun orogenic belt: implications for crustal evolution during oceanic subduction and continental collision

Abstract

ABSTRACT The study of Permian-Triassic granitoids, which widely occur within the East Kunlun Orogenic Belt (EKOB), is relevant for a better understanding of the evolution of the Paleo-Tethys Ocean and regional crustal evolution. This study focuses on the Permian-Triassic granitoids from the Nagengkangqieer region, east segment of EKOB. The Late Permian granodiorite (ca. 252 Ma) shows high-K calc-alkaline and metaluminous features with moderate SiO2 (63.31–67.04 wt.%), relatively high Mg# (48–52), and low Sr/Y ratios. The granodiorite is enriched in large ion lithophile elements (LILEs) and depleted in high field strength elements (HFSEs) and shows enriched isotopic compositions (εNd (t) = −5.53 to −5.14, εHf (t) = −5.6 to −1.1). These geochemical characteristics imply that the Late Permian granodiorite was derived from a mixing source of mafic lower crust and mantle materials (<30%). The Middle Triassic porphyritic monzogranite (ca. 239 Ma) and monzogranite (ca. 239 Ma) are high-K calc-alkaline to shoshonitic and weak peraluminous with high SiO2 (70.58–76.54 wt.%) contents and low Mg# (19–36). They display relatively higher FeOT/MgO ratios, lower P2O5 contents, and more enriched isotopic compositions (εNd (t) = −7.87 to −6.71, εHf (t) = −13.7 to −1.2) than those of granodiorites. These findings indicate that the Triassic granites belong to fractionated granites derived from partial melting of the lower continental crust. In combination with the results of this study and the spatio-temporal distribution of regional magmatic, metamorphic, and sedimentary events, we conclude that Late Permian granodiorite formed in a subduction setting, whereas the Middle Triassic granites were formed in a syn-collision setting. Closing of the Paleo-Tethys Ocean occurred in the Middle Triassic (ca. 240 Ma). Additionally, only a small proportion of juvenile materials (<30%) was involved in magmatism and contributed to continental crustal growth during Late Permian subduction, whereas crustal reworking dominated regional magmatism and resulted in a higher crustal maturity.