Petrogenesis and tectonic implication of the Middle Triassic pegmatite from the Mokeri area, eastern segment of the East Kunlun Orogen (China)

Abstract

The newly discovered pegmatite, along with associated syenogranite and gabbroic diorite in the Mokeri area of the East Kunlun Orogen (EKO), were comprehensively investigated utilizing geochronological, whole rock and mineral chemical analysis. Zircon U-Pb dating results indicate that the pegmatite and syenogranite were emplaced almost simultaneously at 236 ± 4 Ma and 236 ± 2 Ma, respectively, while the gabbroic diorite was emplaced at 240 ± 3 Ma. The Mokeri pegmatite and syenogranite are chemically classified as high-K calc-alkaline, weakly peraluminous I-type granites. They exhibit similar geochemical compositions with high SiO2 (73.70 – 78.57 wt%), low MgO (0.04 – 0.11 wt%), Fe2O3T (0.23 – 2.00 wt%) and TiO2 contents (0.02 – 0.20 wt%), as well as low Nb/Ta (3.81 – 12.05) and Zr/Hf ratios (12.64 – 31.04), indicating their common crustal magma source. Geochemical and mineralogical evidences suggest an evolutionary relationship between the Mokeri syenogranite and pegmatite, characterized by higher SiO2 and Na2O contents, as well as elevated Rb/Sr and Rb/Ba ratios, alongside lower Fe2O3T, MgO and TiO2 concentrations in the pegmatite. The pegmatite exhibits a high differentiation index (DI) of 94 (mean value), which signifies a continuous sequence of magma evolution in conjunction with the moderately differentiated syenogranite (DI = 90). Plagioclase components in the pegmatite (An7 to An18) are more evolved than those in syenogranite (An19 to An31). Garnet, derived from two pegmatite dykes, belongs to almandine-spessartine series and exhibits a progressive enrichment in manganese with increasing distance from the parental body—syenogranite. The Mokeri gabbroic diorite exhibits compositional depletion of Nb, Ta, and Ti contents and enrichment of Sr, Ba, and Pb contents, resembling the observed patterns in the island arc basalts (IAB). The elevated ratios of Th/Yb, Ba/Zr and Rb/Y suggest it derivates from an enriched lithospheric mantle modified by subducted slab-fluid. The Mokeri pegmatite-syenogranite-gabbroic diorite association was formed during a transitional phase between syn-collision and post-collision extension (240 – 236 Ma), indicating the termination of collision between the East Kunlun Terrane and Bayan Har Terrane at 236 Ma. These magmatic events likely represent a tectonic-magmatic response to slab break off during the post-collision.