Petrogenesis and Geodynamic Implications of Late Triassic Mogetong Adakitic Pluton in East Kunlun Orogen, Northern Tibet: Constraints from Zircon U-Pb-Hf Isotopes and Whole-Rock Geochemistry

Adakites or adakitic rocks usually show special geochemical signatures and are petrological probes to reveal the tectono-magmatic evolutionary history of paleo-orogenic belts. Here, we present a comprehensive study on the zircon U-Pb geochronology, whole-rock geochemistry, and zircon Lu-Hf isotopes of Mogetong adakitic pluton in East Kunlun orogen, Northern Tibetan Plateau, to constrain its petrogenesis and tectonic setting, and thus to reveal its implications for the Paleo-Tethyan orogeny. The studied pluton comprises of quartz monzonite porphyry with zircon U-Pb crystallization age of ca. 215 Ma, which is coeval to their diorite enclaves (ca. 212 Ma). The quartz monzonite porphyries have intermediate SiO2 (63.31-65.74 wt%), relatively high Al2O3 (15.52-16.02 wt%), K2O (2.83-3.34 wt%), and Sr (462-729 ppm), but low Y (9.14-15.7 ppm) and Yb (0.73-1.39 ppm) with high Mg-# (47-55), Sr/Y (30-57) and La/Yb ratios resembling typical high-K calc-alkaline and high Mg-# adakitic rocks. Zircon Lu-Hf isotopes show that the studied samples have weakly juvenile zircon Lu-Hf isotopes (epsilon Hf(t) = 1.80-4.03) with older model age (1.00-1.14 Ga). The relative low content of Cr (14-59 ppm) and Ni (8-30 ppm), as well as the petrological, geochemical, and Lu-Hf isotopic data, indicates that the Mogetong adakitic rocks were generated by partial melting of thickened lower crust with a certain contribution of the underplated mantle-derived magma in slab break-off setting. This study shows that the Late Triassic adakitic magmatism in East Kunlun orogen may be the response of tectonic transition from oceanic subduction to post-subduction extension, and the reworking of ancient continental crust with subsequent variable crust-mantle magma mixing is the major mechanism of continental crust evolution in the Paleo-Tethyan orogenic belt.