Petrogenesis and Geological Implications of the Middle Triassic Garnet-Bearing Two-Mica Granite from Jialuhe Region, East Kunlun

Late Paleozoic to Early Mesozoic I-type granodiorites enriched in mafic microgranular enclaves are widespread in Eastern Kunlun orogen, which therefore has been considered as a natural laboratory for studying magma mixing between mantle- and crust-derived melts. In contrast, coeval peraluminum to strong peraluminous granitoids are rarely reported in this region. Recently, we identified a set of garnet-bearing two-mica granites in the Jialuhe region at eastern part of the East Kunlun Orogen. Hereby we present a synthetical mineralogy, petrology, zircon U-Pb geochronology, and lithogeochemistry and Lu-Hf isotope study on it, aiming to constrain its petrogenesis and tectonic implications. The rock outcropped as multiple sub-parallel dykes in the field, intruding into granodiorite. It mainly consists of quartz, plagioclase, k-feldspar, biotite and muscovite, with minor garnet as accessory minerals. LA-ICP-MS zircon U-Pb isotopic dating yields an age of 242.0±1.4 Ma, indicating that the garnet-bearing two-mica granites were formed in the Middle Triassic. Chemical analyses show that the rocks have characteristics of high SiO₂(74.49%~75.24%), K₂O(4.21%~4.33%), but low P₂O₅(0.02%~0.03%) contents, belonging to peraluminous to strong peraluminous series (A/CNK=1.08~1.12), showing a high degree of differentiation. They are relatively enriched in elements such as U, Th, Pb, Rb, but depleted in Nb, Ta and other related elements. Note that, P and Ti are strongly depleted, which may indicate early crystallization of apatite and titanomagnetite. The rocks also have a low total amounts of rare earth elements (61.55×10^-6~119.05×10^-6), and show a weak negative Eu anomaly (δEu=0.51~0.65). Their zircon ε_Hf(t) values ranged from -8.19 to -2.78 (mean of -5.61), with a two-stage model ages (T_DM2) of 1.3 Ga to 1.6 Ga Ma and whole-rock Nb/Ta ratios are 8.22~9.67 (close to continental crust ratio of 10.91). These features are similar to those of nearly coeval biotite monzogranite in the region, indicating that the rocks may be originated from remelting of lower crust. The biotite-rich enclaves are widely developed in the rocks, suggesting that the wall rock materials are captured during magma ascent, with obvious assimilation and contamination. Thus, we propose that the garnet-bearing two-mica granitic dykes belong to high-fractionated and peraluminous to strongly peraluminous I-type granites series, which are products of felsic magma produced by partial melting of lower crust undergoing long-term fractionation and being subjected to surrounding rocks assimilation during its ascent.