Identification of the Early Jurassic mylonitic granitic pluton and tectonic implications in Namling area, southern Tibet

Abstract A number of studies revealed that the Gangdese magmatic belt of southern Tibet was closely related to northward subduction of the Neo-Tethys oceanic lithosphere and Indo-Asian collision. However, pre-Cretaceous magmatism is still poorly constrained in the Gangdese magmatic belt, southern Tibet. Here, we conducted systematically geochronology and geochemistry studies on a newly-identified granitic pluton in the middle Gangdese magmatic belt (Namling area), southern Tibet. Zircon SHRIMP II U-Pb dating for one representative sample gives a weighted age of 184.2 ± 1.8 Ma (MSWD=1.11), corresponding to emplacement and crystallization age of the granitic pluton in the Early Jurassic (Pliensbachian). High SiO2 (68.9‒72.1 wt.%) contents and intermediate Mg# values (35‒38) together suggest that the newly-identified granitic pluton was probably formed by partial melting of crustal material with minor injection of mantle-derived magma, precluding an origin from melting of meta-sedimentary rocks that are characterized by low Mg# and high zircon δ18O values (>8‰). Geochemically, the newly-identified granitic pluton belongs to typical I-type granitic affinity, whereas this is inconsistent with aluminium saturation index (ASI=A/CNK ratios) and geochemical signatures. This suggests that zircon oxygen isotopes (4.30‰‒5.28‰) and mineral features (lacking Al-rich minerals) are reliable indicators for discriminating granitic origin. Significantly depleted whole-rock Sr-Nd-Hf isotopic compositions and zircon eHf(t) values indicate that the granitic pluton was derived from partial melting of depleted arc-type lavas. In addition, the granitic pluton shows zircon δ18O values ranging from 4.30‰ to 5.28‰ (with a mean value of 4.77‰) that are consistent with mantle-derived zircon values (5.3‰±0.6‰) within the uncertainties, indicating that the granitic pluton might have experienced weak short-living high-temperature hydrous fluid-rock interaction. Combined with the Sr-Nd-Hf-O isotopes and geochemical signatures, we propose that the newly-identified granitic pluton was originated from partial melting of depleted mafic lower crust, and experienced only negligible wall-rock contamination during ascent. Integrated with published data, we also propose that the initial subduction of the Neo-Tethys oceanic lithosphere occurred no later than the Pliensbachian of the Early Jurassic.