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

• 作者: Yuanku Meng Walter D.Mooney Runlong Fan Jinqing Liu Youqing Wei
• 来源期刊: 地学前缘（英文版）
• 年，卷（期）: 2021,（1）
• 所属期刊栏目: Research Paper
• 页码范围: 13-28
• 页数: 16页
• 语种: 英文

摘要

A number of studies revealed that the Gangdese magmatic belt of southern Tibet was closely related to the 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 sys-tematically 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 of184.2±1.8 Ma(MSWD = 1.11), corresponding to emplacement and crystallization age of the granitic pluton in the Early Jurassic(Pliensbachian).High SiC2(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 εHf(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％o to 5.28‰(with a mean value of 4.77‰)that are consistent with mantle-derived zircon values(5.3‰±0.6％o)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.