中天山库米什一带早古生代花岗岩的锆石U-Pb年代学、岩石地球化学及其地质意义

Zircon U-Pb Geochronology, Lithogeochemistry and Geological Implications of the Early Paleozoic Granites at Kumishi, Central Tianshan Terrane, Xinjiang

新疆中天山库米什一带广泛发育一套片理化花岗岩-花岗片麻岩组合,目前学术界对这套岩石的地球化学特征、构造背景以及成矿潜力仍不清楚。为此,对这套花岗岩开展了锆石U-Pb年代学、主微量元素地球化学以及Sr-Nd同位素研究。结果显示,这套花岗岩主要形成于中晚奥陶世(460~455 Ma),显示富硅、富铝、富碱、低镁、低磷、贫钛的特征,在成因上属于I型花岗岩。具有富集端元的Sr-Nd同位素特征,TDM年龄为1.6 Ga,暗示岩浆源区可能为中元古代的古老地壳物质,且形成于低温、中等压力(0.8~1.3 GPa)环境。此外,还显示相对陡峭的右倾稀土元素配分模式,伴有中等程度的负铕异常,富集大离子亲石元素,亏损Nb、Ta、Ti、P等高场强元素,具有弧岩浆的地球化学特征,暗示其可能形成于俯冲环境。通过对比可知,中晚奥陶世花岗岩在形成时代、野外产状、化学组成、岩石成因以及构造背景等方面,均显著不同于库米什地区二叠纪的成钨花岗岩,因而推测其成矿潜力可能相对较小。

A set of granite-granitic gneiss assemblages is widely developed in the Kumishi region of the central Tianshan terrane, Xinjiang. The geochemical characteristics, tectonic setting and metallogenic potential of this assemblage are still unclear. Therefore, zircon U-Pb geochronology, major and trace element geochemistry and Sr-Nd isotope study is carried out on this granite-granitic gneiss assemblage. The results show that the granites were mainly formed in the Late Ordovician(460~455 Ma), and they are enriched in silicon, aluminum, and alkali, but depleted in magnesium, phosphorus, and titanium, with presence of magnetite and sphene, and belong to type I granites in genesis. These granites display enriched Sr-Nd isotope characteristics with TDMage of 1.6 Ga, which indicates that the magma may be sourced from Mesoproterozoic old crustal material. In addition, the parental magma was likely formed at low temperature and moderate pressure(0.8~1.3 GPa) conditions. They show a relatively steep right-leaning REE distribution pattern with moderate negative Eu anomalies, enrichment of large ion lithophile elements and depletion of Nb, Ta, Ti, P and other high field strength elements, indicating that they may be formed in a subduction environment. In comparison with previous studies, the late Ordovician granites are significantly different from the Permian tungsten-productive granites in the Kumishi area in terms of formation age, field occurrence, chemical composition, petrogenesis and tectonic setting. Therefore, it is proposed that the mineralization potential of the granite is relatively poor.