帕米尔东缘杜格里富碱斑岩锆石U-Pb定年、地球化学特征及构造意义

帕米尔构造结是陆陆碰撞造山的典型地区之一,位于帕米尔东缘的杜格里富碱斑岩是揭示帕米尔新生代构造演化的重要对象。岩体侵位于下二叠统粉砂质板岩中,主要由正长斑岩和石英正长斑岩组成。通过锆石LA-ICP-MS U-Pb定年获得岩体的成岩年龄为10.76±0.09 Ma,表明杜格里富碱斑岩形成于中新世中期。地球化学分析结果显示,杜格里岩体具富碱、高钾、准铝质—弱过铝质的特征,微量元素高度富集Rb、Sr、Ba等大离子亲石元素(LILE),而亏损Nb、Ta、Ti等高场强元素(HFSE),稀土元素总量较高,球粒陨石标准化配分型式呈右陡倾的LREE富集型,且无明显负Eu异常,反应源区为加厚镁铁质下地壳,并经历过古俯冲洋壳板片流体的交代作用和富集地幔物质的加入。综合分析认为,杜格里富碱斑岩形成于后碰撞阶段伸展构造体制下,是青藏高原西北缘构造转换带对印度-欧亚大陆碰撞造山过程响应的记录。

LA-ICP-MS Zircon U-Pb Dating and Geochemistry of the Dugeli Alkali-rich Porphyries on the Eastern Margin of Pamir and Their Tectonic Implications

The Pamir syntaxis is one of the typical continental collision orogeny. The Dugeli alkali-rich porphyries on the eastern margin of Pamir is of great significance to reveal the Cenozoic tectonic evolution in this area. The rock mass intrudes into silty slate of the Early Permian and is mainly composed of syenite porphyry and quartz syenite porphyry. The LA-ICP-MS zircon U-Pb dating of the Dugeli porphyry yielded an age of 10.76±0.09 Ma, revealing that the rocks formed in the Middle Miocene. Geochemical analysis indicates that the Dugeli porphyry belongs to the metaluminous to weakly peraluminous A-type granite with features of alkali-rich, high-potassium. Trace elements show high enrichment of LILE, such as Rb, Sr, and Ba, and depletion of HFSE, such as Nb, Ta and Ti. The total REE content in the porphyry is high, and the REE chondrite- normalized distribution pattern shows distinct right leaning, light REE enrichment, with no obvious negative Eu anomaly. These features suggest that the magma source is the thickened mafic lower crust, which underwent metasomatism of ancient subducted oceanic slab fluids and was injected by enrichment mantle material. It is concluded that the Dugeli alkali-rich porphyry was formed in extensional tectonic setting during the post-collision stage and records the response to the India-Eurasia collision orogeny in the tectonic transition zone of northwest of the Qinghai-Tibet Plateau.