东昆仑骆驼峰地区中晚三叠世花岗岩年代学、地球化学及构造意义

骆驼峰地区位于东昆仑造山带昆北弧后裂陷带,其内发育大量记录古特提斯洋演化的岩浆岩。本文对该地区花岗闪长岩和钾长花岗岩进行了年代学和地球化学研究,用以限定古特提斯洋的构造演化。LA-ICP-MS锆石U-Pb测年结果表明花岗闪长岩形成于中三叠世(233Ma),钾长花岗岩形成于晚三叠世(218Ma)。花岗闪长岩和钾长花岗岩分属于中钾钙碱性-高钾钙碱性系列和钾玄岩系列,A/CNK值为0.92-1.02,具有偏铝质花岗岩类属性。两类岩石均具有负铕异常、稀土总量低、轻稀土明显分馏和铕弱异常的特点。花岗闪长岩富含大离子亲石元素、亏损高场强元素,具有火山弧花岗岩特征,为古特提斯洋俯冲消减作用的产物。钾长花岗岩具有相对较高Nb、Ta和Rb含量,显示出同碰撞花岗岩的特征,暗示古特提斯洋在早三叠已经闭合进入陆陆碰撞阶段。两类岩石记录了古特提斯洋俯冲到消亡的构造体制的转换。

Geochronology, Geochemistry and Tectonic Significance of Middle and Late Triassic Granites in the Luotuofeng Area, East Kunlun

The Luotuofeng area is located in the North arc back fracture zone of the eastern Kunlun orogenic belt, which contains a large number of magmatic rocks that record the evolution of the Paleo-Tethys Ocean. Geochronology and geochemistry of two types of granites (granodiorite and moyite) in this area are studied in order to define the tectonic evolution of the Paleo-Tethys Ocean. LA-ICP-MS zircon U-Pb dating shows that granodiorites (233Ma) were formed in the middle Triassic, and the moyite (218Ma) were formed in the late Triassic.The two type of rocks are high potassium calc-alkaline series, with a A/CNK value of 1.21 and 1.95, belonging to the meta aluminous granitoids. All of them are characterized by low total amount of rare earth elements, obvious fractionation of light rare earth and weak negative Eu anomaly. Granodiorite is rich in large ion lithophile elements and depleted high field strength elements. It has the characteristics of volcanic arc granite and is the product of subduction of the Paleo-Tethyan Ocean. Moyite have relatively high Nb, Ta and Rb contents, showing the characteristics of syn-collisional granites, suggesting that the Paleo-Tethyan Ocean closed into the land-land collision stage in the early Triassic. The two types of rocks record the transformation of the tectonic system from subduction to extinction of the Paleo-Tethyan Ocean.