帕米尔东北缘穷阿木太克岩体年代学、地球化学特征及地质意义

分布于帕米尔东北缘羌塘地块上的穷阿木太克岩体,与班公湖怒江洋的闭合有着密切关系。岩体主要由英云闪长岩、花岗闪长岩和二长花岗岩组成。LA-ICP-MS锆石U-Pb测年结果显示,穷阿木太克岩体的形成年龄为(107.0±1.2) Ma(MSWD=0.55),属于早白垩世晚期。地球化学结果显示,主量元素具有富Ca、富K、富碱等特点,岩石Al含量较高,为弱过铝质花岗岩系列,具典型的钙碱性特征。稀土元素具有轻稀土富集、重稀土亏损的右倾型特征,并显示弱的负铕异常。微量元素表现出大离子亲石元素Rb、Th、U、K高度富集和高场强元素Nb、Ta、Ti、Hf强烈亏损。岩石在成因上可能是地壳下部基性岩石经熔融或部分熔融作用形成的。结合本区所处的构造环境,早白垩世晚期穷阿木太克岩体可能形成于羌塘地块与冈底斯地块之间同碰撞(挤压环境)向碰撞后(伸展环境)的转化阶段,为后造山花岗岩类,岩石系列从早到晚由中钾钙碱性系列向钾玄岩系列演化。

Geochronological and Geochemical Characteristics and Tectonic Implications of Qiong' Amutaike Pluton in Northeastern Pamir

Qiong' amutaike pluton located at Qiangtang block in the northeastern Pamir has a close relationship with Bangong Lake-Nujiang suture zone. The granitic rocks are composed mainly of tonalite, granodiorite, and monzogranite. The zircon U-Pb SHRIMP dating of Qiong' amutaike pluton yields the concordant age of (107.0±1.2) Ma (MSWD=0.55), indicating a Late Early Cretaceous. The geochemical analysis shows that the major elements are characterized by high CaO (2.08%-4.21%), K₂O (2.16%-5.13%), alkali (K₂O+Na₂O=5.19%-8.00%), and slightly high Al₂O₃ (13.79%-16.62%), suggesting that the granites belong to the typical calc-alkaline series. The samples are enriched in LREE (light rare earth elements) and depleted of HREE (heavy rare earth elements) with slight Eu anomalies (δEu=0.51-0.76). The chondrite normalized REE pattern curve shows right-sloping type. The trace element studies demonstrate an enrichment of Rb, Th, U, K and depletion of Nb, Ta, Ti, and Hf. The granites might be the product of basic rocks from melting or partial melting of the lower crust. Based on the tectonic environment of the region, we suggest that Qiong' amutaike pluton and evolved into the rich-potassium series from the mid-potassium calc-alkali rock series, generated in a transitional setting of syn-collisional (compressional) and post-collisional (extensional), indicating the collision between Qiangtang massif and Gangdisi massif. These rocks are post-orogenic granites.