藏北阿索地区早白垩世基性岩脉地球化学及年代学特征:对班公湖－怒江洋闭合时限的约束

班公湖－怒江洋闭合时限是中特提斯洋演化争论的焦点之一，而基性岩脉对于识别构造环境转变具有重要意义.本文对阿索地区侵入构造混杂岩带中的基性岩脉进行了1件LA-ICP-MS锆石U-Pb年龄测定和10件全岩地球化学元素分析.辉长岩脉获得了102.90±0.86 Ma（MSWD=1.4）的谐和年龄，全岩地球化学分析显示基性岩脉属于亚碱性系列，钙碱性辉长闪长岩类，样品轻微富集轻稀土元素，富集大离子亲石元素Th、U、Pb等，亏损高场强元素Nb、Ta等.基性岩脉来源于被俯冲沉积物和板片流体改造过的亏损地幔源区，由尖晶石二辉橄榄岩高程度部分熔融形成，未经历明显的地壳混染过程.结合构造环境判别图解和前人研究结果，认为阿索地区的基性岩脉可能形成于早白垩世晚期伸展环境之中.系统收集研究区及附近120~95 Ma之间的岩浆岩数据，发现在110~100 Ma之间班公湖－怒江缝合带及两侧处于伸展环境之中，但不同区域伸展事件的时代有所不同，原因可能与班公湖－怒江洋的不均一闭合相关.班公湖－怒江洋最后的闭合时限可能在103 Ma左右. 

Geochemistry and Chronological Characteristics of the Early Cretaceous Mafic Dikes in the Asa Area,North Tibet: Constraints on the Closure Time of the Bangong-Nujiang Ocean

The closure time of the Bangong-Nujiang Ocean is one of the focal points of the Meso-Tethys Ocean evolution debate, and the mafic dikes play an important role in the identification of tectonic setting transformation. Here we report zircon LA-ICP-MS U-Pb ages and whole-rock geochemical data from the mafic dikes invaded in the tectonic mélange. Zircon U-Pb dating using LA-ICP-MS techniques yields the concordant age with a weighted mean 206Pb/238U age of 102.90±0.86 Ma (MSWD=1.4) for the gabbro dike. The whole-rock geochemical characteristics from the mafic dikes suggest that these rocks are calc-alkaline series. The dikes are enriched in large ion lithophile element (LILE) and show negative Nb and Ta anomalies with slightly enrichment of light rare earth element (LREE). The geochemical compositions suggest that the dikes are originated from a depleted mantle source metasomatized by slab-derived components and formed by the high degree of partial melting of spinel peridotite with the lack of mixing of crustal materials. Combined with the tectonic environment discriminant diagram and previous research results, we believe that the dikes formed in the late Early Cretaceous extension environment. We found that the Bangong-Nujiang suture zone and the two sides are in an extension environment between 110-100 Ma by collecting data on magmatic rocks (120-95 Ma) near the study area. The times of regional extension differ from area to area, which may be related to the diachronism of closure time of Bangong-Nujiang Ocean. The final closure time of Bangong-Nujiang Ocean may be around 103 Ma.