东昆仑祁漫塔格花岗片麻岩记录的岩浆和变质事件

东昆仑青海祁漫塔格尕林格一带原定为金水口群的眼球状花岗片麻岩,实际为新元古代早期形成的花岗岩.采用SHRIMP和LA-MC-ICP-MS两种方法对其中的锆石进行了测试,获得的年龄分别为938±5Ma和938±2Ma,代表了花岗岩的形成时代.花岗岩地球化学特征显示为S-型,属于钙碱性系列的弱过铝-过铝质花岗岩,εNd(0)为-9.4～-11.7,εNd(t=938Ma)为-0.6～-3.2,显示低的负值,tDM为1.6 ～2.1Ga,推测其源岩与白沙河岩组类似.东昆仑东段、柴北缘以及阿尔金均有1000～900Ma的花岗岩形成,表明这次岩浆活动比较广泛,可能与我国西部不同陆块间的汇聚有关,是我国西部新元古代克拉通基底形成的反映,同时也响应于全球Rodinia超大陆的形成.花岗片麻岩中1粒锆石边部获得了416±11Ma的年龄,与区域上志留-泥盆纪花岗岩形成时代一致,代表了新元古代花岗岩发生变质作用的时代,其中白云母40Ar/39Ar的坪年龄和等时线年龄为406±2Ma,代表了变质花岗岩的冷却年龄,这些年龄表明新元古代花岗岩卷入了古生代中期与祁漫塔格洋/海盆关闭有关的造山事件.

Magmatic and metamorphic events recorded in granitic gneisses from the Qimantag, East Kunlun Mountains, Northwest China

SHRIMP and LA-ICP-MS U-Pb analyses of zirons from granitic gneisses from Jinshuikou Group in the East Kunlun Mountains Qimantag belt indicate magmatic crystallization at 938±5Ma and 938±2Ma. Major elements, trace elements, and Sm-Nd isotopic data also indicate that all these Early-Neoproterozoic granites are characterized by the features of crust-derived and typical continent-continent syn-collisional granites. We infer that source of the granites similar to Baishahe Formation of the Proterozoic Jinshuikou Group. (Meta-) granitoids of similar ages, 1000~900Ma, occurred in further eastern segment of East Kunlun, the North Qaidam terrane and the Altyn Tagh Fault, suggesting this magmatic activity is widespread, that might be related to the assemble of different blocks, marking basement formation of Neoproterozoic craton in the Northwest China; this event might be the response of the formation of Rodinia Supercontinent. Zircon rims in one of these samples yield younger ages (416±11Ma) compatible with zircon from Silurian-Divonian granites. ⁴⁰Ar/³⁹Ar dating of muscovite from granitic gneisses is 406±2Ma. These ages revealed distinct metamorphic and cooling events of Early-Neoproterozoic granites during Silurian-Divonian. This tectono-thermal event is related to the closure of the Qimantag oceanic basin or marginal sea basin.