西昆仑苏纳克早古生代花岗岩锆石U-Pb年龄、地球化学特征及其地质意义

苏纳克花岗岩位于西昆仑造山带库尔良裂谷带内,岩体岩性为黑云母二长花岗岩,其中侵入有花岗闪长岩脉。LA-ICP-MS锆石U-Pb定年结果显示,黑云母二长花岗岩和花岗闪长岩脉的结晶年龄分别为(477.0±2.1)Ma和(466.7±2.0)Ma,为早—中奥陶世岩浆活动的产物。苏纳克黑云母二长花岗岩主量元素组成具有高硅(SiO₂含量(质量分数,下同)为72.75%～76.44%)、富碱(Na₂O+K₂O值为8.58%～9.12%)的特征,A/CNK值为1.02～1.11,为弱过铝质、高钾钙碱性系列。球粒陨石标准化稀土元素配分模式表现为平缓的右倾型特征,并呈“V”字型,且具有显著的负Eu异常。微量元素组成明显富集Rb、K等大离子亲石元素及Th、U等高场强元素,强烈亏损Ba、Sr、Eu、P、Ti等。整体上,苏纳克黑云母二长花岗岩具有高分异I型花岗岩的地球化学特征,是西昆仑早古生代原特提斯洋俯冲过程中的产物。

Zircon U-Pb Dating and Geochemical Characteristics of Early Paleozoic Granite in the Southern Sunake of West Kunlun,Northwest China and Their Geological Significances

Sunake granite, located in Kuerliang rift zone of West Kunlun orogen, is composed of biotite monzogranite, in which granodiorite dikes are intruded. The LA-ICP-MS dating yields zircon U-Pb ages of(477.0±2.1)Ma and(466.7±2.0)Ma from the biotite monzogranite and granodiorite, respectively, which are the products of tectonic magmatism during Early-Middle Ordovician. The major elements of Sunake biotite monzogranite are characterized by high silica(the contents of SiO₂ are 72.75%-76.44%)and alkali(the contents of Na₂O+K₂O are 8.58%-9.12%)with the A/CNK of 1.02-1.11, showing that they are weakly peraluminous and high-K calc-alkaline series. The chondrite-normalized REE pattern presents a gentle right-leaning and “V” shape with significant negative Eu anomalies. The trace elements are enriched in Rb, K, Th and U, and evidently depleted in Ba, Sr, Eu, P and Ti. Overall, Sunake biotite monzogranite has the geochemical characteristics of high fractionated I-type granite, which is the product of Proto-Tethys Ocean subduction in West Kunlun during Early Paleozoic.