东昆仑西段白日其利早泥盆世A2型花岗岩的成因及其地质意义

The genesis and geological significance of Early Devonian A2-typegranite in Bairiqili area, western East Kunlun Mountains

查明白日其利地区花岗岩成岩时代、地球化学特征及构造背景,对探讨东昆仑古特提斯洋构造演化具有重要意义。笔者等对钾长花岗岩开展了岩石学、锆石U-Pb年代学、Lu—Hf同位素及岩石地球化学研究。钾长花岗岩年龄为408.9±0.9 Ma,表明其形成于早泥盆世。岩石地球化学主量元素显示富硅、富钾和低镁的特征,相对亏损P、Nb、Ti、Zr等高场强元素,富集Rb、K、Ba等大离子亲石元素,属于过铝质—准铝质岩石。微量元素特征显示强烈的负Eu异常(δEu=0.05~0.06),稀土配分模式图显示海鸥型模式,具有A型花岗岩的特征。εHf(t)值介于-5.52~3.53之间,均值为-0.75,总体显示源岩以壳源物质为主。构造图解指示该岩体为A2型花岗岩,形成于碰撞后伸展背景。结合区域岩浆岩证据,推断东昆仑造山带在晚志留世应为碰撞向伸展转换阶段,早泥盆世已经完全进入伸展阶段。

This research paper investigates the diagenetic age,geochemical characteristics,and tectonic setting of granite in the Bairiqili area,contributing to our understanding of the Paleo-Tethys Ocean’s tectonic evolution in East Kunlun.We conducted a comprehensive study of the petrology,zircon U-Pb dating,Lu—Hf isotopic analysis,and whole-rock geochemistry of moyite samples.Our zircon U-Pb dating indicates the granite formed during the Early Devonian,specifically at 408.9±0.9 Ma.The granite exhibits high levels of SiO2 and K2O but low MgO,enriched in large ion lithophile elements such as Rb,K,and Ba,and depleted in high field strength elements including P,Nb,Ti,and Zr,classifying it as a peraluminous to quasi-aluminous rock.The trace elements reveal a marked negative Eu anomaly,while the rare earth element distribution presents a distinct‘seagull’pattern,characteristic of A-type granites.The calculatedεHf(t)values range from-5.52 to 3.53,suggesting a predominantly crustal origin for the source materials.According to the tectonic discrimination diagrams,the studied granite is identified as A2-type,which typically forms in post-collisional extensional environments.When correlating these findings with regional magmatic activity,we infer that the East Kunlun orogenic belt was likely in a collision—extension phase during the late Silurian and had transitioned fully into an extensional regime by the Early Devonian.