西秦岭西段共和盆地周缘西功卡花岗闪长岩体锆石UPb年龄、地球化学特征及其构造意义
Zircon UPb Ages, Geochemical Characteristics and Tectonic Implications of Xigongka Granodiorite Around Gonghe Basin, West of Western Qinling Mountains

摘要： 西功卡花岗闪长岩体位于西秦岭西段的共和盆地周缘。对该岩体进行了系统的岩相学、锆石UPb年代学和岩石地球化学研究。锆石LAICPMS UPb年龄为(2272±73) Ma(MSWD=20)，代表岩体侵位时间。西功卡岩体具有高硅(SiO2=6986%~7094%)、富钾(K2O= 320%~408%)、富碱(K2O+Na2O=561%~703%)、弱过铝质(A/CNK=099~120)特征，属于高钾钙碱性Ⅰ型花岗岩。西功卡岩体富集K、Rb、Ba、Th、U等大离子亲石元素，亏损Nb、Ta、Ti、P等高场强元素，轻重稀土元素分馏较明显(LREE/HREE=1104~1313)，具有中等Eu负异常(δEu=072~ 084)。岩石地球化学特征表明，西功卡花岗闪长岩主要由壳源物质熔融形成，结合前人研究成果，笔者认为，西功卡岩体形成于晚三叠世，与区域其他岩体年龄基本一致，均形成于后碰撞构造环境，为同一构造-岩浆事件的产物。研究表明，西秦岭造山带西段共和盆地周缘于晚三叠世进入后碰撞构造演化阶段，该认识对西秦岭造山带构造-岩浆事件演化过程提供了新的约束。
Abstract: Xigongka granodiorite is located in Gonghe Basin, west of the Western Qinling mountains Orogenic Belt, and the main composition of rock is gray, finegrained, block granodiorite. In this paper，we carried out the petrography, zircon UPb geochronology and geochemistry of Xigongka granodiorite. Zircon LAICPMS UPb dating yielded a weighted mean age of (2272±73) Ma, which is interpreted as the emplacement age of the Xikongka intrusion.The Xigongka intrusion is weakly peraluminous and highK calcalkaline with A/CNK values of 099-120 and high contents of SiO2(6986%-7094%)，K2O(320%-408%)，K2O+Na2O(561%-703%), and belongs to Itype granite. It is characterized by enrichment in LILE(such as K, Rb, Ba, Th, U), and by depletion in HFSE(such as Nb, Ta, Ti, P) with high LREE/HREE ratios of 1104-1313 and moderate negative Eu anomalies (δEu=072-084). The geochemical characteristics of the Xigongka granodiorite suggested that it was partial melting of crustal components. In combination with previous studies, the Xigongka intrusion was formed in the Late Triassic，which is consistent with the ages of other intrusions in the region. Therefore, they were all formed in the same tectonic environment of postcollision. This finding indicated that the west part of Western Qinling Orogen turned into the tectonic environment of postcollision in the Late Triassic and it also provided new constraints on the tectonomagmatic evolution of Gonghe Basin in Western Qinling Orogen