晋蒙地区西施沟复式岩体锆石U-Pb年龄、地球化学特征及岩石成因

Zircon U-Pb Ages,Geochemical Characteristics and Petrogenesis of the Xishigou Complex Pluton across the Border Between Shanxi and Inner Mongolia

本文对晋蒙地区的西施沟复式岩体进行了LA-ICP-MS锆石U-Pb定年、岩石地球化学特征研究,探讨了其岩石成因及其对华北克拉通北缘中生代构造岩浆演化的启示。该岩体的岩石类型从中性岩至酸性岩连续变化且以中性岩为主,内部广泛发育微粒闪长质包体,具明显的岩浆混合成因;锆石U-Pb定年结果显示其形成年龄为160.19±0.50Ma,为中晚侏罗世岩浆活动的产物;岩石地球化学数据表明,属于准铝(弱过铝)质的高钾钙碱性系列I型花岗岩;稀土总量(ΣREE)中等,轻重稀土分馏明显,无明显负铕异常;均富集轻稀土及大离子亲石元素(Ba、Rb、K),亏损高场强元素(Nb、Ta、P),具明显的陆壳成分特征;其形成可能与中晚侏罗世古太平洋板块的俯冲有关,为华北板块陆内造山作用的产物。

In this work,LA-ICP-MS zircon U-Pb dating and geochemical characteristics of the Xishigou complex pluton across the border between Shanxi and Inner Mongolia were studied. Its petrogenesis and implications for Mesozoic tectonomagmatic evolution in the northern margin of North China Craton were discussed. The rock types vary from intermediate to acidic rocks,dominated by intermediate rocks. Microgranular mafic enclaves are widely developed in the pluton,indicative of an obvious magmatic mixing origin. Zircon U-Pb dating shows their formation ages at(160.19±0.50)Ma,indicating that they are the product of magmatic activity in the middle Late Jurassic. Geochemical data suggest that the pluton is a metaluminous to peraluminous I-type granite and belongs to the high-K calc-alkaline series. The rock has medium ΣREE and high fractionation degree of LREE and HREE,without pronounced negative Eu anomalies. It also exhibits LREE and LILE(Ba,Rb,K)enrichment,and HFSE(Nb,Ta,P)depletion. These features are similar to geochemical characteristics of continental crust composition. The formation of the pluton may be related to the subduction of the Pacific Plate in the middle Late Jurassic,a product of intracontinental orogeny in the North China plate.