蚌埠荆山和涂山岩体的年代学、地球化学特征及其动力学意义

对蚌埠地区的荆山和涂山花岗岩进行了详细的年代学和岩石地球化学研究,分析结果表明:花岗岩的SiO2含量较高,介于71.22%～73.76%之间,富K2O、Na2O,贫TiO2、CaO、MgO,为弱过铝质岩石。K2O-Na2O图解上,所有数据点都落在"I"型花岗岩区域,结合呈右倾的微量元素标准化蛛网图,可知该区富集大离子亲石元素,同时具有明显的U、Pb、Ba、Sr正异常,Ti的负异常,暗示蚌埠岩体的形成背景可能为造山后的伸展环境。CamecaIMS1280SIMS锆石U-Pb年龄分析表明,主要有800Ma左右的新元古代年龄、三叠纪相关的220Ma和晚侏罗世的165Ma这三种主体年龄。根据上述地球化学资料,推断蚌埠岩体的母岩浆来源于火成岩的部分熔融,岩浆源区为与华北板块碰撞后继续向北伸展的部分扬子板块下地壳火成岩物质,其中,800Ma左右的新元古代年龄解释为扬子板块下地壳物质的继承锆石年龄,三叠纪的继承锆石理解为三叠纪时期华南、华北板块的碰撞导致,晚侏罗世的岩浆活动推测由于古太平洋板块北西向的斜向俯冲诱发扬子板块下地壳的物质部分熔融所致。

Geochronology, Geochemical Characteristics and Geodynamic Significance of the Jingshan and Tushan Granites in the Bengbu Uplift

The Bengbu uplift is located on the southeastern margin of the North China block, about 200km north of the Dabie orogen in the southwest of the Su-Lu orogen, east of the Tan-Lu Fault Zone. The genesis and geochronology of the Bengbu granites can reveal the deep-subduction direction of the Dabie-Sulu ultrahigh-pressure metamorphic belt and influence scope and help to realize the tectonic evolution of the eastern North China block in the Mesozoic. Considering these, we particularly analyze the Bengbu Jingshan and Tushan granitic rocks by geochronology and geochemistry methods. The Jingshan and Tushan granites in this area are mainly biotite monzonitic granites, which have very high SiO_2 (71.22%～73.76%), relatively high K_2O and Na_2O, and low TiO_2, CaO and MgO contents, belonging to slight peraluminous rocks. All samples are projected onto the area of I-type granites in the diagram of K_2O-Na_2O, combining with a right-deviating trace elements spider diagram, with obviously positive anomalies of U, Pb, Ba, Sr and negative Ti anomaly, which all prove that the Bengbu granites could be formed under the post-orogenic extensional environment. Furthermore, Cameca IMS 1280 SIMS zircon U-Pb dating shows that there are three major age groups:～800Ma,～220Ma and ～165Ma. According to the above-mentioned geochemical data, we believe the parent magmas of the Bengbu granites were originated from the partial melting of igneous rocks and magma source of lower crust of the Yangtze craton. As for ages, new Proterozoic ages of about 800Ma are interpreted as the inherited zircon ages of the Yangtze craton lower crust,～220Ma Triassic ages are due to the Triassic collision of the South China block and North China block, and late Jurassic magmatism are presumed owing to partial melting of the Yangtze craton lower crustal materials induced by oblique subduction of the paleo-Pacific plate.