华北板块北缘勃隆克A型花岗岩锆石U-Pb定年及其地质意义

内蒙古翁牛特旗勃隆克岩体位于华北板块北缘,侵位于上侏罗系火山岩地层中。详细的岩相学研究显示,勃隆克花岗岩具有粒状结构、蠕虫结构和文像结构,块状构造,部分斜长石已绢云母化、泥化。对勃隆克花岗岩进行了LA-ICP-MS锆石U-Pb定年,获得134.0±1.8Ma和134.9±4.1Ma的侵位年龄,表明其形成于早白垩世。地球化学特征显示,花岗岩属于高钾钙碱性系列,有较高的SO2(74.1%~75.6%)、Na2O+K2O(8.98%~9.2%)、Rb(210×10-6~225×10-6)含量和10000×Ga/Al(2.69~2.80)、Rb/Sr(5.8~18.9)值,具有较低的CaO、MgO、Ba和Sr含量。铝饱和指数A/CNK=0.99~1.03,属于偏铝质或过铝质A型花岗岩。稀土元素球粒陨石标准化图解显示,轻稀土元素相对富集,负Eu异常明显;在原始地幔标准化图解上,Ba、Sr、Nb、Ta、P、T强烈亏损,富集Rb、Th、K、Hf等元素,与华北板块北缘早白垩世A型花岗岩类似。结合区域构造演化,认为勃隆克花岗岩形成于伸展构造背景。晚中生代,华北板块北缘构造体制经历了重大的转变,地壳从挤压体制转为岩石圈减薄和地壳伸展体制,软流圈物质上涌导致上覆地壳长英质物质的部分熔融形成勃隆克A型花岗岩。

Zircon U-Pb dating of Bolongke A-type granite on the margin of northern North China Plate and its geological significance

Bolongke granite,emplaced in the Upper Jurassic volcanic strata,is located on the north margin of the North China Craton(NCC).Detailed petrographical observation reveals that the granite occurs in the forms of euhedral and subeuhedral crystals and exhibits mymekitic and pegmatitic texture and massive structure.Sencitization and argillation exist in part of plagioclases caused by weathering.The LA-ICP-MS analysis of zircons yielded empkcement ages of 134.0±1.8Ma(MSWD=1.8)and 134.9±4.1Ma(MSWD=2.0),indicating that the Bolongke granite was fomied in the Early Cretaceous.A geochemical study of the intrusion suggests that it belongs to high-K calc-alkaline series and is characterized by depletion of Ba,Sr and enrichment of such elements as Rb,Th,Pb and Hf,with obviously negative Eu anomalies.To sum up,the intrusion shows the charactejstics of A-type granite.Tectonic discrimination diagrams indicate that Bolongke granite was fomed in an anorogenic extension environment.Combined with the achievements obtained by previous researchers,the authors tentatively hold that the emplacement of Bolongke granite belonged to the Mesozoic magmatic events in the North China Craton and was produced by lithospheric thinning.