河北康保地区花岗岩独居石电子探针定年

Electron microprobe dating of monazite in granite from Kanbao area Hebei province

已有年代学研究表明,康保地区西阿公单元含石榴二长花岗岩形成于中元古代。我们采用近年来新发展的独居石电子探针Th-U-总Pb定年方法对康保地区西阿公单元十硼地区变质细粒含石榴二长花岗岩样品中的独居石开展了电子探针进行测年分析,计算方法为Suzuki和Adachi等提出的方法。独居石年龄峰值分别为252Ma,265Ma,281Ma和322Ma,表明西阿公单元含石榴二长花岗岩形成时代不是中元古代,其侵位时间应为二叠纪。这一新的年代学结果与Sengor和王荃推测的华北板块与西伯利亚板块"晚二叠世碰撞"的模式一致。康保地区西阿公单元含石榴二长花岗岩具过铝花岗岩特征,代表古蒙古洋消失,是华北板块与西伯利亚板块碰撞造山阶段多期岩浆作用的结果,为确定古亚洲洋的闭合时间提供了重要依据。

The geochronological data documented in previous studies imply that metamorphosed monzogranites with garnet within unit Xi＇agong located in Kangbo area could be displaced in Mesoproterozoic era. We analyzed monazite grains separated from the monzogranite samples with garnet using EPMA Th-U-total Pb dating method to determine the ages of these samples, and the ages are calculated from the analytical data using the methods offered by Suzuki and Adachi et al.. The new obtained monazite Th-U-total Pb ages yield four peak ages of 252Ma, 265Ma, 281Ma and 322Ma respectively, the results show that fine-grained monzogranites with garnet in unit Xi＇agong were not formed in proterozoic era but in Permian, which is consistent with the late Permian collisional model between the North China and the Siberian plates. The fine-grained monzogranite samples with garnet in unit Xi＇agong are characterized by their peraluminous nature, indicating the disappear of Mongolia ocean, these monzogranitoids were formed in multi-episodes of magma activities during collision and orogenic processes between the North China and the Siberian plates, which provides important chronological evidence for the closure of the Paleo-Asian Ocean.