兴蒙造山带南缘东段和龙地区Ⅰ型花岗岩锆石U-Pb定年、地球化学特征及其地质意义

Zircon U-Pb dating,geochemistry and geological significance of the Itype granites in Helong area,the eastern section of the southern margin of Xing-Meng Orogenic Belt

本文对兴蒙造山带南缘东段和龙地区八家子岩体和勇新岩体进行了岩相学、地球化学、LA-ICP-MS U-Pb定年和Sr-Nd同位素研究,以便制约该区的区域构造演化。锆石测年结果显示,八家子石英闪长岩结晶年龄为257.8±0.9Ma,形成于晚二叠世早期,勇新二长花岗岩的结晶年龄为243.2±1.1Ma和238.5±2.8Ma,形成于中三叠世。八家子石英闪长岩为钠质钙碱系列、偏铝质的Ⅰ型花岗岩类,富集轻稀土(LREEs)和大离子亲石元素(LILEs,Cs、Ba、K和Sr),亏损重稀土(HREEs)和高场强元素(HFSEs,Nb、Ta和P),结合Sr-Nd同位素特征显示其为壳源成因,形成于俯冲环境下,是玄武质下地壳部分熔融的产物。勇新二长花岗岩富硅、铝和钾,为高钾钙碱性系列、偏铝质-弱过铝质的Ⅰ型花岗岩,富集轻稀土(LREEs)及大离子亲石元素(LILEs,Cs、K、Sr),亏损重稀土(HREEs)和高场强元素(HFSEs,Nb、Ta、P和Ti),结合Sr-Nd同位素特征显示其为壳源成因,形成于同碰撞环境下,是玄武质下地壳部分熔融的产物。花岗岩的侵位反映了研究区晚二叠世早期古亚洲洋板块俯冲于华北克拉通之下,中三叠世华北克拉通与兴凯地块碰撞拼合,说明古亚洲洋应闭合于晚二叠世末期-早三叠世期间。

Petrography, geochemical data, LA-ICP-MS zircon U-Pb dating and Sr-Nd isotopic have been obtained from Bajiazi pluton and Yongxin pluton in Helong area, with the aim of constraining the tectonic nature of the eastern section of the southern margin of Xing-Meng Orogenic Belt. LA-ICP-MS zircon U-Pb age data indicate that the quartz diorites from Bajiazi pluton formed during the early period of Late Permian and monzogranites from Yongxin pluton formed during Middle Triassic. Furthermore, Bajiazi quartz diorites belong to calc-alkaline, metaluminous I-type granitoids with feature of N% O content is higher than K2O. They also show strong enrichment in LREEs and LILEs （Cs, Ba, K and Sr）, depletions in HREEs and HFSEs （Nb, Ta and P）. Combined the Sr-Nd isotopic data indicate the primary magma for Bajiazi pluton could be mainly derived from partical melting of the marie lower ernst under the tectonic setting of subduction. The Yongxin monzogranites belong to high-K tale-alkaline, metaluminous-weakly peraluminous I- type granitoids with features of rich in SiO2, Al2O3 and K2O. They also show strong enrichment in LREEs and LILEs （ Cs, K and Sr）, depletions in HREEs and HFSEs （Nb, Ta, Ti and P）. Combined the Sr-Nd isotopic data indicate the primary magma for Bajiazi pluton could be mainly derived from partieal melting of the marie lower ernst under the tectonic setting of syn-eollision. The emplacement timing of Bajiazi pluton and Yongxin pluton reflect the Paleo-Asian oceanic plate had been subdueting beneath the eastern section of the northern margin of the North China Plate during the early period of Late Permian, and the collision between North China Plate and Khanka Massif took place at the Middle Triassic, which suggests that the Paleo-Asian Ocean closed is between the end of Late Permian and Early Triassic.