内蒙古北部乌日尼图地区阿仁绍布辉长岩体的形成时代、地球化学特征及成因

Age dating, geochemistry and petrogenesis of the Arenshaobu gabbro masses in the Wurinitu region, northern Inner Mongolia

关键词:  岩石成因  锆石U-Pb定年  辉长岩  阿仁绍布  内蒙古北部

Key words:  petrogenesis  zircon U-Pb age dating  gabbro  Arenshaobu  northern Inner Mongolia

摘要:

本文系统地报道了内蒙古北部乌日尼图地区阿仁绍布辉长岩的主量元素、微量元素和稀土元素特征,重点讨论了阿仁绍布辉长岩体的岩石成因、原始岩浆的源区特征及其地质意义。研究结果表明,阿仁绍布辉长岩体SiO₂含量为49.62%,Al₂O₃为17.34%,CaO为8.94%,TiO₂为1.44%,P₂O₅为0.26%;岩石富集LILE、亏损HFSE。岩体的Mg#值(0.28~0.68,平均为0.52)较高,反映原始岩浆具幔源原生玄武岩浆特征,在上升过程中经历的结晶分异程度较低。U-Pb锆石年龄测定结果为309±2Ma,代表了阿仁绍布辉长岩体的结晶年龄,为晚石炭世。计算出石英结晶温度为928℃,斜长石结晶温度为1206℃;岩浆冷凝矿物共结晶时的深度大于10km,其结晶条件代表岩体侵位时的物理环境,是在较深的岩浆房中结晶的。阿仁绍布辉长岩具有大陆边缘弧的地球化学特征,可能反映在岩浆形成之前,地幔源区发生了俯冲交代富集作用。

Abstract:

The present paper deals, in detail, with the major element, trace element and rare earth element characteristics of the Arenshaobu gabbro masses in the Wurinitu region, northern Inner Mongolia, with the stress on the petrogenesis, origins of the primitive magmas and their geological implications. The Arenshaobu gabbro masses have SiO₂ contents of 49.62%, Al₂O₃ contents of 17.34%, CaO contents of 8.94%, TiO₂ contents of 1.44%, and P₂O₅ contents of 0.26%. The gabbro masses are enriched in LILE, and depleted in HFSE. The higher Mg# values (0.28-0.68, with an average of 0.52) suggest the origin of the mantle-derived primitive basaltic magmas with lower degrees of crystallization and differentiation during the ascending processes of the magmas. The zircon UPb ages are dated at 309±2 Ma, indicating that the crystallization ages of Arenshaobu gabbro masses may be dated back to the Late Cretaceous. The crystallization temperatures are measured at 928℃ for quartz, and 1206℃ for plagioclase. The depths for the magmatic colling and mineral cocrystallization are more than 10 km, and the crystallization of the Arenshaobu gabbro masses occurred in the deeper-seated magma chambers. The continentalmargin arc geochemical signatures of the Arenshaobu gabbro masses have mirrored that the subduction, replacement and enrichment in the mantle source areas might have taken place prior to the formation of the magmas.