内蒙古贺根山蛇绿岩中玄武岩锆石U-Pb年龄、地球化学特征及其地质意义

Zircon U-Pb age and geochemical characteristics of basalt of the Hegenshan ophiolite in Inner Mongolia and the geological significance

贺根山蛇绿岩(套)中发育有气孔杏仁状玄武岩,为蛇绿岩套的组成部分。通过对其锆石U-Pb测年,其加权平均年龄为395.9Ma±3.0Ma,结合区域地质背景,认为贺根山蛇绿岩(套)形成时代为中泥盆世—早石炭世。玄武岩为亚碱性系列,具有LREE亏损、类似N-MORB的稀土配分模式,同时具备大洋玄武岩和岛弧玄武岩特征,认为贺根山蛇绿岩(套)应形成于弧后盆地;通过与现代典型Mariana洋内弧后盆地和Okinawa陆缘弧后盆地的玄武岩以及同属中亚造山带的新疆库尔提洋内弧后盆地蛇绿岩对比,发现贺根山玄武岩同Mariana玄武岩和库尔提蛇绿岩更加类似,由此认为贺根山蛇绿岩(套)很可能形成于洋内弧后盆地环境,而非大陆边缘弧后盆地环境。

Stomatal -amygdaloidal basalt occurs in Hegenshan ophiolite. The weighted Zireon U-Pb age of basalt is 395.9±3.0Ma. Combined with regional geological background the ophiolite should be considered to be formed in the Middle Devonian -Early Carboniferous period. The basalt belongs to of the basalt which belongs to the sub alkaline rock series. The Chondrite normalized REE patterns of the basalt is of characteristics of LREE depletion, similar to N MORB. while it has both character of oceanic basalt and arc basalt, so the tectonic setting of Hegenshan ophiolite may be back arc basin. Compared with modern intra oceanic arc Mariana Trough basalts, continental marginal arc Okinawa Trough basalts and mafic rocks of the Kuerti ophiolite which formed in paleo -infra -oceanic back arc basin, the Hegenshan basalt show geochemical characteristics similar to Mariana and Kuerti basalts, we suggest that the Hegenshan ophiolite was likely produced in an infra oceanic back arc tectonic setting instead of continental margin back -arc basin.