祁连山哈拉湖地区奥陶纪岛弧火山岩及其构造意义

Ordovician island arc volcanic rocks in Halahu area of Qilian Mountain and their tectonic significance

通过1∶5万区域地质调查,对祁连山哈拉湖地区火山岩进行了野外地质、岩石学、锆石U-Pb同位素年龄、地球化学等研究。LA-ICP-MS锆石U-Pb同位素定年结果显示,该火山岩年龄为466.3±2.4Ma(n=9,MSWD=1.4),形成于中奥陶世。岩石地球化学研究表明,哈拉湖地区火山岩为低钾拉斑玄武系列,绝大多数样品的亲石元素Rb、Th、U、Ce、Zr、Nd和稀土元素La、Sm富集明显;高场强元素(Sr、P、Ti)强烈亏损,Zr、Hf微弱富集,Ba明显亏损。总的特征显示,少数火山岩具有过渡型洋脊玄武岩的特征,大多数火山岩样品显示岛弧火山岩的特征。此外,哈拉湖地区的岛弧火山岩与晚奥陶世的岛弧花岗岩伴生在一起。这些特征表明,与俯冲有关的奥陶纪岛弧岩石可能与拉脊山地区古大洋的闭合有关。

Based on 1∶50000 regional geological survey, the authors studied volcanic rocks of Hulahu area in Qilian Mountain in the aspects of field geology, petrology, geochronology and geochemistry. Zircon LA-ICP-MS U-Pb dating results show that the Halahu volcanic rocks were formed in Middle Ordovician(466.3±2.4 Ma, MSWD=1.4). Geochemical data show that the Halahu volcanic rocks belong to low-K tholeiite series, and are characterized by enrichment of LILEs(Rb, Th, U, Ce, Zr and Nd) and LREEs and depletion of Sr, P, Ti and Ba. On the whole, a few samples have features similar to those of T-MORB, but most samples show affinities with island arc related to subduction. In addition, the Halahu volcanic rocks were associated with Late Ordovician arc granitoids. These features indicate that the island arc rocks were associated with Ordovician paleo-oceanic closure in Lajishan area.