北山造山带墩墩山泥盆系的沉积序列及其构造环境研究

The Central Asian orogenic belt (CAOB) is one of the largest accretionary orogenic collages in the world with the highest rate of Phanerozoic continental growth and significant metallogenic importance. The Beishan orogen, located in the southern CAOB, and developed between the Tianshan orgoen and Xing'meng orogen, provides a good chance to study the accretionary orogenesis of the CAOB. The Dundunshan Devonian vocano-sedimentary rocks are distributed to the southern of the Hongliuhu-Xichangjing ophiolitic melange, recording the tectonic transition from the Early Paleozoic to the Late Paleozoic. The studies on sedimentary succession and tectonic setting of the vocano-sedimentary can provide key evidence for constraining the Late Paleozoic tectonic evolution of southern Beishan. This subject focuses on the Dundunshan Devonian strata and aims at constraining the sedimentary succession and tectonic setting. This project would present sedimentary structure of sedimentary rocks, detrital modal analyses of framework grains, analyses of heavy minerals, whole-rock major and trace element compositions, Sr-Nd isotopic characteristics, and new zircon U-Pb ages and Hf isotope for the Dundunshan Devonian strata, and evaluates their sedimentary succession, sedimentary provenance and tectonic implications. These results, combined with regional data, provide new constraints on tectonic transition from the Early Paleozoic to the Late Paleozoic. Comparison of volcano-sedimentary rocks on Southern Altaids can shed some new light on the crustal growth and tectonic evolution of the southern CAOB.

北山造山带位于中亚造山带南缘，天山、兴蒙之交。其南部墩墩山泥盆系火山-沉积岩系位于红柳河-洗肠井蛇绿混杂带南缘，记录了北山早古生代向晚古生代构造格局演变的重要信息。对火山岩和沉积岩的沉积序列和构造环境的研究是构建北山晚古生代构造演化的关键。本项目拟以墩墩山泥盆系为研究对象：通过沉积岩野外原生沉积结构观测、碎屑矿物统计、重矿物分析，结合火山-沉积岩系的全岩地球化学和Sr-Nd同位素及锆石U-Pb年代学和Hf同位素研究，查明墩墩山火山-沉积岩系的沉积序列，标定其形成时代，分析沉积岩的沉积环境和源区性质，探讨火山岩和沉积岩的演化关系。通过本项目的研究，揭示北山早古生代向晚古生代构造格局的演变过程，综合对比中亚造山带南缘的火山-沉积岩系的特征，为中亚造山带南缘增生造山方式和演化历史提供依据。

对北山造山带墩墩山泥盆系的野外特征、同位素年代学、岩石地球化学特征进行了系统研究。墩墩山泥盆系由三个井组砂岩和墩墩山群火山岩组成，砂岩整合覆盖在火山岩之上，两者形成时代行近，为晚泥盆世。砂岩主要由石英、岩屑组成，长石次之，147Sm/144Nd值变化于0.15 - 0.24，εNd(t)值为-5.0 - -9.1,Nd模式年龄为1.47-1.82Ga,表明其物源区可能由元古代甚至更老的岩石和大量的年轻物质组成，处于不稳定的构造环境。火山岩为一套准铝质到过铝质的钙碱性火山岩，具有负的εNd(t)值和高的Sr同位素含量，同时单颗粒锆石εHf(t)值为+1.4 - +16.4，指示了复杂的源区特征。结合区域特征，推测墩墩山泥盆系产生于向南的俯冲带后撤，形成于增生弧或者转换弧的大地构造环境。墩墩山火山岩独特的地球化学特征，尤其是Nd - Hf同位素解耦现象表明北山造山带泥盆纪时期经历了重要的构造转换过程。本课题还发现了火山岩中的Nd - Hf同位素解耦现象。这在沉积岩中比较常见，但是火成岩中较为罕见。