北祁连山造山带西段钉合岩体与早古生代蛇绿岩仰冲增生时限

The existing data demonstrate that lots of the Early Paleozoic MORB-type ophiolites, Alaskan-type intrusions and granitoids (including granites and diorites) from the western part of the North Qilian Orogen were formed in a relatively short period of ca. 500 Ma. However, it remains unclear WHEN the obduction and accretion of the Early Paleozoic ophiolites occurred, WHY the magmatism of different types and tectonic environments concentrated in such a short period, and WHETHER there was an accretion of newly formed oceanic crusts. In the regions of Zoulang Nanshan and Tuole Shan, a number of granitoid plutons which are highly likely stitching plutons, occur within or around the mafic-ultramafic rocks, providing a precious clue for answering the above scientific questions. This project plans to investigate and clarify the field geological relationships, petrological and geochemical characteristics, formation ages, as well as character and possible tectonic settings of the mafic-ultramafic rocks and potential stitching plutons from the Chaidanuo-Dachadaban and Yeniu Shan of the Zoulang Nanshan region and Sala river-Dachahe Shan-Daban Shan-Langmao Shan-Zhamashi river-Xiaobabao of the Tuole Shan region. On the basis of these work, we will try to confine the timing of obduction and accretion of the Early Paleozoic ophiolites, and illustrate the geological background of the multiple magmatism at ca. 500 Ma. The research will provide important information for further understanding the geological evolution of the western part of the North Qilian Orogen at the early stage of the Early Paleozoic.

已有资料表明，北祁连造山带西段众多MORB型蛇绿岩、阿拉斯加型岩体、花岗质岩石（包括花岗岩和闪长岩）形成于早古生代～500 Ma较短时期内。然而早古生代蛇绿岩何时发生仰冲增生，仍不清楚；短时期内为何发生了大量不同类型和构造环境的岩浆活动，是否存在新生洋壳的增生，尚需探讨。在走廊南山、托勒山一带，基性-超基性岩之中或周围发育许多疑似钉合岩体的花岗质岩体，为解答上述科学问题提供了珍贵的线索。本项目拟主要针对走廊南山的柴达诺山-大岔大坂、野牛山以及托勒山的萨拉河-大查河山-大板山-浪冒山-扎麻什河-小八宝一带的基性-超基性岩和相关的疑似钉合岩体展开研究，重点查明其野外地质关系、岩石地球化学特征和形成时代，判断基性-超基性岩的类型和形成环境，在此基础上，确定该地区早古生代蛇绿岩发生仰冲增生的时限，探讨～500 Ma多种岩浆活动发生的地质背景，为进一步深入理解该地区早古生代早期地质演化过程提供依据。

北祁连造山带发育众多早古生代蛇绿岩和花岗质岩石，蛇绿岩的时代、构造环境、仰冲增生时间和地球动力学背景，存在诸多疑问。本项目通过大量野外考察、岩石地球化学分析、高精度锆石U-Pb定年等方法，对托勒山、走廊南山、老虎山的蛇绿岩和花岗岩（尤其是钉合岩体）进行了详细研究，查明了其野外产状、岩石组合、成分特征、构造环境和形成时代，限定了地幔橄榄岩构造侵位的时间，取得以下主要研究进展和重要结果：1. 玉石沟-红土沟蛇绿岩由方辉橄榄岩、辉长岩、玄武岩和少量辉石岩、斜长花岗岩组成，辉长岩侵入于地幔橄榄岩之中，辉长岩和斜长花岗岩形成于514-498 Ma，与大岔大坂MORB型辉长岩（518.9 ± 3.3 Ma）、托勒山玻安质岩石（510.6 ± 3.9 Ma）的时代基本一致。玉石沟辉长岩、红土沟斜长花岗岩、大岔大坂MORB型辉长岩、托勒山玻安质岩锆石的εHf (t)值介于10-19，指示岩浆来源于亏损地幔源区。2.柴达诺、野牛山花岗岩为S型，包裹或侵入地幔橄榄岩，属钉合岩体，形成于~508 Ma，限定地幔橄榄岩构造侵位时间上限为中寒武世。3. 地球化学、沉积岩特征表明九个泉和老虎山蛇绿岩形成于岛弧而非弧后盆地环境。九个泉玄武岩中的闪长岩侵入体形成于518.3 ± 3.6 Ma，表明九个泉蛇绿岩在早寒武世已经发育。老虎山蛇绿岩的斜长花岗岩侵入于方辉橄榄岩之中，形成于455.2 ± 2.9 Ma，限定地幔橄榄岩的构造侵位时代早于455 Ma。老虎山闪长岩形成于438.8 ± 1.4 Ma，与典型的岛弧安山岩成分相似，表明早志留世北祁连仍然存在岛弧岩浆活动。4.走廊南山北缘-走廊过渡带的部分花岗岩为S型，形成于464-454 Ma。走廊盆地A型花岗岩形成于426.1 ± 2.8 Ma，与龙首山正长岩（427.4 ± 3.5 Ma），与和走廊南山北缘金佛寺花岗岩（424.0 ± 1.6 Ma）的时代基本一致，表明北祁连洋盆在中志留世已经闭合，进入到后碰撞伸展阶段。本项目研究对恢复北祁连造山带早古生代构造演化以及地球动力学背景提供了坚实的依据，有助于构建新的蛇绿岩形成模式。