东北大兴安岭乌尔旗汗蛇绿岩及其构造演化

Timing, position and process of final suturing of the eastern segment of the Central Asian Orogenic Belt (CAOB) is a remnant debated issue in recent years. Numbers of studies on petrology, paleontology and geophysics, which are related to subduction and collision from four blocks between each other (Erguna, Xing’an, Songnen and Jiamusi blocks), in NE China, had been done. However, the study on ophiolites, that response to the surrounding orogenic process, is still poor. The Wuerqihan ophiolite tectonically has a well-exposed and lies in the recent controversial suture zones of Xinlin-xiguitu suture. Its rock association characteristics is an important record for the evolution of Xinlin-xiguitu Ocean. Up to now, very few studies exist about the association, geochemistry and tectonic evolution of Wuerqihan ophiolite. In this study, we attempt to reveal its tectonic setting, association and chronology. We will perform field survey, petrographical and mineralogical observations as well as comprehensive studies on geochemistry and isotope chronology, including whole-rock geochemistry, in-situ LA-ICP-MS mineral trace-element analysis and EPMA major-element analysis, Sr-Nd isotope analysis, Re-Os dating, zircon U-Pb dating, and Hf isotope. Combining with the previous studies, our results will establish the tectonic evolution model of the Wuerqihan ophiolite and discuss its tectonic setting, and provide a significant evidence on revealing the collision location, time process of Xinlin-xiguitu suture zone, as well as the subduction direction of Xinlin-xiguitu ocean between the Erguna and Xing’an blocks.

中亚造山带东段（东北地区）各地块的闭合位置、时间和演化过程一直是地学界争论的焦点问题。前人基于地球物理、岩浆活动以及古生物地层等方面的研究取得了一些进展，但从蛇绿岩角度的研究仍相对薄弱。乌尔旗汗蛇绿岩位于备受争议的新林—喜桂图缝合带的南段，代表了洋壳的完整序列，记录了新林—喜桂图洋盆开启至消亡的重要信息，然而，由于其岩石组合、年代学及地球化学性质等研究程度较低，制约了整个新林—喜桂图缝合带的构造演化研究。本申请拟通过岩性-构造填图、岩石学、全岩地球化学、构造年代学以及同位素示踪等方法，揭示乌尔旗汗蛇绿岩的岩石组合与构造环境；开展地幔橄榄岩Re-Os定年，辉长岩及斜长花岗岩的锆石U-Pb定年，建立蛇绿岩演化的年代学序列，恢复乌尔旗汗蛇绿岩的构造演化历史，确定新林—喜桂图缝合带南段位置、俯冲极性及形成时代，揭示南北段蛇绿岩的本质特征及内在联系，重建新林—喜桂图洋盆的构造格局。

大兴安岭地区位于中亚造山带东段，该地区发育多条蛇绿混杂岩带，主要包括呼玛、新林、阿里河、头道桥、乌尔旗汗、阿尔山及二连浩特-贺根山蛇绿混杂岩带。本项目对乌尔旗汗地区蛇绿混杂岩的岩石组合、地球化学特征及形成时代进行了详细研究。乌尔旗汗地区蛇绿混杂岩主要分布于扎敦河、乌奴耳及乌川东地区，岩石组合包括辉橄岩、蛇纹岩、变玄武岩、变辉长岩、变辉绿岩、辉绿玢岩和硅质岩，其中变玄武岩（细碧岩）和放射虫硅质岩常紧密伴生，局部变玄武岩中发育淬碎角砾岩，表明属枕状熔岩，具有典型“大洋地壳残片+地幔岩”组合。获得辉长岩或辉绿岩锆石中U-Pb加权平均206 Pb/238 U年龄分别为341 ± 1 Ma，344 ± 2 Ma和336 ± 4 Ma。在地球化学上，蛇绿混杂岩中镁铁质岩可分为两类：第Ⅰ蛇绿岩的SiO2含量为47.47-50.34%，Al2O3含量为13.41-16.24%，与洋中脊拉斑玄武岩相近，TiO2含量为0.91-2.75%，略高于典型洋中脊玄武岩和弧后盆地玄武岩BAB，轻稀土元素轻微亏损，总体属MORB-like型蛇绿岩；第Ⅱ类蛇绿岩的SiO2含量为45.86-50.91%，但Al2O3含量明显偏高（16.46-20.27%），与高铝岛弧玄武岩相近，并且具较低的TiO2（0.46-0.76 %），轻稀土为略富集特征，(La/Sm)N在1.27-2.22之间，总体属IAB-like型蛇绿岩。鉴于该区未报到过弧前玻安岩的相关信息，初步认为这种兼有MORB-like和IAB-like特征的蛇绿岩通常形成于弧后盆地。结合前人研究以及本项目近期识别出的早古生代岩浆弧，我们认为乌尔旗汗蛇绿混杂岩的成因机制可能至少有两种：（1）嫩江洋向西俯冲，在多宝山-蘑菇气-大石寨地区形成早古生代岩浆弧，导致新林-喜桂图缝合带南部再次拉开，形成弧后盆地；（2）兴安地块为增生杂岩地体，该蛇绿混杂岩可能为洋壳持续俯冲结果，从新元古代持续增生至晚石炭世早期。由此可见，中亚造山带东段蛇绿混杂岩构造环境仍十分复杂，亟待解决新识别与已识别蛇绿混杂岩之间的连接与对比。项目资助以来第一作者发表SCI-6篇，EI-1篇，合著-2篇，专著-1部，获得2项省部级称号；参加国际国内学术会议10次。