Ordovician intrusive rocks from the eastern Central Asian Orogenic Belt in Northeast China: chronology and implications for bidirectional subduction of the early Palaeozoic Palaeo-Asian Ocean

The eastern segment of the Central Asian Orogenic Belt is traditionally called the Xing’an Mongolia Orogenic Belt (XMOB). Ordovician intrusive rocks exposed in the XMOB, from north to south, are the Abaga-East Ujimqin Qi-Duobaoshan belt, the Sonid Zuoqi-West Ujimqin Qi belt, and the Damaoqi-Baimaimiao-Tulinkai belt, respectively. Zircon U–Pb dating and geochemical data are presented for the intrusive rocks in East Ujimqin Qi and West Ujimqin Qi, Inner Mongolia. The intrusive rocks from East Ujimqin Qi consist of gabbro, diorite, and granodiorite. LA-MC-ICP-MS zircon U–Pb ages range 446 to 461 Ma. Geochemical data suggest that the gabbros and diorites from East Ujimqin are a tholeiitic series, both of arc-related and N-MORB (mid-ocean ridge basalt) signature, indicating a back-arc basin setting. The granodiorites have a shoshonitic series and arc-related signature. Rare earth element (REE) patterns and trace element characteristics suggest gabbros, diorites, and granodiorites are petrogenetically correlated. These intrusive rocks from East Ujimqin Qi have high light REE, Th, and U concentrations, suggesting the effect of middle–upper continental crustal contamination. Major oxides display positive or negative correlations, with increasing MgO or SiO2, indicating that fractional crystallization occurred during magma evolution. Geochemical data of diorite from West Ujimqin Qi indicate a tholeiitic series, arc-related signature. Zircon U–Pb dating yielded an age of 441.8 ± 1.5 Ma. Integrated with the regionally exposed Mid–Late Ordovician plutons and metasedimentary strata, we concluded that the northward subduction of the Palaeo-Asian Ocean (PAO) that occurred beneath the southern margin of the South Mongolian Micro-continent along the Sonid Zuoqi-Xilinhot gave rise to early Palaeozoic igneous rocks from the Abaga–East Ujimqin Qi–Duobaoshan and the Sonid Zuoqi–West Ujimqin Qi belts. Southward subduction beneath the North China Craton generated the Damaoqi–Baimaimiao–Tulinkai belt. The results support the bidirectional subduction model of the PAO in the early Palaeozoic.

传统上，中亚造山带（Central Asian Orogenic Belt）的东段被称为兴安蒙古造山带（Xing’an Mongolia Orogenic Belt，XMOB）。出露于兴安蒙古造山带的奥陶纪侵入岩自北向南依次为阿巴嘎-东乌珠穆沁旗-多宝山带、苏尼特左旗-西乌珠穆沁旗带以及达茂旗-白庙-图林凯带。本文针对内蒙古东乌珠穆沁旗与西乌珠穆沁旗的侵入岩开展了锆石U-Pb定年与地球化学数据测试分析。东乌珠穆沁旗的侵入岩主要由辉长岩、闪长岩与花岗闪长岩组成，激光多接收电感耦合等离子体质谱（LA-MC-ICP-MS）测得的锆石U-Pb年龄介于446 Ma至461 Ma之间。地球化学特征显示，东乌珠穆沁旗的辉长岩与闪长岩属于拉斑玄武岩系列，兼具弧相关属性与正常洋中脊玄武岩（mid-ocean ridge basalt，N-MORB）的地球化学特征，指示其形成于弧后盆地构造背景；花岗闪长岩则属于钾玄岩系列，且具有弧相关地球化学特征。稀土元素（Rare earth element，REE）配分模式与微量元素特征表明，辉长岩、闪长岩与花岗闪长岩具有同源成因联系。东乌珠穆沁旗的上述侵入岩具有较高的轻稀土元素、钍（Thorium，Th）与铀（Uranium，U）含量，指示其受到了中-上大陆地壳的混染作用影响。主量氧化物随MgO或SiO₂含量升高呈现正相关或负相关关系，表明岩浆演化过程中发生了分离结晶作用。西乌珠穆沁旗闪长岩的地球化学数据显示其属于拉斑玄武岩系列，且具有弧相关地球化学特征，锆石U-Pb定年结果给出其年龄为441.8 ± 1.5 Ma。结合区域范围内出露的中晚奥陶世深成岩体与变沉积岩系，本文认为：沿苏尼特左旗-锡林浩特一线，位于南蒙古微大陆南缘之下的古亚洲洋（Palaeo-Asian Ocean，PAO）向北俯冲，催生了阿巴嘎-东乌珠穆沁旗-多宝山带与苏尼特左旗-西乌珠穆沁旗带内的早古生代火成岩；而华北克拉通之下的南向俯冲作用则形成了达茂旗-白庙-图林凯带。本研究结果支持古亚洲洋在早古生代时期的双向俯冲模式。