内蒙古贺根山蛇绿岩形成时代及构造启示

The age and tectonic implications of the Hegenshan ophiolite in Inner Mongolia

贺根山蛇绿岩位于兴蒙造山带北缘,发育完整的地幔橄榄岩、堆晶岩和基性熔岩组合,伴生有放射虫硅质岩,但贺根山蛇绿岩的形成时代一直存在争议,给兴蒙造山带北部构造演化阶段划分造成了很大障碍。锆石U-Pb年代学研究表明,贺根山蛇绿岩中辉长闪长岩(341±3Ma)和玄武岩(359±5Ma)结晶年龄为早石炭世早期,同时玄武岩继承锆石峰值年龄为晚泥盆世早期(375±2Ma),这些继承锆石呈短柱状、棱角状,生长环带宽缓,多为补丁状、平坦状,为典型的基性岩浆锆石,表明最迟在晚泥盆世早期洋壳物质已经开始形成。上石炭统格根敖包组火山岩与蛇绿岩局部呈喷发不整合接触,该组的晶屑凝灰岩夹层时代为晚石炭世(323±3Ma),提供了蛇绿岩构造侵位年龄的上限。因此,将贺根山蛇绿岩形成时代定为晚泥盆世-早石炭世,侵位时代为晚石炭世。侵入地幔橄榄岩中的部分基性岩脉时代为早白垩世(132±1Ma、139±3Ma和120±1Ma),它们含有大量继承锆石(144±1Ma^2698±25Ma),继承锆石峰值年龄密切响应了兴蒙造山带北部早白垩世之前复杂的岩浆及构造事件,这些基性岩脉是燕山期伸展环境下的岩浆产物,并非早白垩世蛇绿岩。结合前人的工作成果和区域岩浆岩、地层时空分布特征,建立了兴蒙造山带北部晚古生代构造演化历程:二连贺根山一线早泥盆世处于剥蚀阶段,中泥盆世陆壳拉张出现新生洋盆,晚泥盆世早期洋盆持续扩张形成新生洋壳,早石炭世晚期洋壳开始向北俯冲消减,并持续增生至西伯利亚活动陆缘,晚石炭世洋盆陆续闭合,部分已经构造侵位的蛇绿岩被晚石炭世火山岩不整合覆盖,贺根山蛇绿岩正是该洋盆的残余产物。

The Hegenshan ophiolite is located in the northern part of Xing＇ an Mongolia orogenic belt. It consists of a complete rock assemblage including peridotite, cumulate rocks and marie lava associated with radiolarian cherts. The formation age of the Hegenshan ophiolite has long been controversial, which makes a lot of troubles in dividing the tectonic evolution stages. Zircon U-Pb isotope chronology studies have shown that the crystallizing age of the gabbro diorite （341 ±3Ma） and basalt （359 ± 5Ma） in Hegenshan ophiolite are in the early Early Carboniferous and the peak age of the inherited zircons in basalt is early Late Devonian （375 ＋ 2Ma）. These inherited zircons in basalt are short prismatic and angularity with wide, patchy or planar growth zones which display the characteristics of basic magmatic zircons. All of these indicate that the oceanic ernst already formed during early Late Devonian. The pyroclastic rocks of the Upper Carboniferous Gegenaobao Formation are unconformity with the Hegenshan ophiolite. The age of the tuff in this formation is 323 ± 3Ma which support the minimum age for the emplacement of Hegenshan ophiolite. So we suggested that the age of the Hegenshan ophiolite is Late Devonian to Early Carboniferous and the emplacement time is Late Carboniferous. While the ages of some mafic dikes in Hegenshan ophiolite are Early Cretaceous （ 132 ± 1Ma, 139 ± 3Ma and 120 ± 1Ma）. They contain a lot of inherited zircons （ 144 ± 1Ma - 2698 ± 25Ma）. The peak ages of these inherited zircons closely respond the complex magma and tectonic events before Early Cretaceous in the northern Xing＇ an Mongolia orogenic belt. These marie dikes might form in the extensional environment during the Mesozoic but not the Early Carboniferous ophiolite. Combined with previous achievements as well as temporal and spatial distribution characteristics of regional strata and magmatic rocks we built the tectonic evolution of the northern Xing＇ an Mongolia orogenic belt during the Late Paleozoie. We suggested that the Erenhot and Hegenshan area was in the denudation stage in the Early Devonian and the continental crust expanded to form the oceanic basin in the Middle Devonian, the basin continued to expand and formed the oceanic crust in the early Late Devonian and then the crust began to subduct northward in the late Early Carboniferous and accreted to the Siberian continental margin which resulted in the gradually closing of the ocean basin during the Late Carboniferous, and most emplaced ophiolites were unconformity covered by the volcanic rocks of Late Carboniferous. The Hegenshan ophiolite is the residue of this oceanic lithosphere.