南秦岭构造带志留纪-石炭纪构造古地理演化的古地磁制约

Qinling Orogenic Belt is a compound orogen formed by multiple process of subduction, accretion and collision between the North China Block and South China Block. Key Scientific issues concerning timings of the closure of Shangdan Ocean and the opening of Mianlue Ocean play an important role in understanding the tectonic evolution for Qinling Orogenic Belt. The South Qinling Belt is sandwiched between the Shangdan and Mianlue Sutures, and its paleogeography positions during the Silurian-Carboniferous are essential for understanding the tectonic evolution of both the Shangdan Ocean and the Mianlue Ocean. In this study, we are planning to carry out a systematic paleomagnetic study on Silurian dyke swarms and Devonian-Carboniferous limestones from the South Qinling Belt to provide quantitative or semi-quantitative constraints on the paleopositions of the South Qinling Belt. On the analysis of the paleomagnetic data from the South Qinling Belt and North China Block to the south, relative tectonic relationship of these two blocks/terranes is to be investigated, and the closure time of Shangdan Ocean is to be discussed. On the comparison of the paleomagnetic data from the South Qinling Belt and South China Block, relative tectonic relationship between these two blocks is to be investigated, and the tectonic evolution of the Mianlue Ocean during the mid-Paleozoic is to be discussed.

秦岭造山带是华北和华南板块碰撞形成的复合型造山带，具有多期俯冲、碰撞的特征。目前学术界对于秦岭造山带的构造演化，特别是商丹洋闭合的时限以及勉略洋打开的时限等关键科学问题的认识存在较大争议。南秦岭构造带位于商丹缝合带与勉略缝合带之间，其志留纪-石炭纪的古地理位置对于探讨商丹洋与勉略洋的构造演化至关重要。本项目拟选取南秦岭构造带内出露的志留纪未变质、变形的基性岩墙群以及泥盆纪-石炭纪未变质或极弱变质的灰岩作为研究对象，开展详细的构造古地磁研究，以期获得高质量的志留纪-石炭纪古地磁数据，约束南秦岭构造带志留纪-石炭纪的古地理位置及其变化。通过与华北板块同时期古地磁数据的对比，分析南秦岭构造带与华北板块之间的相对位置关系及其变化，约束商丹洋的闭合时限；通过与华南板块视极移曲线对比，分析南秦岭构造带与华南板块的古位置关系，探讨勉略洋早期的构造演化过程。

秦岭造山带是分割中国南北地质的重要界线，也是中国大陆完成主体最后拼接的主要结合带。自新元古代中晚期以来，秦岭造山带经历了多期次拼合俯冲碰撞过程，其构造演化特别是板块碰撞拼合的缝合带位置及时限等关键科学问题备受关注却仍存较大争议。南秦岭构造带古生代的古地理位置及其演化以及它与华北、华南板块之间的构造关系是认识秦岭造山带构造演化的关键所在。本课题选择南秦岭构造带志留纪岩墙群、泥盆纪-三叠纪沉积地层为研究对象，开展系统构造古地磁工作，约束了南秦岭构造带泥盆纪-三叠纪的古地理位置及其演化。结合华北与华南板块已有同时代高质量的古地磁数据，对南秦岭与华北、华南板块之间的分离聚合过程进行了探讨。得到了以下结论与认识：1）本研究取得的古地磁结果表明，以33.4 °N, 109.5 °E为参考点，南秦岭构造带在晚泥盆世时期位于9.4°N±3.2°，在早石炭世期间位于-1.5°N±2.5°，在早三叠世时期位于23.6°N±2.4°。2）在晚泥盆世-早三叠世时期，南秦岭构造带一直位于华北板块的南缘，表明南秦岭构造带与华北板块之间的商丹洋闭合的时限应该早于晚泥盆世。3）在晚泥盆世-早三叠世期间，南秦岭构造带与华南板块之间一直保持一定规模的纬度差，表明两者之间在该时期一致被勉略洋分割，勉略洋的时代应该为古生代。该项研究为认识秦岭造山带的构造演化提供了定量的古地磁学制约。