西南三江造山带古特提斯弧岩浆岩的时空分布及构造演化新模型

位于青藏高原东缘、东南缘的西南三江造山带的构造几何格架形成于晚古生代-早中生代古特提斯阶段。前人综合1/20万区域地质调查成果创造性地构建了基于板块构造理论的大地构造几何格架初始版本,成为指导三江造山带地学研究的关键基础。二十世纪末引入我国的锆石原位U/Pb测年技术大规模应用于三江造山带研究,积累了大量高精度年代学数据,为升级大地构造几何格架初始版本奠定了良好基础。收集、整理、综合现有测年结果,本文将三江造山带内古特提斯阶段弧岩浆岩划分为两阶段、三个带,分别为晚三叠世玉树-义敦陆缘弧岩浆岩带、二叠纪-中三叠世江达-维西-马登-点苍山陆缘弧岩浆岩带、二叠纪-晚三叠世云县-绿春-哀牢山陆缘弧岩浆岩带。结合高压变质带研究进展、大地构造相时空配置关系提出:(1)玉树-义敦陆缘弧岩浆岩带为甘孜-理塘洋向南西俯冲于东羌塘-中咱陆块之下的产物,俯冲持续时间仅10Myr左右;(2)江达-维西-马登-点苍山、云县-绿春-哀牢山两个陆缘弧岩浆岩带属同一弧岩浆岩带,是龙木措-双湖-昌宁-孟连古洋壳向北、北东俯冲作用的产物,其俯冲时长达70Myr,产生4或5个岩浆岩活跃期。从俯冲作用持续时间及岩浆岩带空间规模分析,龙木措-双湖-昌宁-孟连缝合带代表的古洋盆为三江造山带古特提斯主洋盆,而甘孜-理塘洋规模相对较小。古特提斯主洋盆的闭合具有穿时性,北段闭合于中三叠世,而南段则到晚三叠世才最终闭合。新数据揭示的古特提斯阶段陆缘弧岩浆岩带空间分布特点提出了若干新科学问题,值得进一步深入研究。

Paleotethyan tectonic evolution of the Sanjiang Orogenic Belt, SW China: Temporal and spatial distribution pattern of arc-like igneous rocks

The tectonic geometrical framework of the Sanjiang Orogenic Belt, eastern and southeastern Tibetan Plateau, was formed during Late Paleozoic to Early Mesozoic (i.e., the Paleotethyan stage). Previous workers had constructed an earlier version of the framework in term of plate tectonics based on the 1/200,000 geological mapping results. It has become the robust ground for geological study of the Sanjiang Orogenic Belt. Since the 1990s, in-situ zircon U/Pb dating techniques were introduced into China. Application of them has accumulated a large body of high-quality dating results. It is time to "upgrade" the earlier version. By synthesizing available zircon U/Pb data, we try to divide the arc-like igneous rocks in the Sanjiang Orogenic Belt into three belts, which were formed during two distinctive stages:(1) the Late Triassic Yushu-Yidun continent marginal arc belt; (2) the Permian to Middle Triassic Jamda-Weixi-Madeng-Diancangshan continent marginal arc belt; and (3) the Permian to Late Triassic Yunxian-Lüchun-Ailaoshan continent marginal arc belt. The spatial and temporal pattern of tectonic facies, especially the high-pressure/low temperature metamorphic belts, suggests that:(1) the Yushu-Yidun continent marginal arc was resulted from southwestward subduction of the Garz-Litang ocean beneath the Eastern Qiangtang-Zhongza block, and this subduction lasted only for less than~10Myr; (2) the Jamda-Weixi-Madeng-Diancangshan and the Yunxian-Lüchun-Ailaoshan continent marginal arc belts were formed due to northward or northeastward subduction of the Longmu Co-Shuanghu-Changning-Menglian ocean which had continued for more than 70Myr since the Early Permian, and given rise to four and five pulses of arc-like magamatism along the northern and southern segments of the arc-belt, respectively. The longer subduction period and the larger spatial scale of the arc-belt of the Longmu Co-Shuanghu-Changning-Menglian Ocean compared with the Garz-Litang Ocean suggests that the former is the main Paleotethyan oceanic basin, while the later belongs to a branch of the Paleotethys. The final closure of the main Paleotethys is diachronous:its northern segment closed at Middle Triassic, while the south at Late Triassic. Such a spatial and temporal pattern of the Paleotethyan acr-like igneous rocks rises up several scientific issues that are worthy further studying.