拉脊山早古生代洋盆持续时限研究

Detailed studies on the Ocean Plate Stratigraphy (OPS) can provide important information for the evolution of ancient ocean basins and the reconstruction of ancient plate tectonic framework. In the Lajishan area, the accretionary complexes in Donggoumeikuang and Hetan Village contain fundamental elements of the OPS, including MORB, OIB, radiolarian chert, limestone and terrigenous turbidite. However, previous studies on the early Paleozoic tectonic evolution of Lajishan were mainly focused on geochemistry of Cambrian volcanic rocks, but the OPS was overlooked. In combine with the results of large-scale geological mapping and multi-methods of sedimentology, petrology, radiolarian micropalaeontology, structural geology, geochemistry and zircon geochronology, the aims of this project are to systemically study the component and texture of the early Paleozoic Lajishan accretionary complex and the temporal-spatial architecture of the OPS, and to discuss the possible lasting time of the ocean basin. These results can be used to reconstruct the ocean plate stratigraphy and tectonic evolution of the ocean basin, which are also the critical evidences to define the plate tectonic framework in the South Qilian.

造山带内大洋板块地层的精细研究可为古洋盆演化和古板块构造格局重建提供最直接的资料。拉脊山东沟煤矿和河滩村东沟增生杂岩中包含了大洋板块地层基本岩石单元，包括MORB型和OIB型玄武岩、放射虫硅质岩、灰岩以及陆源碎屑浊积岩。前人对拉脊山早古生代构造演化的研究多集中于寒武纪火山岩地球化学，但是缺乏从古大洋板块地层序列角度的研究。本项目拟在对拉脊山增生杂岩大比例尺地质填图基础上，主要运用沉积学、岩石学和放射虫古生物学，并结合构造地质学、地球化学和锆石年代学等方法，系统研究拉脊山早古生代增生杂岩结构组成，揭示拉脊山地区古大洋板块地层时空格架，并探讨古洋盆可能持续时限。本项目将有助于拉脊山早古生代大洋板块地层重建和古洋盆演化历史研究，并为南祁连古板块构造格局研究提供重要的地质依据。

造山带内增生杂岩的识别与精细研究可为古洋盆演化和古板块构造格局重建提供最直接证据。祁连造山带被普遍认为是原特提斯洋盆俯冲和闭合的产物，然而南祁连地区是否存在古洋盆长期存在争议。本研究通过对拉脊山关键地段早古生代岩石开展大比例尺地质填图和室内综合研究，在东沟、石灰窑和拉脊山口地区识别出一套MORB型玄武岩、灰黑色和红色硅质岩、砂岩、泥岩和混杂岩组合，发育双重逆冲构造、逆冲断层、无根褶皱、紧闭褶皱和透入性面理。另外，在青沙山和东沟地区还识别出连续的由OIB型枕状玄武岩、火山碎屑岩、泥岩、凝灰岩和灰岩组成的火山-沉积组合。上述岩石组合是拉脊山增生杂岩的重要组成部分，分别代表早古生代原特提斯洋内深海和海山OPS残片。空间上，拉脊山增生杂岩与南侧的SSZ型蛇绿岩和岛弧共同构成了原特提斯洋向南俯冲的早古生代洋内沟-弧体系。拉脊山增生杂岩的识别不仅表明拉脊山地区存在早古生代洋盆，同时为造山带古洋盆构造演化研究提供了新思路。