拉脊山增生楔结构、组成与古洋盆重建研究

High-resolution analysis on the texture and composition of accretionary wedge in orogenic belts provides direct evidence for the reconstruction of ancient oceanic basins and plate-tectonic affinities. The Liutaicun-Hetancun accretionary wedge in the Lajishan area comprises fundamental rock units of the oceanic plate stratigraphy, including MORB- and OIB-type basalts, radiolarian cherts, limestones, and trench turbidites. Previous studies on the Paleozoic tectonic evolution of the Lajishan area mainly focus on geochemistry of Cambrian volcanic rocks and lack studies from the accretionary wedge and oceanic plate stratigraphy, which have resulted in a long-lasting controversy over the tectonic evolution of the Lajishan and adjacent areas. Based on large-scale geological mapping and methods in sedimentology, petrology, radiolarian paleontology, structural geology, mineralogy, geochemistry, and zircon geochronology, this project will systemically study the texture, composition, and deformation of the Lajishan accretionary wedge, reveal spatial and temporal framework of the ancient oceanic plate stratigraphy, reconstruct elements of plate tectonics, and discuss the timing of the opening and closure of the ancient oceanic basin and their relationship between the evolution of the oceanic basin in the Lajisahn and North Qilian areas during early Paleozoic. This project is significant and will deepen the understanding on the evolution of the Lajishan ancient oceanic basin and provide direct evidence for study the relationship between the South Qilian belt and Proto-Tethyan ocean.

造山带增生楔结构与组成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证据。拉脊山六台村-河滩村增生楔包含了大洋板块地层最基本岩石单元，包括MORB和OIB型玄武岩、放射虫硅质岩、灰岩和海沟浊积岩。前人对拉脊山早古生代构造演化研究多集中于寒武纪火山岩地球化学，缺乏从增生楔和大洋板块地层角度研究，导致拉脊山及区域构造演化长期存在争议。本项目拟在大比例尺地质填图基础上，运用沉积学、岩石学和放射虫古生物学结合构造地质学、矿物学、地球化学和锆石年代学等方法，综合区域地质资料，系统研究拉脊山增生楔结构、组成与构造变形，揭示拉脊山地区古大洋板块地层时空格架，重塑古板块构造格局，探讨古洋盆开启与闭合时限以及拉脊山与北祁连早古生代洋盆演化的关系。本项目研究的科学意义在于：深化拉脊山古洋盆演化过程，为南祁连构造演化与原特提斯洋之间关系研究提供直接依据。

增生楔是造山带中的重要构造单元之一，其结构与组成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证据。拉脊山构造带是祁连造山带的重要组成部分，前人对该其构造演化研究多集中于寒武纪火山岩地球化学，缺乏从增生楔和大洋板块地层角度研究，导致拉脊山及区域构造演化长期存在争议，影响了原特提斯洋古地理重建与中央造山带结构认识。本项目采用大比例尺地质填图以及岩石-沉积-构造-古生物等多学科研究方法，系统开展了拉脊山六台村-河滩村增生楔结构、组成与构造变形等内容研究。研究显示，拉脊山增生楔由中寒武世-早奥陶世洋壳物质构成，包括硅质岩、凝灰岩、页岩、硅质泥岩、灰岩、火山质砾岩、枕状玄武岩等，呈现连续单元和混杂岩两类，发育双重逆冲构造和各种褶皱变形。空间上，它们自北而南依次为海沟、大洋盆地、海山OPS和SSZ型蛇绿岩，呈现出向南逐渐远离大陆边缘且洋盆逐渐加深的古地理格局。拉脊山西沟蛇绿岩中辉长岩及其就位形成的斜长角闪岩锆石U-Pb年龄分别为535-525Ma和521Ma，且增生楔含有早-中奥陶世放射虫化石和早、中寒武世三叶虫化石，说明增生楔形成于晚寒武世-中奥陶世，且原特提斯洋在535Ma之前就已经存在。晚奥陶世–早志留世花抱山组和药水泉组河流相沉积与下伏晚寒武世-早奥陶世海山OPS、正常OPS和蛇绿岩及其北侧同时期岛弧带之间为角度不整合接触，是晚奥陶世-早志留世时期弧-陆碰撞及原特提斯洋盆闭合的沉积响应。区域上，拉脊山和北祁连增生楔分别位于中祁连微陆块南、北两侧，是寒武-奥陶纪时期原特提斯洋在中祁连微陆块南、北两侧同时发生北向和南向俯冲的结果；它们在随后与中祁连微陆块发生弧-陆碰撞，导致俯冲极性反转并发生洋-陆俯冲，于晚奥陶世（~445Ma）发生陆-陆碰撞导致洋盆闭合。这些认识为深化拉脊山古洋盆演化过程以及南祁连构造演化与原特提斯洋之间关系研究提供直接依据。