班怒带两条主要蛇绿岩带之间成因联系：来自沉积记录的约束

The Bangong-Nujiang suture zone (BNSZ) is an important tectonic belt in central Tibet and previous studies have been mainly focused on petrology, geochemistry, and geochronology of the scattered ophiolitic fragments from the suture zone. However, as compared to other classic suture zones, sedimentary records and their coupling relationships with the suturing processes are less well-studied, which is an important cause of the controversies around the BNSZ evolution. One of the controversies is related to the genetic relationship between the two major ophiolitic mélange belts (Bangonghu-Dongco-Dingqing and Shiquanhe-Laguoco-Namco ophiolitic belts) occurring along the suture. Herein, this proposed project aims at providing critical constraints on the issue by carrying out following studies on sedimentary records relevant to the two ophiolitic belts (Dongco and Laguoco) in Gaize, Tibet: (1) Reconstructing sedimentary successions and stratigraphy for the coherent strata associated with the ophiolites, interpreting depositional environments, and analyzing spatial-temporal evolution of sedimentary basins and corresponding tectonic regimes; (2) Studying sedimentary characteristics and lithologic assemblages of sedimentary blocks from the ophiolitic mélanges and reconstructing ocean plate stratigraphy; (3) Conducting provenance analysis of the sediments with special attention to the ophiolitic provenance, in order to understand the genesis and emplacement mechanism of the ophiolite; (4) Comparative analyses of sedimentary records of the two ophiolitic mélange belts ( i.e., provenance, basin evolution and ocean plate stratigraphy), in order to clarify their genetic relationships. The foreseen results are also of significance for understanding subduction processes related to the demise of the Bangong Tethyan ocean and evaluating the role of the Bangong-Nujiang ocean in the Tethyan ocean evolution.

对于班怒带的许多重要认识均来自于蛇绿岩岩石学、地球化学及年代学研究工作。然而，与许多经典缝合带相比，对于沉积记录及其与洋盆演化过程之间的耦合关系研究仍是薄弱环节，这是班怒带研究存在诸多争议的重要原因之一。争议之一为班怒带两条主要蛇绿岩带（班公湖-洞错-丁青和狮泉河-拉果错-纳木错蛇绿岩带）之间的成因联系。针对此争议，本项目计划对这两条蛇绿岩带在改则地区的相关沉积记录开展研究：（1）重建蛇绿岩带相关有序地层与混杂岩沉积残片的沉积序列，分析盆地性质及时空演化过程，反演沉积记录对班怒洋盆演化各阶段的响应机制；（2）开展沉积物源分析，重点关注蛇绿岩物源信息，为蛇绿岩就位时代和机制提供具体依据；（3）对比分析两条蛇绿岩带相关沉积记录异同并厘清二者成因联系。研究成果可望为在相关地区进一步开展深部构造-岩浆作用与沉积作用的耦合研究提供参考，并为理解班怒洋在特提斯洋演化过程中的地位提供重要时空断面。

基于前期研究基础，在2019-2022年期间对班公湖怒江缝合带西段（改则-狮泉河地区）典型蛇绿混杂岩开展了精细的研究工作。基于野外地质填图、沉积学、地层学和构造地质相关数据收集，以及室内研究工作（岩相学、年代学、岩石地球化学等），本项目基本完成预期研究内容并实现了设定的研究目标。项目示例性的建立了班怒带改则-洞错地球上三叠统-侏罗系综合地层剖面（包括有序地层及复原自蛇绿混杂岩的洋壳地层层序），系统分析了其所对应的沉积环境、盆地性质的时空演化特征。结合沉积物源分析成果，复原各构造单元及沉积盆地在班怒洋盆中所处构造位置。通过分析不同阶段沉积盆地性质及其所反映的相对海平面变化特征和构造背景，结合区域地质事实，反演了班怒洋晚三叠世-早中侏罗世时期俯冲消减过程的时空演化。相关研究结果为解释项目所提出的关键科学问题“班怒带主要蛇绿混杂岩带之间的成因关系“提供了确切的解释，即，北侧的日土-改则-洞错蛇绿混杂岩带与南侧的狮泉河-拉果错蛇绿混杂岩带并非起源于同一蛇绿岩体的异地推覆体，而是班怒洋俯冲过程中不同时间阶段的俯冲增生产物。项目研究成果为研究班怒带以及青藏高原形成过程提供了非常详实的研究资料及关键认识。通过本项目的开展，培养博士生、硕士生及本科生10余人；并发表论文12篇，其中SCI论文9篇（包括Geophysical Research Letters, Gondwana Research等重要期刊论文），EI论文1篇，中文核心2篇，会议摘要2篇。参加国际会议三人次，其中两人次参加2019年GSA年会的分会场报告，一次为2022年冈瓦纳年会邀请大会报告。基于项目也可望在“班公湖-怒江成矿带找矿勘查”及“区域地震地质灾害防治”等方面形成一定的社会实践应用和成果转化。