班公湖-怒江洋早白垩世构造演化：来自复理石沉积的制约

The Bangong-Nujiang suture zone (BNSZ) is an important tectonic boundary and metallogenic belt in the Tibetan Plateau, and its Early Cretaceous evolution has always been concerned by most geologists. In recent years, as the discovery of large amounts of mineral resources and the ore-forming period was determined to be Early Cretaceous, the Early Cretaceous evolution of the Bangong-Nujiang Ocean (BNO) has been one of the focuses of the research on the BNSZ and the Tibetan Plateau. However, different models for the Early Cretaceous evolution of the BNO have been proposed by different scholars based on various volcanic rocks and a few littoral-neritic sedimentary deposits. The bathyal to abyssal flysch deposits in the BNSZ are poorly researched, although they are important for restoring the evolution of the BNO. Based on years of field research, applicants firstly identified a typical Early Cretaceous flysch deposit in the BNSZ. This project is intending to carry out research on the clastic rocks and volcanic rocks within this Early Cretaceous flysch deposit, and comprehensively use multidisciplinary research methods such as petrology, stratigraphic sedimentology, whole rock geochemistry and isotopic chronology. Our aim was to analysis their tectonic background and tectonic environment when they deposited, and identify the provenance of their clastic rocks. Finally, combined with geological data, we will restrict and restore the Early Cretaceous evolution of the BNO from the perspective of sedimentology.

班公湖-怒江缝合带作为青藏高原重要的构造界线和成矿带，其早白垩世构造演化历来备受关注。尤其是近年来，随着大量矿产资源的陆续发现和成矿期被确定为早白垩世，班公湖-怒江洋早白垩世构造演化也成为了该条缝合带，乃至整个青藏高原研究的热点之一。然而，依据不同类型的岩浆岩和部分滨浅海相沉积岩等证据，不同学者提出了班公湖-怒江洋不同的早白垩世演化模型。缝合带内的次深海-深海复理石沉积，尽管对恢复大洋演化具有重要意义，但研究程度较低。通过多年的野外考察，申请者首次在缝合带中段厘定出具典型鲍马层序的早白垩世复理石沉积。本项目拟通过对该套早白垩世复理石沉积的碎屑岩和火山岩夹层等开展详细研究，综合运用岩石学、地层沉积学、全岩地球化学、同位素年代学等研究手段，明确其沉积时的构造背景和环境，查明其碎屑岩的物源区，最终结合区域地质资料，从沉积学角度约束和反演班公湖-怒江洋早白垩世时期的构造演化。

班公湖-怒江缝合带是青藏高原中部重要的构造界线和成矿带，对它的深入研究，不但可以为恢复和反演特提斯构造域的演化以及青藏高原早期的形成和发展提供重要依据，还可以寻找更多储备矿产，服务国民经济。尽管自20世纪80年代以来，前人已经对该条缝合带做了大量的研究，但有关班怒带的很多地质问题仍在争论之中，尤其是其所代表大洋的闭合时限。本项研究对扎嘎、麻布加错、多昌以及卡玛等地区复理石砂岩和火山岩夹层的锆石U-Pb定年结果，确定了班公湖-怒江缝合带中西段早白垩世阿普特期复理石（约126-113Ma）的存在。在空间上，阿普特阶复理石的露头范围从东向西跨度至少300公里。复理石的浊积相与物源分析工作表明，阿普特期复理石岩石组合上主要与典型浊积相中的C（朵叶相），D（朵叶前缘相），部分G（深海平原相）和 F （斜坡相）对应；而这些阿普特期复理石主要是早期增生楔再旋回的沉积产物，接受了少量同期的南羌塘南缘弧火山物质，这些浊积相和物源特征可以和海沟环境中的纵向海底扇相对比。通过野外构造变形特征的研究，这些阿普特期复理石经历了俯冲期的构造变形，与蛇绿岩和放射虫硅质岩等构造混杂在一起构成增生杂岩体。这一构造混杂现象在麻布加错地区最明显，多昌以及卡玛虽然没有强烈的构造混杂特征，但复理石岩层严重弯曲变形，内部发育大量俯冲期的褶皱和逆冲断层等增生楔内部小构造系统。.早白垩世阿普特期海沟沉积的复理石的发现，是相继前人报道了早白垩世中晚期蛇绿岩、洋岛及放射虫硅质岩等直接的洋壳物质之后的又一重要证据，该复理石及其火山岩的存在，明显指示了至少在早白垩世阿普特期班公湖-怒江洋仍在向北俯冲，此时洋盆尚未闭合。