神农架地区中元古界混合沉积特征及其发育模式

神农架地区中元古界主要发育碳酸盐岩沉积,局部夹少量陆源碎屑岩。乱石沟组、大窝坑组、矿石山组及台子组位于中元古界中部,混合沉积较为发育,其混合沉积类型及其主控因素尚缺乏系统研究,对陆源碎屑来源及台子组石英砂岩形成环境仍存在较大争议。根据岩石类型和沉积构造组合对神农架地区乱石沟组—台子组沉积相进行分析,并结合岩石薄片及阴极发光分析结果对混合沉积类型、陆源组分特征及来源进行研究,以明确中元古代神农架地区沉积背景及其沉积演化历史。岩石薄片及阴极发光分析表明,神农架地区乱石沟组—台子组陆源碎屑主要来自由大量花岗岩、少量变质岩和沉积岩构成的古陆。乱石沟组—台子组岩石组合特征及混积岩类型存在较大差异,沉积环境、海平面变化及构造活动是控制混积岩分布及混合沉积类型的主要因素。乱石沟组及大窝坑组沉积早—中期,神农架地区毗邻古陆,扬子北缘构造活动相对稳定,地形平缓且水深较浅,以广泛发育“相混合”式混合沉积为特征；大窝坑组沉积晚期及矿石山组沉积早期,扬子北缘开始进入强烈拉张时期并导致相对海平面上升,神农架地区与古陆拉分形成相对独立的碳酸盐台地,以发育缺乏陆源碎屑砂的清水碳酸盐沉积为特征；矿石山组沉积晚期及台子组沉积时期,扬子北缘的持续拉张导致神农架地区进入较深水环境,早期滨岸沉积物崩塌形成重力流搬运至神农顶一带,形成“间断式”混合沉积。乱石沟组—台子组沉积时期处于中元古代重要的构造演化阶段,台子组沉积之后,神农架地区彻底与古陆分隔,不再发育“相混合”式混合沉积,乱石沟组—台子组沉积时期为扬子台地北缘由窄大洋向开阔洋盆演化的重要时期。

Characteristics and Development Model of Mesoproterozoic Mixed Deposits in Shennongjia Area: An example from Shennongding section of Luanshigou Formation-Taizi Formation

The Mesoproterozoic in the Shennongjia area mainly developed carbonate deposits with a small amount of local terrigenous clastic rocks. The Luanshigou, Dawokeng, Kuangshishan and Taizi Formations in the mid-Mesoproterozoic contain relatively mixed deposits. To date, little or no systematic research has been conducted on the mixed sediment types and their main influences. Major controversies continue regarding the source of terrigenous clastics and the formation environment of the quartz sandstone of the Taizi Formation. The present study analyzed the sedimentary facies of the Luanshigou and Taizi Formations in the Shennongjia area based on rock type and sedimentary structure. A combination of rock flakes and cathodoluminescence （CL） analysis was used to investigate the mixed sedimentary types and the characteristics and sources of the terrigenous component to clarify the sedimentary background and sedimentary evolution history in the Mesoproterozoic. This analysis showed that the terrigenous detritus was mainly generated from ancient continents, and comprises a large amount of granite with some metamorphic and sedimentary rocks. The assemblages and mixture types in the rocks of the Luanshigou and Taizi Formations are very different, with sedimentary environment, sea level change and tectonic activity being the main influences on the distribution and types of sediment mixes. In the early to middle sedimentary period of the Luanshigou and Dawokeng Formations, the Shennongjia area was adjacent to the ancient land, and tectonic activity was relatively stable at the northern margin of the Yangtze Craton, with gentle terrain and shallow water as indicated by the extensive development of mixed-facies-type deposits. In the late depositional period of the Dawokeng Formation and early depositional period of Kuangshishan Formation, the northern margin of the Yangtze Craton began to enter a period of intense extension, leading to a rise in sea level. The Shennongjia area and the ancient continent were separated into distinct carbonate platforms, as evidenced by clear-water carbonate deposits lacking terrigenous clastic sand. In the late sedimentary period of the Kuangshishan and Taizi Formations, the continuous extension at the northern margin of the Yangtze Craton led to the environment of the Shennongjia area entering a relatively deep-water phase. Early coastal sediments collapsed to form gravity flows to the top of the Shennongjia area, forming intermittently mixed deposits. The sedimentary period of the Luanshigou and Taizi Formations was a significant tectonic evolutionary stage of the Mesoproterozoic. Following the deposition of the Taizi Formation, the Shennongjia area was completely separated from the ancient land with the result that mixed-facies deposition no longer took place. The sedimentary period of the Luanshigou?Taizi Formations was an important period of evolution from a narrow sea to an open ocean basin at the northern margin of the Yangtze Craton.