Analysis of petrological characteristics and origin of siliceous rocks during the earliest Cambrian Yurtus Formation in the Aksu area of the Tarim Basin in Northwest China

硅质岩成因涉及硅质来源、沉积环境、形成方式以及成岩阶段等问题,直接的岩石学证据在解决硅质岩成因类型中往往能够起到决定性作用.塔里木盆地下寒武统玉尔吐斯组底部硅质岩段岩石组合类型复杂多样,沉积现象丰富,其成因至今无法达成一致.为了揭示该套硅质岩的岩石学特征及沉积环境,笔者对3条野外剖面(库勒、肖尔布拉克西沟西、于提希)进行了详细的测量、描述,并系统观察该套薄层状硅质岩在偏光显微镜、阴极发光显微镜、扫描电子显微镜下的结构特征.综合前人研究,同时结合玉尔吐斯组硅质岩中非硅质成分(残余矿物、伴生矿物、填隙物以及混入物成分)、结构组分特点,笔者分别提出了中观尺度下(露头)“颜色-构造-成分”综合分类方案以及显微镜尺度下“成分-结构组分-成因”综合分类方案,并在宏观尺度下将硅质岩段分为5段,中观尺度下分为6种岩石类型,显微镜尺度下分为7种岩石类型.早寒武世早期,南天山洋初始裂陷,位于其南部的阿克苏地区遭遇大规模海侵,同时由于海底扩张作用,热液活动频繁,为微生物繁盛奠定了物质、温度和空间基础.富硅质网状脉和上覆薄层状硅质岩的垂向叠置关系表明硅质岩在台内裂洼槽带至内-外缓坡带由幕式低-中温、富硅-磷-钡-钒的海底热液系统以硅质充填下伏网状裂缝、大范围的喷流沉积以及交代原始沉积物等方式形成.笔者认为该套薄层状硅质岩的形成并非由一种成因主导,而是多种成因相互协调、共同作用的结果.同时,微生物群落的聚磷作用以及对(成岩)重晶石形成的促进作用进一步证实了微生物活动在硅质岩的形成过程中具有重要意义.The origin of siliceous rocks is related to the silicon source,deposition environment,formation mode and diagenetic stage.In addition,direct petrological evidence often plays a decisive role in the determination of the siliceous rock formation types.The siliceous rock member of the lowermost Cambrian Yurtus Fm.,in the Aksu area of the Tarim Basin in Northwest China shows various complex rock combination types and abundant sedimentary structures,though the cause of this complexity remains unclear.In order to reveal the petrological characteristics and sedimentary environments of the siliceous rocks,the authors made detailed measurements and descriptions of three outcrops (Kule Sec.,Xiaoerbulake Xigouxi Sec.,and Yutixi Sec.),and systematically observed the rock texture characteristics of the Yurtus Fm.bedded chert under polarizing microscope,cathode luminescence microscope and scanning electron microscope (SEM).Based on the previous research,combined with the characteristics of non-silica constituent (residual minerals,associated minerals,matrix and mix-in components) and structural components in the Yurtus Fm.siliceous rocks,the authors put forth a comprehensive classification scheme of "color-structure-composition" on the mesoscale and "component-structure-origin" on the microscope scale.In the Early Cambrian,the Aksu area suffered a large-scale transgression during the initial stages of the southern Tianshan Ocean formation;at the same time,hydrothermal activities happened with increasing frequency due to seafloor spreading,laying the foundation for microbes to flourish under the favorable material,space and temperature conditions.The vertical stacking relationship between the silicadominant vein swarms and overlying bedded chert suggests that the siliceous rocks were formed (from the fault depression sag to inner-outer ramp) by an identical episodic low-middle temperature,Si-P-Ba-V-rich subseafloor hydrothermal fluid conduit system in the way of vein intrusion,widespread venting sedimentation and replacement of primary deposits.The authors believe that the thin bedded silica rocks do not have a single origin,rather they formed through the coordination and interactions of various causes;and microbial activities played an indispensable role in the formation of the silica rocks,as evidenced by their contribution to phosphorus enrichment and promotion of (early diagenesis) barite formation during the metallogenic process.中国石油勘探开发研究院院级项目5245-2642