一类新型沉积岩:地幔热液喷积岩——以中国新疆三塘湖地区为例

A New Type of Sedimentary Rocks: Mantle-Originated Hydroclastites and Hydrothermal Exhalites,Santanghu Area,Xinjiang,NW China

中国西部新疆三塘湖地区的二叠系芦草沟组是一套以內源碎屑为特征的新型沉积组合—地幔热液喷流沉积岩(简称喷积岩)。该套岩石呈毫米级纹层状,沉积厚度大,岩石类型多样,是地球深部岩浆-热液流体脉动式喷流沉积的累积物,表现出富含幔源热液矿物和岩浆微屑的特点。按物质来源、形成方式和结构构造可划分为:以岩浆喷发爆破的岩浆碎屑为特征的喷爆岩;以岩浆溢流显示岩浆矿物及结构构造为特征的喷溢岩;以热液喷流显示"黑、白烟囱"型矿物组合为特征的喷流岩;以岩浆矿物碎屑、热液喷流物质和湖水均匀混合沉积为特征的喷混岩以及与热液活动相关的嗜热嗜毒生物岩。该套岩石组合形成于陆内裂谷欠补偿深水湖盆中。

The Lower Permian Lucaogou Formation in Santanghu area, Xinjiang, NW China contains new types of sedimentary deposits mantle-originated hydroclastites and hydrothermal exhalites, which are characterized by a subsurface source. The rocks occur as mm-laminae with a great total thickness and have a variety of composition, texture, and structures. They contain abundant mantle-originated hydrothermal minerals and microscopic hydroclastic magmatic fragments, and can be classified into five types on the basis of sediment source and grain composition, sedimentary texture and structure, and interpreted forming mechanisms： 1） fall-out phreatomagmatic hydroclastite, formed by fall out of fine magmatic ash generated by hydroclastic explosion; 2） surge phreatomagmatic hydroclastite, formed by base surge of coarse magmatic fragments generated by hydroclastic explosion; 3） hydrothermal exhalite, formed by chemical precipitation from hydrothermal exhalative fluids and containing typical minerals associated with ＂black and white smokers＂; 4） peperite composed of admixtures of hydroclastic, hydrothermal, and normal lacustrine chemical sediments; and 5） microbialite composed of biogenic minerals produced by thermophilic and toxin-loving microbes. All these rocks were deposited in a starved lake in an intracontinental rift basin. The grain composition suggests that hydroclastic and hydrothermal processes were originated from a great depth and related to mantle-derived magmas. The interlamination of these rocks suggests that hydroclastic, hydrothermal, and normal lacustrine processes were alternating and episodic.