新疆三塘湖薄层状岩浆-热液白云质喷流沉积岩

Thin Bed Magmatic-hydrothermal Dolomitic Exhalative Sedimentary Rocks in Santanghu Basin,Xinjiang

新疆三塘湖地区的晚石炭世—中二叠世发育陆内裂谷型盆地,主要发育形成于欠补偿湖盆背景下的湖相碳酸盐岩与凝灰质泥岩互层的纹层状沉积岩,局部夹有火山碎屑岩以及热液喷流岩夹层。选取其中一类特殊的、以白云石和蒙脱石为主要造岩矿物构成的夹层为研究对象,通过岩石学、矿物学以及地球化学探讨其形成机理。研究结果表明,其矿物成分单一,形态自形并部分具有港湾状熔蚀的构造,显示火成岩特征。岩石整体呈薄层状,层内无明显的成层性,代表高密度流体中的沉积物快速混杂堆积。由大量细小矿物组成的斑状团块结构,则代表了在高温、高密度的碎屑流体中形成的次生火山泥粒。地球化学结果展现出幔源为主,并被壳源流体混染的复杂过程。综合分析认为,该类岩石为碳酸岩水下火山喷发作用生成的高密度碎屑流体沉积形成,并且被同期或较晚期热液改造后的一类特殊沉积岩。

The Santanghu area is an intracontinental starved lacustrine rift basin from later Carboniferous to the middle Permian,and is mainly composed of interlaminated carbonate rocks and tuffaceous shale with little intervals of pyroclastic and hydrothermal exhalative rocks. This paper focuses on one special interval type which mainly contains of dolomite and smectite,and research for its forming mechanism on the basis of petrology,mineralogy and geochemistry.The simple mineral composition and euhedral crystal with partly embayed boundaries suggest an igneous origin. The rock is thin-bedded without inner stratified structure,indicating fast and mixed deposition processes from a density flow. Aggregates formed by tiny minerals may be caused by accretionary lapilli formed in clastic flow with high density and high temperature. Finally,features of geochemistry show a mainly mantle-derived origin with mixed crustal material. Comprehensively,this type of rock is deposited from subaqueous high-density pyroclastic flow fed by carbonatite eruptions,and altered by contemporaneous or later hydrothermal fluid.