甘肃酒西盆地青西凹陷下白垩统下沟组湖相喷流岩物质组分与结构构造*
Compositions and texture of lacustrine exhalative rocks from the Lower Cretaceous Xiagou Formation in Qingxi sag of Jiuxi Basin, Gansu

甘肃酒西盆地青西凹陷下白垩统下沟组广泛发育的暗色纹层状细粒岩,以其富含铁白云石、钠长石、重晶石、方沸石、黄铁矿和有机质,局部含有微量萤石、透闪石、石盐和呈碎屑状、浸染状产出的微量闪锌矿、黄铜矿、方铅矿等矿物组分吸引着众多沉积地质学家和石油地质学家的高度关注。在以往的研究中,大多数研究者认为此类细粒岩的成因与火山活动有关,将其确定为火山物质蚀变作用的产物。通过地质背景、物质组分、岩石结构构造、包裹体和地球化学特征的研究,提出此类细粒岩属于新型的湖相“白烟型”喷流岩,可识别出6种热水矿物组合、3种主要结构类型和4种常见的原生沉积构造。6种热水矿物组合包括: Ⅰ钠长石—铁白云石(或铁白云石—钠长石)组合,Ⅱ重晶石—钠长石—铁白云石组合,Ⅲ石英—重晶石—钠长石—铁白云石组合,Ⅳ石英—方沸石—钠长石—铁白云石组合,Ⅴ地开石—铁白云石组合,Ⅵ单一铁白云石组合。3种主要结构类型包括: ①泥晶—微晶结构,②内碎屑结构,③石盐假晶结构。4种常见的原生沉积构造包括: ⅰ纹层状和条带状构造,ⅱ网脉状充填构造,ⅲ旋涡状喷管结构,ⅳ同生变形层理结构。在物质组分和岩石结构分析的基础上,划分出网脉状充填型、水爆型、盆地沉积型和区域扩散型4种喷流岩产状类型,建立了下沟组湖相喷流岩分带性沉积模式,探讨了喷流岩研究意义。
The widely developed laminated, fine-grained sedimentary rocks of the Lower Cretaceous Xiagou Formation in Qingxi sag of Jiuxi Basin, Gansu Province are rich in ankerite,albites,barites,zeolites and pyrites,and are poor in fluorites,tremolites,halites and clastic chalcopyrites,galenas and sphalerite. The hydrothermal minerals and favorable hydrocarbon conditions have been attracting the attention of many sedimentary geologists and petroleum geologists. In previous studies,most geologists believed that these fine-grained sedimentary rocks were associated with the volcanic activity,or were the products of volcanic material alteration. However, through a systematic study on the geological background,compositions,rock textures,rock structures,inclusions and geochemical features,we believed that these fine-grained sedimentary rocks were the lacustrine white-smoke type exhalative rocks. Six kinds of hydrothermal mineral assemblages were identified,including: Ⅰ albites-ankerites(or ankerites-albites),Ⅱ barites-albites-ankerites,Ⅲ Analcime-albite-ankerites,Ⅳ quartz-albite-ankerites,Ⅴ dickite-ankerites and Ⅵ the single ankerite. Three main types of rock textures were identified,including: ① the microcrystalline textures,②the intraclastic structure and ③ the halite pseudocrystal textures. Four common types of primary sedimentary structures were divided,including: ① the laminated and banded structures,② the network vein filling structures,③ vortical nozzle structures and ④ the syngenetic deformation bedding structures. On the basis of the rock composition,textures and structures,we have identified four