柴北缘托莫尔日特滩间山群碎屑岩组的沉积环境与构造背景分析

Sedimentary environment and tectonic setting of the clastic formation of the Tanjianshan Group in the Tuomoerrite area, North Qaidam

柴北缘构造带断续出露的寒武-奥陶纪滩间山群火山-沉积岩系是早古生代时期原特提斯洋俯冲造山作用的产物,也是中国西部块状硫化物和造山型金矿床的重要含矿岩系。前人已对滩间山群火山岩组开展了大量的地球化学和同位素年代学研究,而碎屑岩组的沉积序列和沉积相研究相对薄弱。我们在柴北缘托莫尔日特地区地质填图过程中,对该地区的滩间山群碎屑岩组开展了系统的沉积组合序列和沉积相研究。托莫尔日特地区滩间山群碎屑岩组是由富含火山碎屑物质的含砾砂岩、砂岩、粉砂岩、泥岩、沉凝灰岩、硅质凝灰岩、硅质岩及少量砾岩共同组成。可分为下岩性段和上岩性段,其中下岩性段为一套形成于海底扇外扇环境以沉凝灰岩为主的沉积组合;上岩性段包含海底扇内扇、中扇和外扇沉积,且以中扇环境的沉积为主。垂向上,它们呈现出粒度向上逐渐变细特征;空间上,具有向北西方向火山物质成分逐渐增多、砾岩和砂岩厚度变薄且砾岩消失的变化特征。砂岩主要为岩屑长石杂砂岩,其中岩屑主要是安山岩和少量玄武岩、英安岩、凝灰岩;长石主要为斜长石;石英碎屑相对缺失。砾岩中砾石以安山岩和硅质岩为主,并含有少量灰岩和凝灰岩砾石。底冲刷面、正粒序、平行层理和波纹层理等沉积构造普遍发育,呈现出典型的浊流沉积特征;同时在局部露头见有滑塌构造和波痕。区域上,这套沉积组合序列位于寒武-奥陶纪岛弧火山岩的南侧;古水流分析显示,它们的碎屑物质主要来自于其北东方向。这些结果表明,柴北缘构造带东段的滩间山群碎屑岩组是一套成熟度极低且与寒武-奥陶纪岛弧火山活动密切相关的沉积组合。

Volcano-sedimentary system of the Cambrian-Ordovician Tanjianshan Group in the northern margin of Qaidam basin was formed by the subduction of the Proto-Tethys Ocean during Early Paleozoic and hosted massive sulfides and orogenic gold deposits in western China.Much attention had been paid to geochemistry and isotopic dating of the volcanic Formation of the Tanjianshan Group,but the sedimentary sequence and facies of the associated clastic rocks received less care.Systemic studies on the sedimentary sequence and facies in the processes of the geological mapping demonstrate that the clastic Formation in the Tuomoerrite area consists of pebbly sandstone,sandstone,siltstone,mudstone,tuffite,siliceous tuff,chert and minor conglomerate.This formation can be subdivided into two lithologic units.The lower unit is mainly dominated by tuffite deposited in the lower submarine fan environment,whereas the upper unit is deposited in the upper,mid,and lower fan environments.In contrast,the rock assemblages of the upper unit are mainly composed of the mid-fan deposits.The upper unit is characterized by fining-upward sequence and increasing abundance of volcanic composition northwestward,but the thickness of sandstones decreases and conglomerate disappears to northwest.Sandstones mainly comprise lithic greywacke.Lithic fragments contain andesite,basalt,dacite,and minor tuff and limestone.Feldspar fragments are dominated by plagioclase grains,and less monocrystalline quartz grains with an igneous origin occur in several samples.Clasts of conglomerates are dominated by angular andesite,chert and minor limestone and tuff.Irregular basal erosional surface,normal grading,parallel laminations and ripples are widespread show typical of turbidite.Slumping folds and wave marks can also be observed in the local outcrops.Regionally,volcano-clastic rocks distributed in the southern margin of Cambrian-Ordovician Keke island arc with a southwestward paleocurrent.These results demonstrate that the clastic formation is closely related to the Cambrian-Ordovician island arc.