准噶尔盆地东部中二叠统幔源热液沉积白云岩*

准噶尔盆地东部吉木萨尔凹陷中二叠统芦草沟组,沉积于陆内裂谷背景下岩浆—热液活动强烈的咸化湖盆中。在芦草沟组中发现一类与湖底热液活动有关的幔源热液沉积白云岩,包括方沸钠长白云岩、硅质白云岩与含水铵长石钠长白云岩。作者对该类白云岩开展微米级尺度的岩相学研究,结合同位素地球化学研究,探讨成矿流体性质与详细成因机理。岩矿特征和同沉积变形构造指示白云岩形成于沉积期。白云石为有序性较差的原白云石,呈微米级球状、葡萄状晶体,与方沸石、水铵长石、钠长石及玉髓等热液矿物彼此镶嵌结晶,高温下发生了白云石对钠长石碎屑的交代作用;白云岩锶同位素(平均为0.705 687)显示成矿流体中幔源组分较多;白云岩具有重δ¹³ C_PDB (平均为6.94‰)和轻δ¹⁸ O_PDB (平均为-8.12‰)。基于成矿流体由幔源热液与湖水混合的假设,利用δ¹⁸ O_PDB计算估计,研究区白云岩的形成温度较普通云质岩高25~50 ℃。研究表明白云石为幔源热液喷出湖底后直接沉淀形成,并提出可能的成因模式。热液沉积白云岩是一种不可或缺的白云岩成因类型,研究成果对促进人们理解白云石的形成规律、推动完善成因岩石学理论具有重要意义。

Mantle-originated hydrothermal-sedimentary dolostone in the Middle Permian in eastern Junggar Basin,China

The Middle Permian Lucaogou Formation in Jimusar sag,eastern Junggar Basin,China,deposited in a salt-lake within an intracontinental rift basin,in which magmatic-hydrothermal activities were intensive. Mantle-originated hydrothermal-sedimentary dolostone in lake bottom are discovered. Three types of dolostones are found,including analcime-albite dolostone,silicic dolostone and buddingtonite-albite dolostone. The ore-forming fluid characteristics and detailed formation mechanism are discussed based on micrometer-scale petrographical and isotopic geochemical research. Both petrographic feature and syndepositional deformation have indicated these dolostones are formed in a depositional stage. The dolomites are relatively poor-ordered proto-dolomites,with micron-sized spherical or aciniform morphology,and coexist with hydrothermal minerals that include analcime,buddingtonite,albite and chalcedony. Albite clasts were replaced by dolomites under high temperatures. The remarkably low strontium isotopic composition of the dolostones(av. 0.705687)suggests abundant mantle-originated material in the ore-forming fluid. The dolostones have positive δ¹³ C_PDB values(av. 6.94‰)and negative δ¹⁸ O_PDB (av.-8.12‰). The calculated formation temperature of the dolomites by using δ¹⁸ O_PDB is about 25-50 ℃ higher than that of the dolomitic rocks in Lucaogou Formation,based on the assumption that the fluid is the mixture of lake water and mantle-originated hydrothermal fluid. We conclude a possible genetic model that the dolomites precipitated directly from the mantle-originated hydrothermal fluid once it erupted to the lake bottom. We argue that the hydrothermal-sedimentary dolomite is an indispensable genetic type and this study is of great significance for both deciphering the formation rules of dolomite and supplementing the theory of petrogenesis.