二连盆地白音查干凹陷下白垩统湖相沸石成因:来自矿物学、微量元素特征的证据

Mineralogical and Trace-Element Constrains on the Genesis of Zeolite in Lower Cretaceous Lacustrine Rocks from Baiyinchagan Sag,Erlian Basin,China

二连盆地白音查干凹陷下白垩统暗色沉积物中发育由白色颗粒构成的同沉积纹层、条带,其矿物组合中包含大量成因不明的钠沸石和方沸石.在薄片鉴定和电子探针(EMPA)分析的基础上,通过场发射扫描电镜(FE-SEM)和电感耦合等离子体质谱仪(ICP-MS)法对两种沸石矿物的形貌、微结构、共生矿物组合和微量元素成分进行分析.实验结果表明,钠沸石和方沸石自形程度较高,其中钠沸石为半自形-自形的棱柱状,长轴方向长度最大可达600μm,集合体为纤维状或块状;方沸石呈自形的四角三八面体,粒径变化较大,介于10～300μm.钠沸石和方沸石既能独立生长,也能互相交代,与重晶石、含铁白云石、菱镁矿和菱铁矿以及立方体黄铁矿等矿物紧密共生.与地壳及同区和同层位湖相泥岩微量元素配分曲线比较,含沸石岩石的全岩微量元素富集Pb、Sb、Tl、W、Mo、Bi,亏损V、Cr、Co、Ni,暗示了中高温含深源物质的碱性热液流体的加入.在综合分析沸石矿物形态结构、共生矿物组合及微量元素特征的基础上,认为在异常热流驱动下,中高温热液流体沿着断裂等运移通道喷出或溢流至湖底沉积物表面,与冷的湖水混合直接结晶沉淀形成方沸石、钠沸石和其他共生矿物,属于热水沉积成因.

Abstact： Thick synsedimentary laminaes and strines consisting of white grains were recently described in Lower Cretaceous lacustrine rocks in Baiyinchagan Sag of Erlian Basin,which include enormous quantities of natrolite and analcime. Based on analysis of thin sections and Electron Probe Micro-anlyzer（ EPMA）,micro-morphology,texture and trace element composition of natrolite and analcime were determined using Field Emission Scanning Electron Microscopy（ FE-SEM） and Inductively Coupled Plasma-Mass Spectrometry（ ICP-MS）. FE-SEM observations show that natrolite comprises subhedral-euhedral prismatic crystals that extend along onedismensional axial with the maximum size of 600 μm and forms fibrous and massive aggregates. Analcime is composed of euhedral tetrakisoctahedron crystals ranging from 10 μm to 300 μm in diameters. In most cases,natrolite and analcime occur together and mutually replace. They coexist with barite,ferriferous dolomite,pyrite,magnesite and siderite as their paragenetic minerals. Compared with crust and mudstone of the same strata in research area,zeolite-bearing rocks contain high values of Pb,Sb,TI,Mo,W,Bi and have a severe depletion in V,Cr,Co,Ni,which suggest hydrothermal fluids including mantle-derived materials involved during sedimentary processes. Mineralogical and geochemical investigations of assemblages of natrolite and analcime suggests a result of direct precipitation during the mixing of hydrothermal fluids and lake water at the bottom of lake related to continental rift,in which faults act as migration pathways and hydrothermal fluids are derived by an abnormal heat convection system.