三塘湖盆地二叠系陆相热水沉积方沸石岩特征及成因分析

摘要

在新疆三塘湖盆地野外露头及钻井中二叠统芦草沟组（P2l）厚层黑色泥岩中发现多层白色纹层状方沸石岩。根据主要造岩矿物特征,将其大致分为两类,第一类为方沸钾长白云岩,主要造岩矿物包括细粒方沸石、泥晶钾长石（透长石、正长石）、泥晶白云石、泥晶铁白云石等,碳酸盐矿物以高的Fe、Mn含量为特征;第二类为方沸硅质岩与硅质方沸石岩,以细粒方沸石及泥晶石英为主,含少量碳酸盐。扫描电镜观察方沸石以自形的四角三八面体少见,多为它形块状或无定形胶体状。样品主量元素以富SiO2、Al2O3、Na2O、贫TiO2为特征,微量元素Th、Nb、Ti强烈亏损,Sr/Nd比值为44.73～831.78,Nb＊介于0.02～0.17之间。∑REE总量偏低7.33～20.619μg/g,LREE/HREE为4.77～11.20,δEu 0.60～0.99略具负异常,δCe 0.96～1.04,基本无亏损,稀土配分曲线为较平缓的右倾。方沸石岩微量元素、稀土元素配分曲线与研究区早、中二叠世玄武岩、粗面岩及上地壳有较大差异。87Sr/86Sr介于0.705 118～0.706438之间,低于同时期全球海水及中上二叠统海相碳酸盐的87Sr/86Sr比值,143PNd/144Nd介于0.512 454～0.512 713之间,εNd介于-0.81～3.84之间,表明造岩热液可能含有幔源组分。通过详细的岩石学、地球化学及Sr、Nd同位素研究,认为这是一类与陆相裂谷盆地湖底热泉喷流作用有关的热水沉积岩组合,热液组分除了有壳源物质及大气水等的混入外,可能还有深部物质的参与。%Santanhu Basin,located in northeast of Xinjiang,China,was developed a contenental rift basin from Late Carboniferous to Middle Permian.On the outcrop and the drilling cores of the basin,layers of sublacustrine hydrothermal deposits from which six of them were sampled,were found in the thick mudstones in Lucaogou Formation（P2l）,Middle Permian.The hydrothermal samples can be divided into two types： the first type is formed of analcime,sanidine,orthoclase,dolomite and ankerite.The carbonates were detected of concentrating iron and manganese.The second type is formed of analcime,quartz,and carbonates.Most analcime was observed to be xenomorphic block or amorphous gelatinoid otherwise from automorphic tetragonaltrisoctahedron under the SEM.Major elements of all samples were characterized as being rich in SiO2,Al2O3,Na2O,and poor in TiO2.Trace element of the samples show enrichemnt of highly incompatibility elements,such as Ba,Rb,K,Ta,Sr,Zr and Hf,and are lack of Th,Nb,and Ti relatively.The ratios of Sr/Nd = 44.73~831.78,Nb＊=0.02~0.17.ΣREE= 7.33 ~20.619 μg/g,LaN/YbN=2.50 ~16.00,LREE/HREE=4.77~11.20.δEu varies from0.6 to 0.99 which shows negative anomalism to primitive mantle,however,δCe value which varies from 0.96 to 1.04 shows the similarity to primitive mantle.The major and trace elements（includes REE） reveal the distinct differences with the basalts and trachytoids of Permian in the same basin,and also disagree with the average value of Upper Crust.87Sr/86Sr = 0.705 118~0.706 438,143Nd/144Nd =0.512454~0.512713,εNd=-0.81~3.84.Isotope composition indicates that mantle materials may participate in the hydrothermal fluid with which the hydrothermal minerals were deposited.By the analysis of the lithology,geochemistry,and Sr,Nd isotope,the characteristics of the samples suggest that they are an assemblage of hydrothermal rocks which are related to the hot springs exhalation at the lake bottom in Santanghu continental rift basin.Besides the Crust and meteoric components,the hydrothermal fluid of the rocks was probabely composed of the substance sourced from deep earth.