山西沁水盆地西缘寒武系碳酸盐岩的元素地球化学特征及其古环境意义

Characteristics of elemental geochemistry of the Cambrian carbonate rocks and their palaeoenvironmental implication in western margin of Qinshui Basin,Shanxi Province

山西省沁水盆地寒武系碳酸盐岩出露规模可观,但对其沉积学、地球化学和古地理学方面的研究成果却很少。笔者在野外调查的基础上,对沁水盆地西缘寒武系进行了岩石学和元素地球化学研究。结果表明,研究区寒武系碳酸盐岩主要包括张夏组和崮山组灰岩及三山子组下部白云岩,属浅海台地相沉积,可分出局限台地、开阔台地、台地边缘礁滩3种沉积相类型。自生元素(CaO、MgO)与陆源元素(SiO2、Al2O3和Fe2O3)变化范围较大,垂向上有2次升降旋回,反映沉积环境比较动荡;K2O含量较高,Na2O和Sr含量较低;ΣREE+Y总量较高,变化范围较广,轻稀土富集,重稀土亏损,Ce和Eu呈弱负异常,标准化曲线呈略微左倾-平坦型,符合正常海相沉积的特点;δ^13C值在-1.96‰~0.41‰之间,为低负值到低正值,总体先升高后降低,δ^18O在-9.96‰^-5.33‰之间,总体自下而上呈阶梯状上升。相关地球化学参数指示研究区主要为海相咸水沉积,古海水温度属温暖的亚热带气候环境下的海水温度,总体上由暖到热再转冷,古气候经历了由相对潮湿-相对干热-相对潮湿的转变过程,古水深发生浅-深-浅的变化,古海平面出现了1次大规模的升降。古环境各要素间耦合关系较好,并对沉积相类型具有一定的制约关系。上述成果对沁水盆地及邻区寒武纪岩相古地理研究和油气勘探,乃至对华北克拉通寒武纪古环境、古生物和古生态演化史的重建等具有重要的理论和实际意义。

A set of significant Cambrian carbonate rocks are found in Qinshui Basin of Shanxi Province. On the basis of field investigation,the petrology and element geochemical composition of Cambrian rocks in the western margin of Qinshui Basin are studied. The results show that the Cambrian carbonate rocks are mainly composed of limestone from the Zhangxia and Gushan Formations,and dolostone from the lower part of Sanshanzi Formation. They are mainly shallow platform facies deposits which can be divided into three types: Restricted platform,open platform and platform margin reef and ramp. There are large variation ranges for the authigenic elements( CaO,MgO) and terrestrial elements( SiO2,Al2O3 and Fe2O3). They have two vertical upward and downward cycles,reflecting the sedimentary environment with high turbulence. The content of K2O is high,but the contents of Na2O and Sr are relatively low. The contents of ΣREE+Y are relatively high and vary widely,and the LREE are much richer than the HREE. A slight negative anomaly of Ce and Eu is found. The normalized curves show a slightly leftward-flat trend,which conforms to the characteristics of the typical marine sediments. The δ^13C value is in the range of-1. 96‰ ~ 0. 41‰,showing an overall change of initial increase and then decrease trend. The δ^18O value ranges from -9. 96‰ ^-5. 33‰,showing a ladder-like upward trend from bottom to top. Related geochemical parameters indicate that the study area was a saltwater marine environment. The paleo-seawater temperature belonged to a warm subtropical climate environment,generally changing from warm to hot and then to cold trends. The paleoclimate experienced a transition of relative humidity-relative dry and heat environmen-relative humidity. The paleo-water depth underwent a variation of shallow-deep-shallow. The ancient sea level experienced a large-scale transgression-regression process. A good coupling relationship between different palaeoenvironment elements is found,showing a certain restriction on the types of sedimentary facies. The results in this paper are very significant for the studies of Cambrian lithofacies palaeogeography and petroleum exploration in the Qinshui Basin and the adjacent regions. It provides a significant theory and practical guidance in the reconstruction of Cambrian palaeoenvironment, palaeotology and palaeoecology evolution of the North China Craton.