塔里木盆地柯坪地区上寒武统表生溶蚀型藻格架白云岩的地球化学特征及意义

Geochemistry characteristics and their implications of hypergene dissolved algal framework dolostone in the Upper Cambrian in Keping area,Tarim Basin

采用野外观察、室内薄片鉴定及多项地球化学分析技术方法,对塔里木盆地柯坪地区上寒武统表生溶蚀型藻格架白云岩的特征及成因进行了研究。宏观上,藻格架白云岩呈丘状、透镜状夹于潮坪相白云岩层间,由于差异性溶蚀,发育了大量表生溶蚀孔。微观上,藻格架由富藻的泥粉晶白云石组成,而架间孔由浅色的亮晶白云石充填。藻格架泥粉晶白云石呈他形—半自形,镶嵌结构,具有暗红色—橙红色的阴极发光,较高的Na、K含量,较低的Fe含量;δ13C为-0.572‰～0.124‰、平均值-0.116‰,δ18O为-5.391‰～-4.983‰、平均值-5.240‰,表明其形成于准同生阶段盐度较高的相对氧化环境中。架间充填的亮晶白云石胶结物,呈半自形—自形中细晶,具有昏暗的阴极发光或者不发光,较低的Na、K含量,较高的Fe含量,δ13C值为-0.662‰～-0.251‰、平均值为-0.460‰;δ18O值为-6.639‰～-5.939‰、平均值-6.267‰,表明其形成于相对还原的埋藏环境。稀土元素分析结果表明,二者均具有LREE轻微富集、HREE亏损、Eu负异常特征,与海相泥晶灰岩稀土元素配分模式相似,揭示了其白云化流体均来自于原始的海水。在溶蚀作用方面,亮晶白云石胶结物相对泥粉晶白云石藻格架更易于溶蚀。前者在大气水表生溶蚀过程中,主量元素Ca、Mg丢失显著,Mg/Ca值由0.955降至0.007,微量元素Na、K丢失相对明显,Na/Ca值由原来的3.8×10-4降为1.9×10-4,K/Ca值由1.1×10-3降至检测限以下,而不改变稀土元素的配分模式。这些特征表明,表生溶蚀过程在元素特征上是一个去白云化的盐度降低过程,而这一过程中基本无稀土元素的带入带出。

Based on field investigation, petrographic observation, and related geochemical analysis, the characteristics and genesis of the hypergene dissolved algal framework dolostone in the Upper Cambrian in Keping area, Tarim Basin, were studied. In macro-scale, hummocky and lenticular algal reef dolos- tone sandwiched between the tidal-flat dolostones. A large number of supergene dissolution pores devel- oped due to the differential dissolution. In micro-scale, the rocks are mainly composed of dark mud-silt- sized dolomite enriched in algae and light-colored sparry dolomite cement. Mud-silt-sized dolomite has subhedral-anhedral shaped mosaic structure, and display dark-orange red luminescence. They have high concentration of Na and K, but lower concentration of Fe. The δ13C ranges between -0. 572‰ and 0. 124‰, with an average value of-0. 116‰; the value of 8180 ranges between -5.391‰ and -4. 983‰, with an average of -5. 240‰ These characteristics indicate that the mud-silt-sized dolomite was formed in contemporaneously stage under relatively oxidizing and higher salinity conditions. The spar- ry dolomite cements filling between algal framework, are anhedral-subhedral shaped and medium-coarse crystalline sized. They show dark cathodolumineseence or non-luminous. The contents of Na and K are low, whereas the content of Fe is high. δ13C values range between -0. 662‰ and -0. 251‰, with an av- erage value of -0. 460‰; δ18O values range between -6. 639‰ and -5. 939‰, with an average value of -6. 267‰. These data indicate that the sparry dolomite cement formed in a relative reduction environment during burial stage. Both mud-silt-sized dolomite and sparry dolomite cements share similar REE patterns with finely crystalline limestone： LREE are enriched, HREE are depleted, and there are Eu negative a- nomalies. It suggests that both of them derived from seawater dolomitization fluid. In dissolution aspect, sparry dolomite cements are more easily dissolved than mud-silt-sized algal framework dolomite. In the su- pergene dissolution process, the major elements, Ca and Mg of the sparry dolomite cements, decreased significantly, and the Mg/Ca value declines from 0. 955 to 0. 007. Trace elements of Na and K were lost obviously. The Na/Ca value declines from 3.8×10-4 reduced to 1.9×10-4, K/Ca value declines from 1.1 ×l0-3 to below the detection limit. Distribution pattern of rare earth elements were not changed. These features suggest that hypergene dissolution process is dedolomitization with salinity decreases, and there were nearly no gains and losses of rare earth elements.