塔里木盆地柯坪地区中寒武统准同生白云岩孔洞充填物期次、地化特征及形成条件*

对塔里木盆地柯坪地区蓬莱坝剖面中寒武统沙依里克组萨布哈白云岩及其溶孔中的多期次充填物进行了岩石学和地球化学分析，以对其沉淀时的流体环境进行恢复。孔洞中的环带状充填物可分为4期，由白云岩向内依次为：（1）乳白色粒状方解石；（2）铁质浸染的杂色柱状方解石；（3）无色透明的石英；（4）紫色粒状萤石。碳氧同位素、微量元素及稀土元素分析结果表明：白云岩具有高的Sr/Ba值5.47，低的U/Th值0.2954（明显小于0.7），高的Zr/Hf值36.86，相对高的稀土总量，相对高的 δ¹⁸O值-7.444‰，盐度指数Z值为120.56，表明它形成于相对低温、富氧和盐度相对较高的流体环境，进一步证实其是形成于蒸发环境的准同生白云岩。第1期方解石具有低的Sr/Ba值2.33，相对高的U/Th值1.3，低的Zr/Hf值20.857，相对居中的稀土总量，相对低的δ¹⁸O值-13.758‰，盐度指数Z值为105.35（小于120），表明它形成于温度相对较高、缺氧、盐度较低的流体条件下。第2期方解石具有具有相对低的Sr/Ba值2.77，相对高的U/Th值1，相对低的Zr/Hf值32.08，相对居中的稀土总量，相对低的 δ¹⁸O值-11.344‰，盐度指数Z值为116.51，表明其形成于温度相对较高、贫氧、盐度较低的流体条件下。萤石具有居中的Sr/Ba值3.15，相对高的U/Th值0.8666，相对低的Zr/Hf值31.875，相对低的稀土总量，其配分模式与岩浆成因萤石较为接近，表明其形成于温度相对较高、缺氧、低盐度的流体条件下。这些特征反映该套地层经历了准同生期白云岩化后的埋藏-抬升-再埋藏-最终抬升至地表的过程。

Generations,geochemistry characteristics and their precipitation conditions of cave fillings of the Middle Cambrian penecontemporaneous dolostone in Keping area,Tarim Basin

In order to rationally analyze and evaluate the developing and preserving condition of penecontemporaneous dolostone caves related with evaporation in burial diagenetic environment，the petrology and geochemistry of the Middle Cambrian dolomites and their cave fillings in the Keping area，Tarim Basin have been studied in details，and the precipitation conditions of the cave fillings have been recovered.It has been discovered that the cave fillings show obvious generational features.There are at least four generations of fillings from the bedrock to the center of the cave：(1) ivory granular calcite;(2)iron dipped variegated columnar calcite;(3)achromatic transparent quartz;(4)purple fluorite.The analytical data of carbon and oxygen stable isotope，trace elements and REE compositions show that the host dolomite has these characteristics such as high Sr/Ba ratio 5.47，low U/Th ratio -0.2954<0.7，high Zr/Hf ratio 36.86，relatively high total REE，relatively high δ¹⁸O -7.444‰，salinity index Z was 120.56，which show a relatively lower temperature，oxygen rich and relatively high saline environment. The calcite fillings of the first generation has these characteristics such as low Sr/Ba 2.33，high U/Th 1.3>1.25，low Zr/Hf ratio 20.857，moderate total REE，relatively low δ¹⁸O -13.758‰，salinity index Z was 105.35<120，which shows a relatively higher temperature，hypoxia and relatively lower saline environment. The second generation has these characteristics such as relatively low Sr/Ba 2.77，high U/Th 1，in the range 0.75～1.25，relatively lower Zr/H ratio 32.08，moderate total REE，relatively low δ¹⁸O -11.344‰，salinity index Z was 116.51<120，which also shows a relatively high temperature，Oxygen-poor and low saline condition. The fluorite fillings of the fourth generation has these characteristics such as relatively low Sr/Ba 3.15，relatively high U/Th 0.8666，in the range 0.75～1.25，relatively low Zr/Hf ratio 31.875，low total REE，and the REE pattern is similar to that of the magmatic origin fluorite，which also shows a relatively high temperature，oxygen-poor and low saline precipitation condition.The precipitation conditions of cave fillings reflect that this set of strata experienced a process of burial-uplift-and then buried-and ultimately uplifting to the surface after the penecontemporaneous dolomitization.