陕西铜川地区延长组长7油页岩无机地球化学特征及古环境恢复

Inorganic geochemical characteristics and paleoenvironment of Chang 7 oil shale in Yanchang Formation,Tongchuan area,Shaanxi Province

为研究陕西铜川地区三叠系延长组长7段油页岩沉积时水体的盐度、氧化还原条件、深度、湖底热液活动情况以及古气候等沉积古环境特征,对该区长7段油页岩岩心样品进行了地球化学特征分析。该区长7段油页岩Sr含量为198.28×10^(-6),Sr/Ba均值为0.39,V/Ni值为4.50,Walker法计算"相当硼"含量大部分样品低于200×10^(-6),最低值接近40×10^(-6),表明长7油页岩形成于微咸—淡水、厌氧还原环境;钴元素定量计算显示其为最大水深40～70 m的深湖—半深湖相;Sr/Cu值平均为4.28,属温湿—半湿润气候条件;P、Cu、Co、Ni、V、Rb等元素富集,REE总量低、HREE富集、Ce表现为负异常,La/Yb—REE图解样品点大部分落在玄武岩、花岗岩和沉积岩的交会区内等,反映沉积时期湖底有热液沉积作用的存在。

The geochemistry of shale core samples from the 7th member (Chang 7) of Triassic Yanchang Formation were analyzed in order to study the characteristics of sedimentary palaeoenvironment such as salinity,redox condition,depth,hydrothermal activity and palaeoclimate when the Chang 7 member was deposited in the Tongchuan area,Shaanxi Province.The content of Sr is 198.28×10^-6,Sr/Ba is 0.39,V/Ni is 4.50,the equivalent boron content by the Walker method in most samples is less than 200×10^-6,and the lowest value is about 40×10^-6.Based on the above characteristics,it can be concluded that the Chang 7 oil shale was deposited in a brackish fresh water and anaerobic environment.The quantitative determination of cobalt indicated that the sedimentary environment was a lake with a maximum water depth of 40-70 m.The average Sr/Cu ratio is 4.28,which means that the paleoclimate was warm-wet and semi-humid.P,Cu,Co,Ni,V and Rb were enriched.The total REE was low,and the HREE was enriched.Ce showed a negative anomaly.Most of the La/ Yb-REE plots were located near the intersection of basalt,granite and sedimentary rocks.All these characteristics strongly confirmed the existence of hydrothermal sedimentation in the lake.