柴北缘九龙山地区侏罗系砂岩中碳酸盐胶结物特征及意义

Characteristics and Significance of Carbon and Oxygen Isotopic Compositions of Carbonate Cements in Jiulongshan Region,North Edge of Qaidam Basin

综合运用岩石学、矿物学和地球化学等方法,对柴达木盆地北缘中段九龙山地区侏罗系砂岩中碳酸盐胶结物的成岩温度、形成环境和物质来源等进行了系统研究。结果表明,九龙山地区侏罗系砂岩中存在4种自生的碳酸盐胶结物类型:方解石、含铁方解石、白云石和铁白云石。测定其碳同位素(δ13C)值分布范围在-15.1‰^-1.3‰之间,平均值为-6.01‰;氧同位素(δ18 O)值分布范围在-17.8‰^-11.2‰之间,平均值为-15.06‰,推算古盐度(Z)值分布范围为87.71~117.77,平均值为107.5。说明九龙山地区侏罗系砂岩在成岩阶段早期有少量有机碳的加入,且成岩流体主要是来自碎屑析水和大气中的淋滤淡水。碳酸盐胶结物的形成温度在78.5~126.55℃之间,平均值为105.86℃。根据成岩特征及形成温度说明侏罗系砂岩的埋藏深度应该大于2 500m,结合目前侏罗系的埋藏深度在1 600m左右,说明九龙山地区侏罗系砂岩在成岩过程中早期被深埋藏后,又经历了后期构造运动的强烈改造,被抬升1 000m左右。

Based on the analyses of petrology,mineralogy and geochemistry,the depositional environment and origin of carbonate cements of Jurassic sandstones in Jiulongshan Region on the northern margin of the Qaidam Basin are studied.The experimental results demonstrate that there were four kinds of autogenic carbonate cements：calcite,iron-bearing calcite,dolomite and ankerite,whose carbon isotope（δ^13C）ranged from-15.1‰to-1.3‰ with an average value of-6.01‰,and oxygen isotope（δ^18 O）ranged from-17.8‰to-11.2‰ with an average value of-15.06‰,and Z-value of paleosalinity ranged from 87.71 to 117.77 with an average value of 107.5.The above results showed that small amount of organic carbon involved in the early diagenesis of Jurassic sandstones in Jiulongshan Region,and diagenetic fluids were mainly leaching freshwater from clast and atmosphere.The temperature to form carbonate cements was 78.5-126.55℃with an average value of 105.86℃.According to paleo-goethermal gradient,the burial depth of Jurassic sandstones should be about 2.5km,but the actual burial depth was less than 1.6km,so it was concluded that the Jurassic Formation in Jiulongshan region had experienced strong tectonic movement and were uplifted about 1.0km after deeply buried.