【摘要】 综合运用岩石学、矿物学和地球化学的研究方法,对柴达木盆地北缘构造带冷湖五号下侏罗统碳酸盐胶结物和沉积环境进行了研究。结果表明下侏罗统碎屑岩主要为岩屑砂岩和长石岩屑砂岩,黏土矿物含量较高,平均可达36.5%,具有高含量的高岭石和伊/蒙混层中蒙脱石含量较低的特征。碎屑颗粒以线接触和凹凸接触为主,主要处于中成岩阶段B期;镜下常见方解石、白云石和铁方解石3种主要的碳酸盐胶结物。碳酸盐胶结物的碳—氧同位素分析结果显示,碳同位素(δ¹³ C)值分布范围为-15.6‰⁹.2‰,平均值为-3.2‰,具双峰分布的特征,第一个峰值出现在4‰左右,集中在下侏罗统小煤沟组上段;第二个峰值出现在-12‰左右,主要集中在下侏罗统小煤沟组下段,代表有机来源碳的加入。氧同位素(δ¹⁸ O)值分布范围为-18.5‰^-8.3‰,平均值为-13.31‰。结合镜下鉴定结果可将碳酸盐胶结物划分为2期,早期碳酸盐胶结物形成于成岩阶段早期,直至早成岩阶段B期结束,主要为无机碳源,但其受生物产甲烷阶段伴生的CO₂影响较大,反映的古盐度偏高;晚期碳酸盐胶结物形成于中成岩阶段A期,主要充填于碎屑颗粒的压裂缝和节理缝中,受到有机质脱羧作用的影响,碳同位素(δ¹³C)值偏低。结合全岩主量元素和微量元素的分析以及元素比值分析,表明柴达木盆地冷湖五号构造带下侏罗统沉积早期气候温暖、湿润,沉积环境以淡水环境为主;晚期气候逐渐变得干热,沉积环境以淡水—微咸水环境为主。更多还原

【Abstract】 Based on the analyses of petrology,mineralogy and geochemistry,the formation phases of the carbonate cements and sedimentary environment in Jurassic clastic rocks in Lenghu V Tectonic Belt were studied.The experimental results demonstrated that the rocks of Lower Jurassic were mainly lithic sandstone and feldspar lithic sandstone with high content of clay minerals,which was up to 36.5%.The clay minerals in the Lower Jurassic were characterized by high content of kaolinite and low S%in I/S.Clastic rocks were mainly line contact and sature line contact,diagenetic stage of the Lower Jurassic clastic rocks was phase B of middle diagenetic stage;calcite,dolomite and ankerite were three main carbonate cements through observation under microscope.The carbon isotope(δ¹³ C)of carbonate cements ranges from-15.6‰ to 9.2‰,and the average value is -3.2‰.The distribution was bimodal,one peak value was 4‰,consisting of samples coming from the upper section in Xiaomeigou Formation;the other one was -12‰,consisting of samples mainly coming from the lower section in Xiaomeigou Formation.The oxygen isotope(δ¹⁸ O)had a wide range,ranging from-18.5‰ to -8.3‰,and the average value was -13.3‰.Combined with the observation under microscope,two phases of carbonate cements were distinguished according to isotope characteristic.The earlier carbonate cements were formed from eogenetic A phase to eogenetic B phase.The source of the carbon was inorganic,but it was mixed by the carbon from methane generation process.This mixing leads to positive bias of the δ¹³ C,and make the indicated paleosalinity higher than the truth.The later carbonate cements were formed in phase A of middle diagenetic stage,and always filled in the cracks in clastic.The carbon came from decarboxylation of organic matters,so its δ¹³ C was lighter.The carbon and oxygen isotopes together with element geochemistry showed that the climate in Low Jurassic was warm and wet,especially in the earlier time,sedimentary environment was fresh water.However,during the late stage,the climate became dry and cold,and sedimentary environment became fresh-brackish.更多还原