鄂尔多斯盆地西南缘蓟县系碳酸盐岩碳氧同位素特征及其地质意义

Characteristics of carbon and oxygen isotopes of the Jixian System carbonate rocks in the southwestern margin of Ordos Basin and their implication

通过对鄂尔多斯盆地西南缘蓟县系白云岩进行碳氧同位素分析、成岩环境分析及古盐度、古温度的恢复,结果表明,研究区蓟县系未经过深埋藏,一直处于近地表,其δ13CPDB和δ18OPDB分布范围较窄,分别为-0.4‰^-0.2‰、-4.7‰^-3.9‰,与研究区稳定存在的潮间带环境特征相一致;得出古盐度值(Z)为124.24~124.74,古水温(T)为19.12~22.67℃,反映该区中元古代温暖的古气候环境.分析表明,在温暖的古气候背景下,蓟县系碳氧同位素变化特征具有一定的正相关性,推测温度可能对碳氧同位素的变化起主要控制作用.垂向上,δ13CPDB、δ18OPDB、Z与T具有波动性变化,但整体变化幅度较小,在一定程度上反映了研究区中元古代稳定的构造环境.综合分析认为,鄂尔多斯盆地西南缘中元古代气候环境温暖并具有稳定的构造环境,这可能是华北板块在中元古代处于较低纬度及研究区在该时期持续热沉降的良好响应.

Analyses of carbon-oxygen isotopes and diagenetic environment of the Jixian System dolomites in the southwestern margin of Ordos Basin showed that the Jixian System had not been buried deeply but was in the near surface,and the distribution ranges ofδ13CPDB,andδ18OPDB were narrow,respectively from-0.4‰to-0.2‰and-4.7‰to-3.9‰,which was consistent with the isotope geochemistry characteristics of the intertidal zone.To reconstruct the paleosalinity and paleotemperature,the values of paleosalinity(Z)and paleotemperature(T)were respectively 124.24 to 124.74 and 19.12℃to22.67℃,manifesting the warm climate of the study area in Middle Proterozoic.Analysis also showed that,under this warm climate,there was a certain positive correlation between the variations of carbon and oxygen isotopes,which might be mainly controlled by temperature.In the vertical direction,δ13CPDB,δ18OPDB,Z and T have undergone fluctuation changes,but the changes in the whole was small,indicating that the study area was mainly in a stable tectonic environment to some extent during the Middle Proterozoic.The comprehensive analysis results reveal that,in the Middle Proterozoic,the southwestern margin of Ordos Basin had a warm climate and was mainly in a stable tectonic environment,which might be the good responses of the low latitudes location of the northern China plate and the continuous thermal subsidence of the study area in the Middle Proterozoic.