川东地区石炭系碳酸盐岩碳、氧、锶同位素特征及其成因分析

摘要

利用微量元素和碳、氧同位素特征对样品有效性作出了检验.在证明样品未受明显蚀变的前提下对川东地区石炭系碳酸盐岩碳、氧、锶同位素进行了分析.在层序地层格架中对比不同体系域、不同岩性的碳、氧同位素特征,87Sr/86Sr比值特征,Z值与古温度特征,分析了碳酸盐岩的成岩环境.低位体系域以膏盐湖及萨巴哈环境为主,炎热干旱,陆源淡水对成岩作用影响有限,去膏化、去白云岩化作用导致次生灰岩和“鸡笼铁丝”构造的发育.海侵体系域以半局限—局限陆棚环境为主,锶同位素特征表明陆源淡水对成岩作用影响增强,广泛沉积白云岩.高位体系域仍以陆棚环境为主,沉积海相灰岩为主,幔源锶含量增加,表明海平面上升,陆源锶的注入明显降低.白云岩碳、氧、锶同位素值差异较大,主要存在4种沉积环境:(1)Z值较低的淡水沉积环境;(2)海相环境下的中—浅埋藏环境;(3)海相环境,温度较低的蒸发潟湖环境;(4)温度最高的高温环境.结合87Sr/86Sr比值特征与岩石学特征相,认为川东地区主要存在淡水、埋藏、准同生、热液等4种白云岩化作用.%The validity of the samples was tested using trace elements and carbon and oxygen isotopes. Carbon, O and Sr isotope data of least altered Carboniferous carbonate rock in the eastern Sichuan Basin were processed. The C, O, and 87Sr/86Sr isotope characteristics, and paleosalinity and paleotemperature of different stratigraphic framework and different lithology were compared, aiming at better constraints on the diagenetic environment of carbonate rocks. The results suggest that it is mainly gypsic saline lake and sabkha environment when lowstand system tract (LST) deposited. The climate was hot and dry, terrestrial source fresh water was limited on impacting the diagenesis, anti-gypsumization and anti-dolomitization cause secondary limestone, with "wire cages" structures developed. When transgressive system tract (TST) deposited, it was mainly semi-limitated or limitated shelf environment. 87Sr/86Sr value suggests that the impact on diagenesis from terrestrial source freshwater was enhanced, with dolomite well developed. It was mainly shelf environment when highstand system tract (HST) deposited, sediments were marine limestone, with mantle source Sr increased, indicating sea level rise, with significantly reduced terrestrial source Sr. Dolomite C, O, and Sr isotopes show big differences indicating four kinds of sedimentary environments: (1) freshwater sedimentary environment with low salinity; (2) intermediate-shallow burial marine environment; (3) marine and low temperature evaporation lagoon environment; (4) high-temperature environment with the highest temperature. Combined with 87Sr/86Sr and petrological characteristics, it is concluded that there are four kinds of dolomitizations: freshwater, buried, penecon-temporaneous, and hydrothermal dolomitizations in the eastern Sichuan Basin.