塔里木盆地塔中地区下奥陶统白云岩成因
- 中国科学院边缘海地质重点实验室 中国科学院南海海洋研究所 广州 510301
- 收稿日期:
1900-01-01
- 修回日期:
1900-01-01
- 刊出日期:
2011-10-10
摘要: 塔里木盆地塔中地区下奥陶统白云岩广泛发育,岩石类型复杂多样,主要包括结晶白云岩、残余颗粒白云岩、残余灰质白云岩、藻云岩、亮边雾心白云岩、环带白云岩及少量膏云岩等结构类型。白云岩交代现象明显,δ18O普遍为中—高负值(-3.2‰~-8.7‰);δ13C值基本为低中负值(-3‰~-0.77‰),Z值大多集中在118~123之间,成岩温度大部分集中于36~51℃范围,属于低温白云岩。白云岩具有低Mg/Ca比值,而Fe/Mn比值变化较大;白云石成分显示高铁白云石和低铁白云石之分;Sr同位素组成(0.708 8~0.709 7)普遍与海水锶同位素组成相当,但也存在个别具有较高的87Sr/86Sr比值(0.710 1~0.710 9)。结合塔中地区奥陶系古地理演变特征,白云岩化类型可划分为准同生期白云石化作用和埋藏白云石化作用。准同生期白云石化作用发生于海底(主要是潮间)成岩环境,主要是在潮坪环境或滩相环境中,其白云岩形成温度较低,具低87Sr/86Sr比值,白云石单矿物具低Fe含量;而埋藏白云岩化存在两种模式,一种为早期浅埋藏过程中的卤水回流模式,其白云岩形成温度亦较低,具高87Sr/86Sr比值,白云石单矿物具高Fe含量;另一类发生于还原性较强晚期埋藏较深的成岩环境,在埋藏阶段白云岩化程度得到进一步加强和调整,其白云岩形成温度较高,白云石捕获的包裹体均一温度较高,盐度较低。
Genesis of Lower Ordovician Dolomite in Central Tarim Basin
- Key Laboratory of Marginal Sea Geology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301
- Received Date:
1900-01-01
- Rev Recd Date:
1900-01-01
- Publish Date:
2011-10-10
Abstract: Lower Ordovician dolomites in the central Tarim basin are of various types and extensively distributed. Seven dolostone types could be documented based on fabrics and structures, which are crystalline dolomite, residualgrain dolomite, residual calcite dolomite, algal dolomite,cloudy centers and clear rims dolomite, clitellum dolomite and gypes dolomite. It's characterised by middlehigh δ18O negative value(3.2%~8.7‰), lowmiddle δ13C negative value(3%~0.77‰), and high Z value of 118~123. Most diagenetic temperature calculated by empirical formulas using δ18O values are between 118 to 123℃, which indicates that the temperature degree of dolomitization is low, and so the kind of dolomite belongs to low temperature dolostone. These dolomites have also characteristics of low Mg/Ca and variable Fe/Mn ratio values. According to mineral composition analyzed by electron microprobe, dolomite monomineral can be divided into high Fe content and low Fe content species. 87Sr/86Sr ratios of most samples are low( among 0.708 8~0.709 7), which are similar to seawater strontium isotope composition, and some are of high ratios between 0.710 1 and 0.710 9. These characteristics reveal that the origin of dolomite in the study area can be classified into penecontemporaneous dolomitization and burial dolomitization. According to Ordovican palaeogeographical evolution, the penecontemporaneous dolomitization arised in the seafloor environment, mainly in tidal flats or beach environment. And this kind of dolomite characterised by low diagenetic temperature, low 87Sr/86Sr ratios, and low Fe content dolomite minerals. The burial dolomitization can be interpreted by two diagentic models. One is seepagereflux dolomitization during early shallow burial stage and characterised by low diagenetic temperature but high 87Sr/86Sr ratios, and high Fe content dolomite mineral. Another is deep burial dolomitization which occured in more strong reducible diagenetic enviornment during late deep burial stage. In general, the degree of dolomitization in burial diagentic enviorment is further enhanced and adjusted. The deep burial dolomite is characterised by high diagenetic temperature and high homogenization temperature of fluid inclusions, but low salinity.