Characteristics and genesis of thick layer dolomites in the Lower Cambrian Xixiangchi Group,eastern Sichuan Basin
The thick layer dolomites of the Cambrian Xixiangchi Group in eastern Sichuan Basin are widely distributed and have exploration potential.Taking the field profiles of Xixiangchi Group in Sanquan,Nanchuan,eastern Sichuan Basin as the research object,and based on the characteristics of petrology,sedimentology,petrogeochemistry and mineralogy,their changes in sedimentary cycles have been analyzed in order to discuss the properties,origin and migration path of the dolomitized fluid.The results show that dolomites of Xixiangchi Group have weak cathodoluminescence.The δ13C and 87Sr/86Sr values overlap with the synchronous seawater section of Xixiangchi Group,which indicates that dolomitized fluid originated from the synchronous seawater,and dolomitization occurred in the penecontemporaneous-shallow burial stage.Geochemical and mineralogical characteristics of the rocks are correlated with sedimentary cycles.Upward-shallowing cycles exhibit a pattern of increasing dolomite abundance,stoichiometry andδ18O,and decreasing cation ordering,suggesting that with the decrease of relative sea level,the temperature,salinity and Mg/Ca of the dolomitized fluid increased,and the degree of dolomitization was high at the top of the cycle.When the relative sea level was relatively high,the saline seawater in the northwestern lagoon of the study area passed through the recently-formed micrite and the previously-formed granular limestone/dolomite with good porosity and permeability,resulting in dolomitization dominated by lateral far-from-source seepage-reflux.As relative sea level gradually fell,the circulation of the lagoon water body gradually deteriorated,and the salinity of the lagoon increased.Granular limestone was formed,exposed and erosed in the paleogeomorphic highs of the intertidal-subtidal zone.Saline seawater in the lagoon was brought to the exposed area by waves and tides,and dolomitization dominated by vertical near-source seepage-reflux occurred with the dolomitized fluid flowing downwards.Multi-stage lateral far-from-source and vertical near-source seepage-reflux dolomitization events continued to superimpose,forming massive dolomites in the intertidal-subtidal zones of the Lower Xixiangchi Group in eastern Sichuan Basin.
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