The Oxygen And Carbon Isotopic Compositions And The Diagenetic Fluid Of Feixianguan Carbonates In Early Triassic, Eastern Sichuan Basin

Stable oxygen and carbon isotopes, as one of the most significant geochemical parameters of carbonates, can serve as vital tools for evaluating both sedimentary and diagenetic fluids. Such isotopic compositions of Feixianguan carbonates (both limestones and dolomites) are obtained based on sedimentary facies, mineralogy and petrology research, and the corresponding evolution curves of Feixianguan marine limestone (seawater) are established. Compared the oxygen and carbon isotope compositions and other geochemical parameters, such as stable strontium isotope and some trace element (Sr, Mn and Fe) contents, of Feixianguan dolomites with that of coeval seawater, the characteristics of dolomite diagenesis fluids (dolomitization fluids) are deduced in this dissertation.The limestone samples of Feixianguan Formation from Liangfengya section, Zhongliangshan, are of micrite or grainy textures as their original matter.δ18O of Zhongliangshan limestones has a predominance value of around -6.5‰, with largest excursions of 2‰(PDB).δ13C values have a relatively broad range, from-1.67 to 4.915‰(PDB). Diagenetic alteration evaluation suggests that most samples with Mn/Sr ratio less than 2 changed little in stable isotope compositions and could represent their coeval seawater composition, relatively. These good samples have high strontium content and low manganese content, with average strontium content of 1166 ppm and manganese 330 ppm. The oxygen and carbon isotopic curves with high resolution, established based on theδ18O andδ13C values of these less altered samples, are of good consistency with the same type curves published by other authors, suggesting that these high resolution curves are reliable and it is some global geologic events that control the evolution trend of oxygen and carbon isotope composition in Feixianguan limestones (seawater).Generally,δ18O values of Feixianguan seawater exhibit an escalating trend, with a very low baseline in P/T boundary, followed by a short-term large scale positive excursion, a low amplitude decrease then, and several small positive shifts during the middle and late Feixianguan. It is considered that the short-term large scale positive excursion close to P/T boundary could be ascribed to the rapid fall of the high temperature of coeval warm seawater after large-scale volcanic eruption occurred during the uppermost Permian in most areas of Sichuan. Feixianguan seawater shows high amplitude positive excursion ofδ13C values. The carbon isotope data suggest the existence of two minimum values with the second one even lower than the first one, next to the P/T boundary, followed by a large scale rising with several small shift cycles. It is believed that the Mass Extinction and the methane leak event directly lead to the existence of the minimumδ13C value and the biological recovery is one of the most significant reasons for escalating trend of carbon isotope curve in Feixianguan Formation.According to micrite-texture characteristics, Feixianguan dolomite, in NE Sichuan, can be divided into three main types---micrite dolomite, grainy dolomite with original texture and crystalline dolomite, from top to bottom in the same shallowing upward cycle profile. All types of dolomites showδ18O value and 87Sr/86Sr higher than that of coeval seawater,δ13C value consistency with that of coeval seawater. Also, these dolomites exhibit extremely low manganese content with value lower than 100 ppm, and relatively low ferrum content with value lower than 1000 ppm. These phenomena suggest that Feixianguan dolomites should precipitate in some high salinity fluids, perhaps close to seawater in geochemical characteristics. However, different types of dolomites show various oxygen and carbon isotopic compositions, strontium isotope, and manganese and ferrum contents, illustrating that different types of dolomites have various dolomitization mechanisms in the context of marine fluids dominating the dolomitization.Micrite dolomites display low fluid-inclusion homogenization temperature (Th), ranging from 40℃to 42℃. It is hard to find two phases fluid-inclusions in grainy dolomites with original texture. Crystalline dolomites have the highest Th in the three types, with the maximum frequency distributed from 110℃to 130℃. The measured salinity of fluid inclusions in host mineral dolomite in crystalline dolomites is as high as 13.52% to 15.96% (EqWt, NaCl), which is significantly higher than that of seawater. Dolomitization fluids deduction results (fromδ18O of dolomite and Th), suggest micrite dolomite and grainy dolomite with original texture might precipitate in some fluids similar with seawater in oxygen isotope and salinity, while crystalline dolomite might deposit in some heavier oxygen isotopic composition and higher salinity fluids. The fine consistency ofδ13C values between dolomites and seawater illustrates that the carbon in dolomite mainly derived from marine calcite (aragonite). In comparison, micrite dolomites and grainy dolomite with original texture show relatively higher manganese and ferrum contents, while crystalline dolomites exhibit much lower manganese and ferrum contents. These geochemical characteristics of all types of dolomites and their deviation from coeval seawater suggest that the dolomitization fluids of micrite dolomites and grainy dolomite with original texture are close to coeval seawater in relatively open system, while the diagenetic fluids of crystalline dolomites are of high temperature and of high salinity in relatively closed system, close to the later marine seawater.Therefore, considering their petrology, sedimentary facies and geochemical parameters, it is proposed that micrite dolomites might form as evaporative pumping model, grainy dolomite with original texture might form as seepage reflux model, and crystalline dolomites might be aroused by buried marine fluids dolomitization.