川东地区华蓥山海相三叠系不同类型岩石对海水信息的保存性评估

通过对川东地区华蓥山海相三叠系不同岩石类型样品的微观组构、阴极发光和微量元素等分析表明:川东地区华蓥山海相三叠系不同的岩石类型样品具有不同的抵抗成岩蚀变能力,其对海水信息的保存程度也不一致;钙质泥岩所记录的有关信息已基本不能代表海水,多数微晶灰岩所记录的有关信息可以较好地代表海水,而粉晶白云岩只有部分样品可以代表海水的...

The capabilities of different Triassic marine rock types for preservation of coeval seawater information in the Huaying Mountain of eastern Sichuan

Different types of marine carbonate rocks have different capabilities of resisting diagenetic alteration and preserving coeval seawater information. Aragonite and high-magnesium calcite minerals, for example, are liable to suffering from relatively strong diagenetic alteration, and their preservation of the coeval seawater information is therefore relatively poor; by contrast, low-magnesium calcite mineral has much better capability for preservation of the coeval seawater information. It is thus thought that the evaluation of marine samples of different rock types for their diagenetic alteration and preservation of the coeval seawater information is a necessary basic work in sedimentary geochemistry and chemostratigraphy. The analyses of rock fabrics, cathodoluminescence and trace elements of Triassic marine samples of different rock types from the Huaying Mountains of eastern Sichuan were conducted in this paper to evaluate their capabilities for resisting diagenetic alteration and preserving coeval seawater information. The results indicate that the calcareous mudstone samples show strong luminescence, their average Mn concentration is higher than 800×10-6, average Sr concentration is only 280×10-6, average Mn/Sr ratio is higher than 3.3, which is the highest average Mn/Sr ratio in all the samples of different rock types. The micrite samples mostly show extremely weak or weak luminescence, with only a few samples showing moderate or strong luminescence, their average Mn concentration is 213×10-6, average Sr concentration is 704×10-6, and average Mn/Sr ratio is 0.44. The very fine-crystalline dolomite shows relatively weak luminescence, their average Mn concentration is 119×10-6, average Sr concentration is 86×10-6, which is the lowest average Sr concentration in all the samples, and average Mn/Sr ratio is 1.46. The grainstone samples show no luminescence, their average Mn concentrations is only 4×10-6, which is the lowest average Mn concentration in all the samples, average Sr concentrations is 1015×10-6, which is the highest average Sr concentration in all the samples, and average Mn/Sr ratio is very low (lower than 0.004). In summary, the Triassic marine samples of different rock types from the Huaying Mountain of eastern Sichuan have different capabilities for resisting diagenetic alteration and preserving coeval seawater information. All the calcareous mudstone samples which have the highest average Mn concentration, relatively low average Sr concentration, relatively high Mn/Sr ratio and strong luminescence could not represent the coeval seawater, so they must be excluded. Most micrite samples which have relatively low average Mn concentration, relatively high average Sr concentration, relatively low Mn/Sr ratio and weak luminescence or moderate luminescence could represent the coeval seawater, but a few of them must be excluded. A few of very fine-crystalline dolomite samples which have relatively low average Sr concentration and relatively high Mn/Sr ratio could not represent the coeval seawater, even though they have relatively low average Mn concentration, weak luminescence or moderate luminescence, but other samples still represent the coeval seawater. The grainstone samples which used to be regarded as rocks liable to suffering from stronger diagenetic alteration have low Mn concentration, high Sr concentration, and no luminescence, so that their diagenetic alteration might have been related to early seawater cementation or deep-buried trapped marine fluids, rather than to meteoric water.