华南栖霞组菊花石假象内正延性玉髓的成因及其地质意义

正延性玉髓常被视为干旱蒸发沉积环境的标志。正延性玉髓亦广泛分布于华南地区栖霞组菊花石 (目前绝大多数为天青石假象 )内。本文在天青石后期矿物交代序列识别的基础上,通过对菊花石内各期次碳酸盐交代矿物的电子探针分析,发现菊花石内正延性玉髓形成于富含硫酸根离子和镁离子的成岩介质条件中。结合已有的正延性玉髓研究报道和栖霞组的沉积、成岩作用环境特征分析,认为华南地区栖霞组菊花石内正延性玉髓的形成环境与高蒸发沉积环境或高盐度成岩环境无关。因此,仅根据正延性玉髓的出现不足以确定沉积或成岩环境的盐度条件。

Length-Slow Chalcedony in Chrysanthemum Stone of Chihsia Formation, South China and Its Geological Implications

Length slow chalcedony (LSC) was commonly used as an indicator of evaporitional environments. LSC was also distributed widely in the chrysanthemum stone, the pseudomorph of a kind of nodular celestite, from the Permian Chihsia Formation of South China. A number of lines of evidence indicated the Chihsia Formation was accumulated in a shallow carbonate platform with normal salinity. Based on microscopic observation, a comprehensive replacement succession has been established for the celestite pseudomorph as: celestite(fine-grained cloudy calcite(LSC) (Fig.Ⅰ B, C, D)(clear granular calcite(euhedral calcite crystal (in LSC) (Fig.Ⅰ B)(clear calcite (Fig.Ⅰ C). The original celestite has also been replaced directly by LSC at places. The latest generation of coarse calcite is related to calcite vein (Fig.Ⅰ C). Electronic microprobe analysis on the replacement calcites (Table 1) shows higher magnesium content in the replacement calcites of early phase ( including fine grained cloudy calcite and clear granular calcite) than those of later phase, indicating a magnesium rich diagenetic environment. In addition, tiny celestite detected by electronic scanning microscope in the replacement calcites of early phase and strontianite in the replacement calcite of later phase indicate the geochemical compositional transition of diagenetic fluids from sulfate rich to carbonate rich. The LSC was thus formed in a fluid rich in sulfate and magnesium, agreeing well with reported cases from evaporitional and non evaporitional environments. The possibility of evaporitional sedimentary environments or saline diagenetic conditions favoring the formation of LSC, thus, was excluded for the Chihsian LSC. The LSC should not be used, by itself, as a reliable indicator of evaporitional environment of deposition.