Dolomitisation under an arid climate at low sea-level: a case study of the Lei 4 Member of the Middle Triassic Leikoupo Formation, Western Sichuan Depression, China

The origin of dolostones from the Lei 4 Member (T(2)l(4)) of the Middle Triassic Leikoupo Formation in the Western Sichuan Depression is unclear. The occurrence and genetic evolution of dolostones in T(2)l(4) were analysed by polarised thin-sections and cathodoluminescence, major- and trace-element, scanning electron microscope, and carbon, oxygen and strontium isotope analyses. The study results are summarised as follows. (1) The dolostones were mainly precipitated in three stages of <40 degrees C, 40-60 degrees C and 60-80 degrees C, corresponding with three paleodepth ranges of 167-300 m, 433-1000 m and 1067-1433 m. The micritic dolostones and the fabric-retentive dolostones (algal dolostone, granular dolostone) mainly formed in a near-surface-shallow-burial environment, and the crystalline dolostones were mainly formed under intermediate burial conditions. (2) Sr-87/Sr-86 ratios are equivalent to that of the Middle Triassic seawater, Sr/Ba and V/Ni ratios >1, Na notably higher than that in coexisting limestones of T(2)l(4), and the dolostones always coexist with evaporative minerals such as gypsum, indicating that dolomitisation fluids mainly originated from evaporative concentrated seawater. (3) The dolostones mainly inherit materials from precursor limestones based on trace-element distribution patterns, and carbon and oxygen isotope values that are consistent with coexisting limestones of T(2)l(4). (4) Mediated by micro-organisms during the syngenetic period, micritic dolostones and some algal dolostones were formed by replacing aragonites and calcites. During shallow burial, concentrated seawater rich in Mg2+ from the supratidal-intertidal zone flowed downward owing to gravity along the platform and replaced the underlying carbonate rocks, promoting continuous growth of the early dolomites. In the intermediate burial period, the Mg2+-depleted dolomitisation fluid caused the early micritic and silt-crystalline dolostones to recrystallise into silt- or fine-crystalline dolostones with larger crystals and altered the fabric-retentive dolostones into crystalline dolostones. (5) The evaporative dolostones deposited in the near-surface environment are characterised by maximal enrichment of Fe, Sr and Na, the highest delta O-18 values, the lowest order degree and the highest Ca/Mg ratios. The reflux dolostones formed in a shallow-burial environment characterised by the lowest Fe, medium delta O-18 values and the lowest order degree. The burial dolostones that developed in the intermediate burial environment are characterised by relative enrichment of Fe and Mn, minimal Na, the lowest delta O-18 values, the highest order degree and medium Ca/Mg ratios.