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Geochemistry and evolution of dolomitizing fluids of the Upper Cambrian-Lower Ordovician dolostones in Central Uplift,Tarim Basin
Author(s): 
Pages: 661-676
Year: Issue:  4
Journal: Journal of Palaeogeography

Keyword:  Tarim BasinUpper Cambrian -Lower Ordoviciandolostonegeochemistrydolo-mitizing fluids;
Abstract: Based on the detailed petrology study as well as isotope data (δ1 3C,δ1 8O and 87Sr /86Sr), the evolution of dolomitizing fluids and genetic mechanism of the Upper Cambrian-Lower Ordovician dolos-tones are investigated in the Central Uplift,Tarim Basin.The results indicate that:The Upper Cambrian dolostones are composed of micritic dolostones,microbial dolostones and (relict) grainy dolostones with well-preserved precursor lithologic fabric.These dolostones have similar δ1 3C values and 87Sr /86Sr ratios to the coeval seawater and slightly higher δ1 8O values,suggesting that the dolomitization occurred during the penecontemporaneous period and the dolomitizing fluid was associated with the slightly evaporitic seawater. The Lower Ordovician dolostones are mainly comprised of fine-crystalline,planar-euhedral to subhedral dolomites.Poor-preserved precursor texture,similar δ1 3C values and 87Sr /86Sr ratios to the coeval seawater as well as slightly depleted δ1 8O values of the dolostone indicated the dolomitization occurred during shallow burial.Some of the dolostones formed in early stage could be altered to fine-coarse crystalline,nonplanar-anhedral dolostones and rift-cavity-filling saddle dolostone cements through recrystallization and tectonically controlled hydrothermal dolomitization during medium to deep burial.The dolomitizing fluids of the burial dolomitization were derived from remnant seawater preserved in the formations,deep mantle hydrothermal fluids and hot brine water in the underlying Middle Cambrian evaporate rocks.These dolostones formed during burial have wide range of 87Sr /86Sr ratios and obviously more negative δ1 8O values due to the com-bined impact of multi-stage and multi-source diagenetic fluids.Generally,these dolostones in the study ar-ea formed in multiple stages of dolomitization with the tendency of forming in early period (large scale re-placement during near-surface and shallow burial),being mature in middle diagenetic stage (partly re-crystallization during medium-deep burial) and modified in late diagenetic stage (partial adjustment of hydrothermal fluid).
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