Application of Stable Strontium Isotope Geochemistry and Fluid Inclusion Microthermometry to Studies of Dolomitization of the Deeply Buried Cambrian Carbonate Successions in West-Central Tarim Basin, NW China

Abstract

Detailed petrographic, geochemical (O-C-Sr isotopes) and fluid inclusion studies of the deeply buried Cambrian carbonates in the West-central Tarim Basin revealed three types of crystalline dolomites (fine-crystalline, nonplanar-a(s), dolomite (RD1), fine- to medium-crystalline, planar-e(s) dolomite (RD2), and medium- to coarse-crystalline, nonplanar-a dolomite (RD3)), medium- to coarse-crystalline, nonplanar-a saddle dolomite cement (CD) and early and later-stage calcite cement. The occurrence of RD1 along low-amplitude stylolites points to link with pressure dissolution by which minor Mg ions were likely released for replacive dolomitization during early- to intermediate-burial seawater dolomitization. The increasing crystal sizes of RD2 and RD3 with irregular overgrowth rims suggests intense recrystallization and replacement upon the RD1 or remaining precursor limestones by dolomitizing fluids during late intermediate burial dolomitization. The overlap of δ¹⁸O, δ¹³C and ⁸⁷Sr/⁸⁶Sr values of RD1-RD3 and CD dolomite with coeval seawater values, suggests that the principal dolomitizing fluids that precipitated these dolomites was connate (Cambrian) seawater preserved in the host limestones/dolomites. Their high ⁸⁷Sr/⁸⁶Sr ratios suggest influx of radiogenic strontium into the Cambrian seawater. Two regimes of fluid flow are recognized in the study area: firstly, influx of magnesium-rich higher-temperature basinal brines along deep-seated faults/fractures, resulting in cementation by CD dolomite. Secondly, the incursion of meteoric waters, mixing with ascending higher-temperature basinal brines, and an increase in Ca²⁺/Mg²⁺ ratio in the fluids probably results in the precipitation of calcite cement in vugs and fractures.

Keywords

Cambrian dolomites / C-O-Sr isotopes / burial dolomitization / West-central Tarim Basin / fluid flow regimes

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