Multiple dolomitization and later hydrothermal alteration on the Upper Cambrian-Lower Ordovician carbonates in the northern Tarim Basin, China

Abstract

Thick Upper Cambrian-Lower Ordovician carbonates were deposited on a shallow marine platform in the northern Tarim Basin, which were extensively dolomitized, particularly for the Upper Cambrian carbonates. The resulting dolomite rocks are predominantly composed of matrix dolomites with minor cement dolomites. Based on petrographic textures, matrix dolomites consist of very finely to finely crystalline, nonplanar-a to planar-s dolomite (Md1), finely to medium crystalline, planar-e(s) dolomite (Md2), and finely to coarsely crystalline, nonplanar-a dolomite (Md3). Minor cement dolomites include finely to medium crystalline, planar-s(e) dolomite (Cd1) and coarsely crystalline, nonplanar saddle dolomite (Cd2), which partially or completely fill dissolution vugs and fractures; these cements postdate matrix dolomites but predate later quartz and calcite infills. Origins of matrix and cement dolomites and other diagenetic minerals are interpreted on the basis of petrography, isotopic geochemistry (O, C and Sr), and fluid inclusion microthermometry. Md1 dolomite was initially mediated by microbes and subsequently precipitated from slightly modified brines (e.g., evaporated seawater) in near-surface to very shallow burial settings, whereas Md2 dolomite was formed from connate seawater in association with burial dissolution and localized Mg concentration (or cannibalization) in shallow burial conditions. Md3 dolomite, however, was likely the result of intense recrystallization (or neomorphism) upon previously-formed dolomites (e.g., Md1 or Md2 dolomite) as the host carbonates were deeply buried, and influenced by later hydrothermal fluids. Subsequent cement dolomite and quartz crystals precipitated from higher-temperature, hydrothermal fluids, which were contributed more or less by the extensive Permian large igneous province (LIP) activity in Tarim Basin as evidenced by less radiogenic Sr in the cement and parts of matrix dolomites. This extensive abnormal hydrothermal activity could also have resulted in recrystallization (or neomorphism) on the previous matrix dolomites. Faults/fractures likely acted as important conduit networks which could have channeled the hydrothermal fluids from depths. However, the basin uplift triggered by the Late Hercynian Orogeny from the Late Permian would have facilitated downward infiltration of meteoric water and dilution of hydrothermal fluids, resulting in precipitation of later calcites in which lighter C and more radiogenic Sr components demonstrate such a switch of fluid properties. This study provides a useful analogue to understand the complicated dolomitizing processes and later hydrothermal alteration intimately related to the Permian LIP activity within Tarim Basin and elsewhere.