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Tectonic and Magmatic Controls on the Hydrothermal Alteration of Dolostones: A Case Study from the Lower Ordovician in Bachu County, NW Tarim Basin

Daizhao Chen¹, Shaofeng Dong¹, Hairuo Qing², Xiqiang Zhou¹, Dan Wang¹, Zenghui Guo¹, Maosheng Jiang¹, and Yixiong Qian³
¹Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China
²Department of Geology, University of Regina, Regina, SK, Canada
³Northwest Exploration and Production Company, SINOPEC, Urumqi, China

Very thick dolostones (up to 2 km) extensively occur in the Cambrian-Lower Ordovician carbonates in Tarim Basin, Northwest China, and their origins, however, are highly controversial. This study presents a case study of dolostones in the Lower Ordovician carbonates associated with tectonic and magmatic activities from the Sancha area, Bachu County, on the western flank of Tarim Basin, where hydrothermally-altered dolostones, marbles and diabase intrusions all occurred within a strip-slip fault zone. Within this zone, the intrusive diabases mostly occurred along the master border faults to the east and west, respectively; the limestones that contact with the intrusions commonly marbleized, the closer the distance, the stronger the marbleization. In contrast, the hydrothermally-altered dolostones were exclusively developed in the antecedent dolostones, particularly for those that contacted with marbles (or slightly crystallized limestones) away from the intrusions. Based on field investigations and petrographic examinations on the hydrothermally-altered dolostones, five distinctive fabric types of dolomite resulting from replacement and/or recrystallization and cementation are identified and categorized further as matrix and cement dolomites, respectively. The matrix dolomites mainly include: (1) fine- to mediumcrystalline, planar-e, floating dolomite, (2) fine- to medium-crystalline planar-s dolomites and (3) medium- to coarse-crystalline non-planar-a dolomites, and cement dolomites include: (1) medium- to coarse-crystalline dolomite and coarse to very coarse-crystalline, saddle dolomites. All these dolomites predated later two-stage crosscutting vein- or vug-filling calcite minerals. Isotopic data reveal a large overlap of oxygen and carbon isotopic values between matrix and cement dolomites, but less radiogenic strontium isotopes of cement dolomites, more or less, comparable to those of intrusive diabases. However, an apparent offset of isotopic data is shown in later calcite, i.e., light O-C isotopes and more radiogenic strontium isotopes. Microthermometry of fluid inclusions indicates the highest homogenization temperature (Th: 177.9-240.7°C) of marbles, lower Th values (98.5-131.4°C) of saddle dolomite and the lowest Th values (82.7-154.1°C) of later calcites. All these data suggest that dolomite recrystallization and precipitation were associated with the hydrothermal activity induced by the intrusive magmatism. CAMEC SIMS U-Pb isotopic dating of zircon yields an age of 290.5 ± 2.9 Ma for the diabases in the Early Permian, which reconciles the relative timing revealed by the paragenetic history. The magmatic emplacement was interpreted to having been linked to large igneous province (LIP) of Early Permian in Tarim Basin. In this case, the widespread magmatic activities could have caused the thermal anomaly in the strata that were penetrated by the intrusions, enhancing the thermal diffusion and hydrothermal fluid migration (although minor thermal fluid amount for the basic magmas) through fracture/fault system, thereby resulting in contact metamorphism (or marbleization), recrystallization of carbonates, dissolution and dolomite precipitation commonly in primary dolostone successions. This case study provides a useful analogue to understand the dolomitization and relevant issues for the deeply-buried dolostones associated with magmatism within the Tarim Basin and elsewhere.

AAPG Search and Discovery Article #90175©2013 AAPG Hedberg Conference, Beijing, China, April 21-24, 2013