Hydrothermal alteration of dolostones in the Lower Ordovician, Tarim Basin, NW China: Multiple constraints from petrology, isotope geochemistry and fluid inclusion microthermometry

Abstract

Massive dolostones pervasively occur in the Lower Paleozoic strata in Tarim Basin, NW China. This study focuses on the hydrothermally-altered dolostones in the Lower Ordovician at Sancha, Bachu County where the hydrothermally-altered dolostones, marbles and diabase intrusions co-occur within a large-scale, NW trending strike-slip fault zone. Within this zone, the intrusive diabases mostly occur along the master border faults to the east and west, respectively. The limestones that got in contact with the intrusions commonly were marbleized and the antecedent dolostones were subject to intense hydrothermal alteration, particularly along fractured portions. Based on field investigations and petrographic examinations on the hydrothermally-altered dolostones, five distinctive fabrics of matrix and cement dolomites are distinguished. Two episodes of vein-filling calcites postdate the hydrothermally-precipitated saddle dolomites. Integrated isotopic geochemistry (C, O and Sr) and fluid inclusion microthermometry suggest that dolomite recrystallization and precipitation were associated with the hydrothermal activity induced by fracturing/faulting and intrusive magmatism. CAMECA SIMS U–Pb isotopic dating of zircon grains from the diabases yields an age of 290.5 ± 2.9 Ma in the Early Permian, which reconciles the relative timing revealed by the paragenetic history. The magmatic emplacement was thus linked to the large igneous province (LIP) of Early Permian in Tarim Basin. In this case, the widespread magmatic activities could have caused the thermal anomaly and fracturing in the strata that were penetrated by the intrusions, enhancing the thermal diffusion and hydrothermal fluid migration (although minor volumes of thermal fluids from the basic magmas) through the fracture/fault systems, thereby resulting in contact metamorphism (or marbleization), recrystallization of matrix dolomites and dolomite precipitation commonly in primary dolostone successions. This study provides a useful analogue to understand the dolomitization and relevant issues for the deeply-buried dolostones associated with tectono-magmatism within the Tarim Basin and elsewhere.