Petrological, geochemical, and hydrothermal characteristics of Ordovician cherts in the southeastern Tarim Basin, NW China, and constraints on the origin of cherts and Permian tectonic evolution

摘要

Cherts occur extensively as stratal wedges embedded in the Middle-Lower Ordovician carbonate successions in the circum-Tangguzibasi areas, south-east Tarim Basin, where thick (up to 2 km), dark grey well-stratified limestones deposited in shelf and platform environments are prominent. Based on detailed core and thin section observations, with petrographic, microthermometric, and geochemical data for the Middle-Lower Ordovician cherts in some key wells, two lithotypes of chert are identified across the marginal zone in the western Tangguzibasi areas: replacement and filled cherts. Replacement chert is characterized by granular micro-crystalline quartz or radially fibrous chalcedony and commonly retains the pre-existing dolomite rhombs and elliptical ooids floating in these cherts. Filled chert, the most common type, is generally rimmed by fine-gained euhedral quartz that grades abruptly into vein-cavity-fill mega-quartz. Compositionally, these chert deposits are strongly enriched in Cr, Zn, Ba and U and generally show apparent right-convex REE patterns with low Sigma REE values (avg. 4.24 ppm), weak negative Ce anomalies (Ce/Ce*: avg. 0.93) and low to intermediate positive Eu anomalies (Eu/Eu*: avg. 1.19), particularly the replacement cherts. Microthermometry of fluid inclusions from coarse quartz reveals high homogenization temperatures from 135 to 195 degrees C for trapped primary fluids with high salinities (11.0-18.9 wt%). These siliceous deposits also have low La/Ho and Th/U, relatively high Y/Ho and Ba/Sr, high Ce/Ce* (similar to 1.06), intermediate to high (Lu/La)(N) (0.5-1.0) and low (La/Ce)(N) (mostly 1.31-1.45). All these data suggest that the cherts formed under the strong influence of hydrothermal fluids originating from detritus-poor hydrogenetic Fe-Mn crusts at deep continental margins. Extensive silicification occurred after intense dolomitization and calcitization, while fracturing/faulting were greatly intensified in the host carbonates, induced by intense extension because of the uplift force of the magma cushion under oblique collision during the Early Permian. Most silica could have directly precipitated from hydrothermal fluids to chalcedony fibres around voids and vein cherts along fracture conduits. These petrological and geochemical data also provide a useful clue to better understand the extensive silica precipitation, intense tectono-hydrothermal activity and significant lithospheric fracturing events as a result of the closure of the South Tienshan and Palaeo-Tethys Oceans during the Early-Middle Permian.