Genesis of fibrous veins in the shales of Longmaxi Formation in Shizhu area, Eastern Chongqing

Fibrous calcite veins (FCV) filled in microfractures in Lower Silurian Longmaxi Formation are widely found in the Eastern Chongqing of Southwestern China. The mechanism and time of fibrous calcite veins is elucidated by the combination of petrographic descriptions integrated with fluid inclusion, stable carbon and oxygen isotopic data and burial history with thermal history evidence. Field observations indicate that the syntaxial fibrous calcite veins are filled in black silty shale/argillaceous siltstone microfractures with bedding-parallel or subhorizon, with width of 0.1-4.0 cm and lateral extended distance of 0.5-8.0 m. Fibrous calcite crystals grow mainly in columnar and fibrous vertical wall rock and are mostly co-precipitated with quartz fibers. The surface of the crystal is clean and the pressure shadow, mechanical twin of calcite and serrated grain boundary between crystals can be seen locally. The fibrous calcites are dark red-orange under cathodoluminescence, and also exhibit low negative values of δ¹³C and extreme negative values of δ¹⁸O, respectively with an average of -1.549‰ and -12.654‰, meaning that calcites may have been obviously affected by the increase of pore-water temperature. Fluid inclusion analyses of two-phase aqueous inclusions in fibrous calcites suggest that calcites occurre at temperatures with an average of 159.5℃. It is inferred that the FCV may have formed during uplift in the Middle-Late Triassic tectonic movement due to fracture-controlled fluid migration and precipitation by syntectonic superhigh pressure and excretion of geofluids in second paleohydrologic system.