Diagenesis and very low grade metamorphism in a 7,012 m-deep well Hongcan 1, eastern Tibetan plateau

Abstract

This study uses clay mineral assemblages, illite “crystallinity” (IC), chlorite “crystallinity” (CC), illite polytypes, the b cell-dimension of K-white mica, mineral geothermo-geobarometers and homogenization temperatures of fluid inclusions to investigate the transition from diagenesis to metamorphism in a 7 km thick Triassic flysch sequence in the well Hongcan 1, eastern Tibetan plateau. The 7,012.8 m deep borehole penetrated flysch of Upper to the Middle Triassic age and represents a unique chance to characterize low temperature metamorphic processes in an unusually thick sedimentary sequence developed on thickened continental crust. Mineral assemblage analysis reveals a burial metamorphic pattern with kaolinite and chlorite/smectite mix-layer phases present in the upper 1,500 m, and illite/smectite mixed-layer phases extending to a depth of 3,000 m. The metamorphic index mineral, graphite, was detected in sedimentary rock below 5,000 m using Raman spectroscopy. There exists a good correlation between IC and CC within the prograde burial sequence; with CC anchizonal boundaries of 0.242 and 0.314°2θ (upper and lower boundaries, respectively) corresponding to Kübler’s IC limits at 0.25 and 0.42°2θ. Illite polytypism also shows an increase in the 2M 1 polytype with increasing depth, with ca. 60 % 2M 1 abundance compared to the 1M type at the surface, to 100 % 2M 1 at the bottom of the borehole. Fluid inclusion analysis show HHC-rich bearing fluids correspond to the diagenetic zone, CH4-rich bearing fluids appear at transitional zone from diagenetic to low anchizone and H2O-rich bearing fluids mark the high anchizone to epizone. Based on chlorite chemical geothermometer, calcite–dolomite geothermo-barometers as well as homogenization temperature of fluid inclusions, a paleotemperature range of 118–348 °C is estimated for the well with a pressure facies of low to intermediate type.