Characteristics and environment indication of mud shale undergone low temperature metamorphism: a case study of Neoproterozoic Binggounan Formation, Hongliugou Ⅰ section, Altyn Tagh fault

The Neoproterozoic Binggounan Formation mud shale in the Hongliugou Ⅰ section on the northwestern margin of the Altyn Tagh fault was deposited in a passive continental margin. They are shelf sediments about 60 m thick, interbedded with siliceous rocks, undergone a low temperature thermodynamic metamorphism, and had hydrocarbon generation potential. The tectonic background, provenance, weathering and sedimentary environment of the mud shale were discussed with regard to stratigraphic sections, mineralogy and geochemical analyses. The mud shale was mainly composed of silica-rich clay rock and mud-rich siliceous shale, followed by mud-silica mixed shale. They have simila-rities in Si, Mg, K, P, Sc, Y, Hf, Th Sc contents compared with the Post Archaean Australian shale (PAAS). The Ti, Mn, Fe, Ta contents and δEun, δCen, ΣREE values are higher, while the Al, Ca, Na, Nb and Zr contents are lower. The primitive sediment sources of the Binggounan mud shale were recycled sedimentary clasts, intermediate mafic and acid intrusive rocks, similar to a normal shale and arenites argillites and ensialic of continental upper crust in composition. The shale has undergone moderate chemical weathering in warm and humid conditions. The formation was divided into three sedimentary cycles from bottom to top, mainly anaerobic and anoxic, and occasionally oxidized. Hydrothermal alteration also occured in the lower section. The mud shale with a clay content of about 40% is rich in trace elements (REE) and organic matter, which was deposited in a suboxic to anoxic section on the shelf, showing a high productivity and hydrocarbon-generation potential.