Petrography and isotopic geochemistry of bedding-parallel fibrous gypsum veins in the Neogene Qingshuiying Formation of the Ningnan Basin, North China: Insights for growth model of antitaxial fibrous veins

摘要

Bedding-parallel fibrous gypsum veins, which are widely developed in the Neogene Qingshuiying Formation of the Ningnan Basin, a Tertiary basin located in the northeastern margin of the Tibetan Plateau, provide valuable information on stress field, fluid origin and host rock deformation during their formation. The systematic petrographic observation and isotope analysis of both of veins and wall rocks were carried out to investigate the formation mechanism of bedding-parallel fibrous gypsum veins. The distribution of fibrous gypsum veins shows vertical heterogeneity, and mainly occurs in the middle and lower part of the Qingshuiying Formation with relatively high gypsum content. Single fibrous gypsum veins consist of a median zone and two fibrous zones. The median zone is characterized by blocky gypsum crystals and multiple host rock fragments formed by crack-seal events. The fibrous zone is composed of parallel gypsum fibres a high angle to the vein walls, indicating shear component. Vein opening is considered to be the result of fluid overpressure caused by horizontal compression in low-permeability mudstone during basin inversion. Fibrous gypsum crystals grow continuously without obvious growth competition between adjacent crystals. This indicates that growth of gypsum fibres occurs in limited space, even without fractures. The main driving force for vein dilation is force of crystallization of gypsum fibre growth, and fluid pressure also has a certain contribution. The sulfur and strontium isotopes of gypsum veins exhibit a high degree of consistency with wall rocks, which indicates the material of the fibrous gypsum veins is derived from the wall rock. A suitable growth model of bedding-parallel fibrous gypsum veins in the Ningnan Basin is established, which explains the process of vein interaction, and the formation of V-shaped or U-shaped solid inclusions.