PRIMARY DOLOSTONE RELATED TO MANTLE-DERIVED EXHALATIVE HYDROTHERMAL ACTIVITIES IN PERMIAN, SANTANGHU BASIN, CHINA

Santanghu Basin located in north-east Xinjiang, China, is a superimposed and reconstructed intracontinental basin from late Carboniferous to Mesozoic-Cenozoic. The region was in the phase of intracontinental rift evolution from late Carboniferous to Permian, in which volcanic-hydrothermal activities were intense. Middle Permian Lucaogou Formation (lower Guadalupian) generates laminated hydrothermal exhalative dolostone which interbed with explosites containing alkaline pyroclasts, such as carbonatite, pyroxenolite and phonolite.

The dolostone in Lucaogou Formation show 0.05-0.15cm laminae. Dolomites and ankerites are cryptocrystalline and cement pyroclasts, such as analcite, alkali feldspar, quartz, tuff detritus and vitric pyroclast. Pyroclasts like analcite are surrounded by 0.001mm thick micritic ankerites while calcites fill residual pores between grains, which indicates ankerite formed earlier than calcite. Part of ankerites show zone structures due to a variety of FeO contents, which reflects the multi-period activities of iron-rich hydrotherm. Geochemical analysis results show that oxygen isotope value fluctuates intensely (d¹⁸O_PDB: -5‰~21.1‰, -11.9‰ on average), indicating the interaction between the hydrothermal fluid and lake water. ⁸⁷Sr/⁸⁶Sr (0.705005 on average) is very close to global mantle’s (0.70350 on average) while is lower than Permian global carbonate’s (0.7067~0.7085) and crust-derived sialic rock’s (0.720 on average), and it suggests that the hydrothermal fluid is mainly derived from mantle and was contaminated by lake water.

The above features reflect that the dolostone of Lucaogou Formation show primary characteristics and associate with the intense mantle-derived volcanic-hydrothermal activities. Alkaline pyroclasts that erupted with magma provided a few Mg²⁺ and Fe²⁺ that contribute to the dolostone formation. In addition, mantle fluid caused serpentinization of the ultrabasic rocks diapir below lower crust, which add numerous of Mg²⁺ and Fe²⁺ to the fluid which then exhaled and mixed with lake water. In this environment, dolomites and ankerites deposited and cemented alkaline pyroclasts.