Petrology and Mineralogy Features of Magmatic-Hydrothermal Explosion Rocks in the Permian Lucaogou Formation of Santanghu Basin and Relative Discussions

Key words: 
• magmatic-hydrothermal explosion rocks /
• hydrothermal fluids exhalative /
• petrology and mineralogy features /
• Lucaogou Formation /
• Sangtanghu Basin

Abstract: In studying the black fine-grained rocks from the Permian Lucaogou Formation of Santanghu Basin, which has long been considered to consist mostly of mudstone, tuff, and carbonate rocks, a series of special laminar sedimentary rocks were discovered. These rocks are neither composed of terrigenous clastic sediments nor intraclast, and they are different from hydrothermal sedimentary rocks (exhalative rocks). Our research team named the series magmatic-hydrothermal explosion rock (MHER). MHER is formed by deep derived magmatic-hydrothermal material (and fluid), which usually forms via a vapor-liquid-solid mechanism, upwelling along the exhalite channels, erupting to the bottom of lake floor and then depositing with the surrounding hydrothermal fluids and water. MHER presents laminated sediments, distributed randomly, and dominantly composed of unstable minerals and metastable minerals with low-grade texture maturity. Based on the observation of a large number of cores and thin sections, in combination with electronic probes and other relative data, this paper classifies explosion rocks into four types according to their different mineral compositions:calcite explosion rock, dolomite explosion rock, diopside explosion rock, and felsic explosion rock. Petrology and mineralogy features of all types are described. MHER developed in an uncompensated lake of an intracontinental rift. The clastics of MHER were more likely derived from magmatic-hydrothermal material or fluid at a variety of depths in the Earth's interior, rather than weathered source rocks or hydrothermal precipitation. This paper also discusses the differences between MHER, sedimentary clastic rock, pyroclastic rock, and exhalaite. Rock-forming material sources, distribution scale, and forming mechanisms can distinguish MHER from the rocks mentioned above.