Origin of dolomitic rocks in the lower Permian Fengcheng formation, Junggar Basin, China: evidence from petrology and geochemistry

Abstract

Although dolomitization of calcite minerals and carbonatization of volcanic rocks have been studied widely, the extensive dolomitic rocks that originated from altered volcanic and volcaniclastic rocks have not been reported. The dolomitic rocks of the Fengcheng Formation in the Junggar Basin of China appear to be formed under unusual geologic conditions. The petrological and geochemical characteristics indicate that the dolomitizing host rock is devitrified volcanic tuff. After low-temperature alteration and calcitization, these tuffaceous rocks are replaced by Mg-rich brine to form massive dolomitic tuffs. We propose that the briny (with −2 ‰ ~ 6 ‰ of δ13CPDB and −5 ‰ ~ 4 ‰ of δ18OPDB) and Mg-rich marine formation water (with 0.7060 ~ 0.7087 of 87Sr/86Sr ratio), the thick and intermediate-mafic volcanic ashes, and the tectonically compressional movement may have favored the formation of the unusual dolomitic rocks. We conclude that the proposed origin of the dolomitic rocks can be extrapolated to other similar terranes with volcaniclastic rocks, seabed tuffaceous sediment, and fracture filling of sill.