A Trachytic Monogenetic Volcano in the Changbaishan Volcanic Field, Xitudingzi Volcano: Petrogenesis, Magma Plumbing System and Implications for Non-basaltic Monogenetic Volcanism

Abstract

Abstract The Changbaishan volcanic field is the largest Quaternary volcanic field in China and includes the most dangerous polygenetic volcano, Tianchi volcano. In addition to the Tianchi volcano, the Changbaishan volcanic field contains two other polygenetic volcanoes (Wangtian’e volcano and Namphothe volcano) and at least 150 monogenetic volcanoes. The compositions of these monogenetic volcanoes range from basalt to rhyolite. Basaltic monogenetic volcanoes are the most abundant and spread throughout the volcanic field, whereas intermediate to evolved monogenetic volcanoes are found only within the Touxi subfield. We know very little about these non-basaltic monogenetic volcanoes, especially the magmatic petrogenesis, volcanological origins and structure of the magma plumbing system beneath them. Therefore, in this study, the Xitudingzi volcano within the Touxi subfield is used as an example to study the petrogenesis of silicic magma and explore the structure of the magma system by combining new 40Ar/39Ar dating, whole-rock geochemistry, Sr-Nd isotopes, and mineral chemistry. The 40Ar/39Ar chronology results showed that the Xitudingzi volcano erupted at 1.17 Ma; this volcano is younger than the other monogenetic volcanoes in the Touxi subfield. The geochemical results indicated that the eruption products of the Xitudingzi volcano are trachytic and formed from the assimilation fractional crystallization (AFC) process of an alkaline basaltic parental magma. The results from the Rhyolite-MELTS and thermobarometry indicated that the magma plumbing system of the Xitudingzi volcano contained two magma chambers at depths of 8 km and 19 km, respectively. The trachytic magma of Xitudingzi was produced by the AFC process from the alkaline basaltic magmas in the deep magma chamber and then transported to the shallow magma chamber. Furthermore, differences in the trace element concentrations between the different units of the Xitudingzi volcano indicated that the shallow pre-eruptive magma chamber was compositionally zoned. Based on the results from our study, the upper crustal magma plumbing system of the Xitutingzi volcano existed independently of the magma plumbing system of the cone-construction stage of the Tianchi volcano, even though the two volcanoes had similar ages of activity as well as the same chemical composition of eruptive products. Based on these results and other existing studies, a relatively thick crust and the developed faults are likely the key factors for the formation of intermediate to evolved monogenetic volcanos in the Touxi volcanic field.