Magmatic Evolution of Changbaishan Tianchi Volcano, China-North Korea: Evidence from Mineral-Hosted Melt and Fluid Inclusions

Data on mineral-hosted melt, fluid, and crystalline inclusions were used to study the composition and evolution of melts that produced rocks of Changbaishan Tianchi volcano, China-North Korea, and estimate their crystallization parameters. The melts crystallized within broad ranges of temperature (1220-700 degrees C) and pressure (3100-1000 bar), at a drastic change in the redox potential: Delta log fo(2) from NNO + 0.92 to +1.42 for the basalt melts, NNO -1.61 to -2.09 for the trachybasaltic andesite melts, NNO -2.63 to -1.89 for the comendite melts, and NNO -1.55 to -3.15 for the pantellerite melts. The paper reports estimates of the compositions of melts that produced the continuous rock series from trachybasalt to comendite and pantellerite. In terms of trace-element concentrations, all of the mafic melts are comparable with OIB magmas. The silicic melts are strongly enriched in trace elements and REE. The most strongly enriched melts contain concentrations of certain elements almost as high as in ores of these elements. The paper reports data on H2O concentrations in melts of different composition. It is demonstrated that the variations in the H2O concentrations were controlled by magma degassing. Data are reported on the Sr and Nd composition of the rocks. The deviations in the Sr isotopic composition are proportional to the Sr-87/Sr-86 ratio and could be produced in a melt with a high enough Sr-87/Sr-86 ratio during a geologically fairly brief time period. The evolution of melts that produced rocks of the volcano was controlled by crystallization differentiation of the parental basalt magmas at insignificant involvement of melt mixing and liquid immiscibility of silicate and sulfide melts. The alkaline salic rocks were generated in shallow-sitting (13-3.5 km) magmatic chambers in which the melts underwent profound differentiation that gave rise to pantellerites and comendites strongly enriched in trace elements (Th, Nb, Ta, Zr, and REE). Data on the composition of the magmas and parameters of their derivation are used to develop a generalized petrologic-geodynamic model for the origin of Changbaishan Tianchi volcano.