10.01.2019
Dr. Olga Andreeva
Magmatic evolution of Changbaishan volcano (China-North Korea): Evidence from melt and fluid inclusions
Abstract
We studied melt, fluid, and crystalline inclusions in minerals from different rock types of Changbaishan Tianchi volcano on the border of China and North Korea. These data were used to reconstruct the compositions, formation conditions and evolution of the parental melts for the trachybasalts, trachybasaltic andesites, comendites, and pantellerites of the volcano. The melts crystallized within broad ranges of temperature (1220–700°C) and pressure (3100–1000 bar), at a dramatic change in redox conditions: logfO2 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 (NNO – logfO2 for the Ni-NiO buffer). The compositions of melts that produced the rock series from trachybasalt to comendite and pantellerite were determined. It was shown that the geochemical characteristics of the mafic melts are almost identical to those of OIB. The silicic melts are strongly enriched in trace elements and REE. The most differentiated melts reach ore concentrations in some elements (Th, Nb, Ta, Zr, and REE). It is demonstrated that the variations in H2O concentrations were controlled by magma degassing.
Sr and Nd isotope systematics was studied. The variations in Sr isotopic composition are proportional to the 87Rb/86Sr ratio and could be produced in a high-87Rb/86Sr melt during a geologically short time period.
The evolution of the melts that produced the volcanic rocks was controlled by crystal fractionation of parental basalt magmas at a minor role of melt mixing and silicate-sulfide liquid immiscibility. The alkaline salic rocks were generated in magma chambers at depths of 13–3.5 km where the initial melts underwent profound differentiation, which led to the formation of pantellerites and comendites strongly enriched in trace elements (Th, Nb, Ta, Zr, and REE). Data on the composition and crystallization parameters of the magmas were used to propose a generalized petrologic and geodynamic model for the origin of Changbaishan Tianchi volcano.
