Experimental Study of the Crystallization Conditions of Ongonites of the Ary-Bulak Massif (Eastern Transbaikalia)

Abstract

Phase equilibria were studied experimentally in three rock samples of the Ary-Bulak massif at T = 800–700°C, P = 1 kbar, water content 10 wt %, and oxygen fugacity corresponding to the Ni–NiO and Mt–Hem buffers. Liquidus phases in melts obtained by melting of Ca-rich ongonites are fluorite, topaz, quartz, and plagioclase (andesine, bytownite). The liquid immiscibility of silicate and F–Ca salt melts described in the Ary-Bulak ongonites was not found in the experimental samples. The liquidus temperature of Ca-rich samples is ≥800°C, which is not typical for highly evolved differentiated granite melts. Based on the obtained factual material, it was concluded that simple experiments on melting–crystallization of rock samples of the Ary-Bulak massif do not fully reproduce its formation. It is possible that an important role in nature was played by irreversible processes that are not taken into account in this experimental series: interaction with host rocks, a sharp significant change in the fluid regime or P-T parameters, etc. The relicts of immiscible silicate and salt F–Ca melts described in natural Ca–F-rich samples also could be caused by one of these processes.