Low-pressure (> 4 MPa) and high-temperature (> 1250 °C) incongruent melting of marly limestone: formation of carbonate melt and melilite–nepheline paralava in the Khamaryn–Khural–Khiid combustion metamorphic complex, East Mongolia

Abstract

Evidence for low-pressure incongruent melting of limestone containing up to 40 wt% of silicate minerals (marly limestone) is reported from the Khamaryn–Khural–Khiid Combustion Metamorphic (CM) complex in East Mongolia. Marly limestone exposed to high-temperature metamorphism during wild coal fires has acquired a mineral assemblage consisting of gehlenitic melilite and Al-diopside-to-kushiroite-dominant clinopyroxene as main phases and rare phases of celsian, spinel, perovskite, geikielite, etc. Melting of silicate minerals and calcite produced silicate melts of different compositions and stoichiometric calcitic (CaCO3) or non-stoichiometric carbonatite-like (CaCO3 + CaO) melts. Coalescence of silicate melt drops was followed by the formation of silica-undersaturated Ca-rich and Na-bearing paralava melts. Melilite-nepheline paralavas discovered in the Khamaryn–Khural–Khiid and Nyalga CM complexes in Mongolia have exceptional features of mineralogy and chemistry due to a rare combination of P–T conditions associated with wild coal fires and combustion metamorphism: a high temperature (> 1250 °C) and fluid pressure above 4 MPa that prevented decomposition of calcite. Such paralavas from the two CM complexes are composed of similar mineral assemblages made up of melilite, clinopyroxene, plagioclase, nepheline, Fe–Ca olivines (Ca-fayalite and kirschsteinite), K-Ba feldspars (celsian and hyalophane), spinel-group minerals, and rhonite-kuratite, with broad composition variations. Paralavas of this kind have never been reported before from anywhere. Their local mineralogical and geochemical specificity may be due to variations in the composition of carbonate protolith and in physicochemical conditions above the coal fire foci (T, P, gas composition, oxygen fugacity, and melt cooling rate). Low-pressure melting of calcite and the formation of carbonate (calcitic or carbonatite-like) melts have implications for phase relations in carbonate rocks altered by high-temperature metamorphism and metasomatism, as well to the origin of carbonatites. Calcite in carbonate sediments exposed to low pressures and high temperatures (above the invariant point Q1 in the CaCO3 phase diagram) does not decompose and can melt. Correspondingly, metamorphic decarbonation reactions do not produce CaO required for the crystallization of many Ca-rich index minerals of spurrite–merwinite facies. The melting point of calcite decreases markedly with increasing H2O content in the H2O–CO2 fluid, which may lead to melting of calcite in carbonate sediments and to formation of calcite-rich carbonatites at P–T crustal conditions.