PHASE RELATIONS IN METACARBONATE ROCKS WITH APPLICATIONS TO NATURAL MINERAL ASSEMBLAGES IN CONTACT METAMORPHISM

The phase relation of metamorphic assemblages is largely controlled by pressure (P), temperature (T) and the composition (X) of fluid that the rock reacts with. P-T-X petrogenetic grids are useful tools to visualize the phase relation and also help to map the metamorphic paragenesis to these physical conditions. To extend the grids of calcsilicate (skarn) systems, we constructed T–X_CO2 (composition of H₂O-CO₂ fluid) grids in the system CFMASHc (CaO-FeO-MgO-Al₂O₃-SiO₂-H₂O-CO₂), with the addition of Fe₂O₃ or TiO₂, and subsystems (CMASHc, CMSHc, CFSHc and CASHc). We adopted internally consistent thermodynamic datasets of Holland and Powell (1998) and compatible activity-composition models for solid solutions. To cover phase parageneses at a variety of geological settings, the grids are constructed for temperature range from 300℃ to 1000℃ at 2 and 4 kbar. The phase assemblages chosen in the grids represent a majority of rock-forming and accessory phases encountered in metamorphosed carbonate rocks.

The univariant curves in the calculated grids are mostly decarbonation reactions that produce calcsilicate mineral assemblages by consuming carbonate. Lower X_CO2 (higher fluid H₂O content) and higher temperatures (closer to intrusion-carbonate contact) favor more carbon release. In addition, phase assemblages vary systematically with increasing temperatures. At a low X_CO2 condition (X_CO2 = 0.1), the index phases appear, in the order from low temperature to high temperature, are talc, tremolite, diopside, olivine ± garnet, and wollastonite. As the pressure increases from 2 kbar to 4 kbar, invariant equilibria shift to lower X_CO2, which means easier for decarbonation reactions to take place.

Such metamorphic zones from low-T to high-T is observed at contact aureoles where the distances to the contact represent a temperature gradient. For example, in skarn at the Mason Valley Mine of Nevada, metacarbonate rock that appears the most proximal to the intrusion (high temperatures) contains garnet, while diopside, tremolite and talc appear with increasing distances to the contact (lower temperatures). The calculated petrogenetic grids, together with pseudosection simulations, could be a powerful approach to unravel P-T conditions and fluid compositions of natural calcsilicate assemblages in contact aureoles.