235 D7. General Topics in Geochemistry and Metamorphic Petrology (Posters)

Wednesday, 12 October 2022
9:00 AM - 1:00 PM
Exhibit Hall F

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

Paper number: 235-1
Program: Technical Sessions
Time: 9:00 AM – 1:00 PM

Authors

Abstract

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–XCO2 (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 XCO2 (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 XCO2 condition (XCO2 = 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 XCO2, 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.

Geological Society of America Abstracts with Programs. Vol. 54, No. 6

doi: 10.1130/abs/2022AM-379680

© Copyright 2022 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.