Phase equilibria in the CKFMAST-COH system applied to garnet-wollastonite calc-silicate rocks: (P-)T-XCO2 evolution and potential for solution models comparison

Calc-silicate rocks are notorious for being amongst the most challenging rocks for petrological models to be employed. The chemical heterogeneity of their protoliths leads to the metamorphic growth of sometimes an exaggerated number of mineral phases. This, coupled with the current lack of activity-composition models directly calibrated to calc-silicate phases, can complicate the thermodynamic modelling of such rocks. In the Variscan chain of Sardinia, garnet-wollastonite calc-silicate rocks hosted in andalusite-bearing metapelites represent good candidates for the modelling of calc-silicate assemblages. The area is characterized by a LP-HT regional metamorphism followed by contact metamorphism during granitoid intrusion, with andradite+wollastonite+vesuvianite growth in the calc-silicate rocks. The observed microstructures combined with mineral chemistry allowed us to reconstruct a prograde evolution characterized by the growth of phyllosilicatesàzoisiteàCa-amphiboleàK-feldspar and Mg-rich clinopyroxene (with minor plagioclase) grossular and epidoteàandraditeàwollastonite and vesuvianite. Minor phases are (relict) quartz and calcite, scapolite, titanite, apatite and johannsenite. We explored isobaric (0.5 GPa) T-XCO2 petrogenetic grids in the CKFMAST-COH system to put further constrains on the T-XFluid evolution underwent by the samples. In the T-XCO2 space, the inferred reactions are crossed, during the prograde path, from ~435 ºC (zoisite-in) to ~640 ºC (wollastonite-in) with an XCO2 decrease from 0.56 to 0.13. The T-X CO2 invariant points associated to the grossular- and wollastonite-forming univariant reactions match with the assemblage Grt+Cpx+Qz+Wo+Ttn modelled in the T-XCO2 phase diagram, which is in turn consistent with the peak mineralogy observed in thin section. Peak temperatures at the wollastonite-in isograd are slightly lower than those reported for the neighbor migmatitic orthogneiss (680-700 °C / 0.32–0.46 GPa, Cruciani et al., 2022). Decompression and heating during granitoid intrusion led to the growth, between 650-720ºC, of andradite on grossular, epidote and clinopyroxene, vesuvianite on grossular and, possibly, further vesuvianite-consuming late wollastonite. Invariant points associated to andradite and vesuvianiteforming univariant reactions depicted from a second isobaric (0.25 GPa) T-XCO2grid again match, in the T-XCO2 phase diagram, with the microstructural relationship and mineral assemblage observed in the samples. Provided the good consistency of our (P-)T-XCO2 data with the regional metamorphic conditions of the area, we modelled our samples using different activity-composition (a-X) solution models. Our preliminary results may provide an useful background for i) a correct a-X models application in the thermodynamic modelling of calc-silicate rocks, and ii) future efforts regarding the calibration of new thermodynamic models for common calc-silicate phases such as for andradite-rich garnets, scapolite or vesuvianite.