2:00 PM - 2:15 PM
[SMP25-14] Effect of syn-metamorphic fluids on HP-LT chloritoid-bearing assemblages: a study case from the Monti Romani (Italy)
Keywords:HP/LT metamorphism, chloritoid, fluid/rock interaction, ferric iron, RSCM thermometry, pseudosection modelling
The Monti Romani Complex (Central Italy) exposes metapelitic – metapsammitic rocks that were metamorphosed during the Alpine Orogeny. HP-LT metamorphism is generally cryptic within these rocks, showing graphite-bearing white mica ± paragonite parageneses, where no Fe-Mg-silicate is evident. Only a handful of samples, collected in correspondence of quartz veins, lack graphite and contain chlorite or zoned chloritoid associated with hematite. Chloritoid zoning, in particular, ranges dramatically from XMg = 0.10-0.14 to XMg = 0.27. Raman Spectroscopy on Carbonaceous Material constraints the thermal peak to 340 – 420 °C, excluding a possible contribution of temperature increase for chloritoid zoning.
Thermodynamic modelling in the system MnO–Na2O–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O (MnNKFMASHTO) fails to reproduce the observed mineral chemistry, if all iron is considered as divalent. The introduction of ferric iron via P–T–X(Fe2O3) phase equilibria modelling reproduces the observed equilibria, mineral chemistry, and modes for values of X(Fe*) = Fe2O3/FeTOT ranging from 0.1 to 0.7. Chloritoid zoning can be explained by a change in the oxidation state of Fe at diminishing pressure and constant temperature, consistent with isothermal exhumation from P = 1.1–1.2 GPa to P = 0.6–0.7 GPa.
Our study suggests that the introduction of an oxidizing aqueous fluid in graphite-bearing metapelite deeply modified mineral stability, catalyzing the growth of Mg-Fe chloritoid and chlorite. This study highlights that the Fe2O3 content has a profound influence on the mineral assemblages that are stable in metapelites. The use of P–T–X(Fe2O3) phase equilibria diagrams is therefore fundamental to derive reliable P–T estimates from the analysis of greenschist- to blueschist-facies parageneses in metasedimentary rocks.
Thermodynamic modelling in the system MnO–Na2O–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O (MnNKFMASHTO) fails to reproduce the observed mineral chemistry, if all iron is considered as divalent. The introduction of ferric iron via P–T–X(Fe2O3) phase equilibria modelling reproduces the observed equilibria, mineral chemistry, and modes for values of X(Fe*) = Fe2O3/FeTOT ranging from 0.1 to 0.7. Chloritoid zoning can be explained by a change in the oxidation state of Fe at diminishing pressure and constant temperature, consistent with isothermal exhumation from P = 1.1–1.2 GPa to P = 0.6–0.7 GPa.
Our study suggests that the introduction of an oxidizing aqueous fluid in graphite-bearing metapelite deeply modified mineral stability, catalyzing the growth of Mg-Fe chloritoid and chlorite. This study highlights that the Fe2O3 content has a profound influence on the mineral assemblages that are stable in metapelites. The use of P–T–X(Fe2O3) phase equilibria diagrams is therefore fundamental to derive reliable P–T estimates from the analysis of greenschist- to blueschist-facies parageneses in metasedimentary rocks.