Phase equilibria and P-T-X(Fe-Mg) relations involving Gl-St-Cd-Als-Bi-Mu-Oz-Kfs-H2O in medium-high grade metapelites (KFASH, KMASH and KFMASH sistems)

Abstract

In different areas of the Hercynian in the Iberian Peninsula some reactions are repeatedly observed in pelites, these reactions are:
Staurolite + muscovite + quartz = Biotite+ Al₂SiO₅+ H₂O
Garnet + muscovite = Al₂SiO₅+ biotite+ quartz
Biotite + Al₂SiO₅ + quartz = Cordierite + K feldspar+Hp
In order to examinate the P-T stability fields of these, and other similar reactions, aH the univariant equilibria in multisystems with Gt-Cd-St-Bi-Mu-Als-Qz-HP, Gt-St-Bi-Mu-Fk-AIs-QzHp and Cd-St-Bi-Mu-Fk-AIs-Qz-H₂O in Kp-FeO-AI₂O r SiO₂-H₂O (KFASH) system have been calculated, and their corresponding P-T grids have been constructed. The expansion of these reactions into divariant surfaces through the P-T-X (Fe-Mg) space was made by studying the assemblage Gt-Cd-St-Bi-Mu-Fk-AIs-Qz-HP in K₂O-FeO-MgO-Al₂O₃-SiO₂-H₂O (KFMASH) with a Fe/Fe+Mg relationship Gt > St > Bi > Cd such as observed in most of natural pelites. A resultant grid was obtained by combining those obtained in the aboye systerns. This grid has been P-T located for P_H20 = P₁₉ near QFM buffer, and excess muscovite and quartz conditions Reaction slopes in this grid were calculated within different P-T surroundings from thermodynamic data as weH as by considering the existing experimental data. In addition to the stability fields of reactions the P-T-X_Fe-Mg theoric relations for three univariant and thirteen divariant reactions have been obtained. The grid confirms the imposibility of staurolite-K feldspar and Garnet-Cordierite-Muscovite coexistence, as weH as the extension of the stability fields for Garnet-Staurolite, Cordierite-Staurolite and Garnet-Cordierite assemblages in muscovite-poor metapelites.