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Calculated phase equilibria for low‐ and medium‐pressure metapelites in the KFMASH and KMnFMASH systems
Author(s) -
Wei C. J.,
Powell R.,
Clarke G. L.
Publication year - 2004
Publication title -
journal of metamorphic geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.639
H-Index - 114
eISSN - 1525-1314
pISSN - 0263-4929
DOI - 10.1111/j.1525-1314.2004.00530.x
Subject(s) - cordierite , staurolite , muscovite , geology , chlorite , feldspar , mineralogy , biotite , kyanite , mineral , geochemistry , quartz , metamorphic rock , materials science , paleontology , ceramic , composite material , metallurgy
Petrogenetic grids in the KFMASH and KMnFMASH model systems calculated with the software thermocalc 3.1 are presented for the P–T range 0.5–12 kbar and 450–900 °C, for assemblages involving garnet, muscovite, chloritoid, biotite, chlorite, staurolite, cordierite, spinel, orthopyroxene, K‐feldspar, Al 2 SiO 5 phases, quartz, water and melt. Based on calculated compatibility diagrams and P–T and T–M Mn [Mn/(Mg + Fe + Mn)] pseudosections for different metapelitic bulk compositions, the principal conclusions are that the addition of Mn to the KFMASH system: (i) enhances the stability of garnet, and, to a lesser extent, aluminosilicates; (ii) reduces the stability of staurolite, cordierite and, to a lesser extent, chlorite; and (iii) extends the medium pressure stability of muscovite and the low‐ P stability field of K‐feldspar. The influence of Mn on individual mineral stabilities is strongly related to rock composition, in particular, to the relative contents of Al 2 O 3 and K 2 O. For metapelites of a range of compositions and M Mn values, P–T pseudosections in the KFMASH system, in most cases, do not adequately predict the mineral assemblages observed in natural assemblages under medium and low‐pressure conditions. In contrast, the P–T pseudosections in the KMnFMASH system generally provide more satisfactory results, suggesting that MnO is one of the non‐KFMASH components that should not be neglected in documenting the phase equilibria of medium‐ and low‐ P metapelites.