The phase relations of muscovite−quartz-bearing pelitic schists containing combinations of garnet (Grt), staurolite (St), chloritoid (Cld), biotite (Bt) and chlorite (Chl) are examined (1) to assess the influence of manganese on natural assemblages, and (2) to constrain the topologies of petrogenetic grids, particularly with respect to the controversial assemblage Cld + Bt. Two field areas were studied: Stonehaven, NE Scotland (P〜4.5 kbar) and the SE Tauern Window, Austria (P〜7 kbar), both characterized by the up-grade progression from typical ‘garnet-zone’ Grt + Chl assemblages to ‘staurolite-zone’ St + Bt ± Grt assemblages via a narrow, complex zone containing Cld + Bt assemblages. In both areas, the following commonly observed chemographic relations hold: Mg/(Mg + Fe): Grt＜St＜Cld≪Bt＜Chl; Mn/(Mn + Fe + Mg): Chl〜Bt≪St＜Cld≪Grt. These compositions yield the MnAFM-discontinuous reaction (Ms + Qtz + H2O in excess): Cld + Chl = Grt + St + Bt.
The distributions of mineral assemblages in both areas are more consistent with the operation of MnAFM reactions than of traditional AFM reactions. Clear correlations exist between Mn content and assemblage in rocks that crystallized at the same P and T.
In the SE Tauern, low-grade Grt + Chl assemblages show a wide range of Mn contents. The crystallization of low-Mn Grt + Chl assemblages down-grade of, but at similar pressures to, low-Mn Grt + Cld + Bt + Chl assemblages implies that the right-hand side of the reaction Grt + Chl = Cld + Bt (Fe,Mg) is stabilized by increasing T. The distributions of assemblages in the areas studied also show differences that are ascribed to P effects. The assemblage Gtt + St + Cld + Chl is common in the SE Tauern but absent from Stonehaven. mn contents of respective minerals in the assemblage Grt + St + Cld + Bt + Chl are higher at Stonehaven than in the SE Tauern, implying that the Cld + Chl = Grt + St + Bt (Mn,Fe,Mg) reaction boundary extends to the low-P side of the [Als,Crd] invariant point in the Mn-free system.
Schreinemakers' rules are used to construct two KFMnMASH grids, in which the Cld + Bt assemblages has markedly different stability limits; one is based on the KFMASH grid of Harte & Hudson (Geological Society Special Publication 8, 323-337, 1979), in which Cld + Bt is stable over a narrow T interval at relatively low P, and the other on the KFMASH grids of Spear & Cheney (Contributions to Mineralogy and Petrology 101, 149-164, 1989) and Wang & Spear (Contributions to Mineralogy and Petrology 106, 217-235, 1991), in which Cld + Bt is stable over wide ranges of P and T. It is argued that available natural-rock data are more compatible with the former.
Key Words: pelites; KFMnMASH petrogenetic grid; chloritoid + biotite; Stonehaven; Tauern Window』
KFMASH phase relations
KFMnMASH phase relations
Stonehaven: petrography, assemblages and metamorphic zonation
SE Tauern Window: Petrography, assemblages and metamorphic zonation
Results from Stonehaven and the SE Tauern
Distribution of assemblages and isograd patterns
Observed MnAFM phase relations
The effects of minor components other than Mn
Discussion - comparison of natural sequences and petrogenetic grids
Derivation of alternative KFMnMASH grids
Comparison of KFMnMASH grids
Implications for the KFMASH grid