『Abstract
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』
Introduction
KFMASH phase relations
KFMnMASH phase relations
Areas studied
Stonehaven: petrography, assemblages and metamorphic zonation
SE Tauern Window: Petrography, assemblages and metamorphic zonation
Mineral compositions
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
Acknowledgments
References