The Effect of Mn on the Phase Relations of Medium-Grade Pelites: Constraints from Natural Assemblages on Petrogenetic Grid Topology

The phase relations of muscovite-quartz-bearing pelitic schistscontaining combinations of garnet (Grt), staurolite (St), chloritoid(Cld), biotitt (Bt) and chlorite (Chl) are examined (1) to assessthe influence of manganese on natural assemblages, and (2) toconstrain the topologies of petrogenetic grids, particularlywith respect to the controversial assemblage Cld +Bt. Two fieldareas were studied: Stonehaven, NE Scotland (p 4•5 kbar)and the SE Tauern Window, Austria (P 7 kbar), both characterizedby the up-grade progression from typical ‘garnet-zone’Grt+Chl assemblages to ‘staurolite-zone’ St+Bt±Grtassemblages via a narrow, complex zone containing Cld+Bt assemblages.In both areas, the following commonly observed chemographicrelations hold: Mg/(Mg+Fe): Grt<St<Cld<<Bt<Chl;Mn/(Mn+Fe+Mg): Chl Bt<<St<Cld<<Grt. These compositionsyield the MnAFM-discontinuous reaction (Ms+Qtz+H₂O in excess):Cld+Chl = Grt+St+Bt. The distributions of mineral assemblages in both areas are moreconsistent with the operation of MnAFM reactions than of traditionalAFM reactions. Clear correlations exist between Mn content andassemblage in rocks that crystallized at the same P and T. In the SE Tauern, low-grade Grt+Chl assemblages show a widerange of Mn contents. The crystallization of low-Mn Grt+Chlassemblages down-grade of, but at similar pressures to, low-MnGrt+Cld+Bt+Chl assemblages implies that the right-hand sideof the reaction Grt+Chl = Cld+Bt (Fe, Mg) is stabilized by increasingT. The distributions of assemblages in the areas studied alsoshow differences that are ascribed to P effects. The assemblageGrt+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+Chlare higher at Stonehaven than in the SE Tauern, implying thatthe Cld+Chl = Grt+St+Bt (Mn, Fe, Mg) reaction boundary extendsto the low-P side of the [AIs, Crd] invariant point in the Mn-freesystem. Schreincmakcrs' rules are used to construct two KFMnMASH grids,in which the Cld+Bt assemblage has markedly different stabilitylimits; 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 relativelylow 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 andPetrology 106, 217–235, 1991), in which Cld+Bt is stableover wide ranges of P and T. It is argued that available natural-rockdata are more compatible with the former. KEY WORDS: pelites; KFMnMASH petrogenetic grid; chloritoid + biotite; Stonehaven; Tauern Window