Mineral Equilibria and P T Diagram for Fe Al Metapelites in the KFMASH System (K 2 O FeO MgO Al 2 O 3 SiO 2 H 2 O)

Petrology, Vol. 13, No. 1, 2005, pp. 73 83. Translated from Petrologiya, Vol. 13, No. 1, 2005, pp. 81 92. Original Russian Text Copyright 2005 by Likhanov, Reverdatto, Selyatitskii. English Translation Copyright 2005 by åäiä Nauka /Interperiodica (Russia). Mineral Equilibria and P T Diagram for Fe Al Metapelites in the KFMASH System (K 2 O FeO MgO Al 2 O 3 SiO 2 H 2 O) I. I. Likhanov, V. V. Reverdatto, and A. Yu. Selyatitskii Institute of Mineralogy and Petrography, Siberian Division, Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090 Russia e-mail: likh@uiggm.nsc.ru Received September 10, 2003 Abstract A new variant of the petrogenetic grid in the KFMASH system proposed for Fe Al metapelites is developed and its P T parameters are verified with the use of natural observations, experimental data, and thermodynamic calculations, with the latter conducted using data from the thermodynamic databases (Berman et al., 1985; Berman and Brown, 1985; Berman, 1988). The comparative analysis of petrogenetic grids for typical and Fe Al metapelites indicates that medium- and high-temperature regions of most of the currently used diagrams are practically identical, except only for the appearance of the stability field of the Grt + Crd + Ms assemblage under low pressures. Notable differences in the topology of the diagrams exist at low and medium metamorphic grades, at T 570 C, at which Fe Al metapelites are characterized by the following succession of mineral assemblages: + + Ms Crd + at low pressures or at moderate pressures. The results of thermodynamic simulations are consistent with data on the metamorphic evolution of low- and moderate-pressure Fe Al metapelites in several metamorphic terranes worldwide. OUTLINE OF THE PROBLEM AND FORMULATION OF THE TASK The accumulation of detailed petrological data on different types of zoning in low- and medium-pressure metamorphic complexes resulted in the identification of new types of phase equilibria and important relations between the variations in metapelite mineral assemblages and the chemistry of these minerals with varying physicochemical parameters. These data turned out to be not fully consistent with currently used petrogenetic grids. This is particularly typical of pelites of unusual Fe- and Al-rich composition, whose metamorphism brings about rare mineral assemblages with chloritoid, Fe-cordierite, and other minerals, such as chloritoid + biotite, chloritoid + biotite + andalusite, and cordierite + garnet + muscovite. For example, one of the key mineral assemblages of metapelites is chloritoid + biotite, which is stable in the petrogenetic diagrams (Harte and Hudson, 1979; Kepezhinskas and Khlestov, 1977) under both relatively low and high pressures within a narrow temperature range. In contrast, in the petrogenetic grid (Spear and Cheney, 1989), this assemblage is stable over a broad P T interval (Fig. 1). These grids also exhibit other differences. For instance, Spear and Cheney (1989) demonstrate that chlorite with biotite is stable on the low-temperature side of the equilibrium chlorite + biotite = garnet + chlorite + muscovite, while Harte and Hudson (1979) believe that the stable assemblage on the low-temperature side of this equilibrium is garnet and chlorite (Fig. 1). The problem of the stability of the garnet + cordierite + muscovite assemblage is of great importance for metamorphic petrology, because this assemblage is shown as unstable in many P T diagrams and is thought to appear only within the stability field of K-feldspar with the participation of Mn- and Fe-rich garnet (Spear and Cheney, 1989). If this assemblage is assumed to be stable, the stability field of the alternative assemblage biotite + andalusite at P 3 4 kbar either disappears (Glebovitskii, 1973) or shifts toward higher temperatures compared with those of the assemblage garnet + cordierite + muscovite (Lepezin, 1978). This is, however, at variance with observations in low-depth metamorphic aureoles (Korikovsky, 1979). These discrepancies are explained by different interpretations of natural observations and the use of different sets of mineralogical (relations between the X Fe values of minerals of variable composition) and thermodynamic data in constructing the P T diagrams (Droop and Harte, 1995). The analysis of literature data indicates that the discrepancies stem mostly from the determining effect of the bulk-rock composition of metapelites on the stability of certain mineral assemblages (Hoschek, 1969). The key point is here the X Fe and X Al of the coexisting minerals. Because of this we attempted to construct a new variant of a petrogenetic grid applicable to the Feand Al-rich metapelite system. MATERIALS Our research was based on studying low-calcic (<1.5 wt % CaO) and moderately potassic (3 4 wt % ä 2 é) metapelites rich in iron and alumina. The excess phase of these rocks at low and medium metamorphic 73

74 LIKHANOV et al. P () Grt + St + Chl (a) (b) (c) () (St) (Grt) (Chl) + + Chl + Grt + (Grt) (Chl) + Chl Grt + T [Als, Crd] [Als, Crd] + [Als, Crd] + (St) (Grt) + Chl + Grt + Gld (Chl) () Grt + St + Chl () (St) Gld+ Grt + St + Chl () () Fig. 1. Configuration of univariant equilibrium lines emanating from invariant points without aluminosilicate and cordierite [Als, Crd] in the petrogenetic diagrams constructed by different authors for typical metapelites in the KFMASH system. (a) (Albee, 1965; Harte and Hudson, 1979; Powell and Holland, 1990; Fed kin, 1970); (b) (Kepezhinskas and Khlestov, 1977; Glebovitskii, 1973; Korikovsky, 1969); (c) (Spear and Cheney, 1989; Wang and Spear, 1991). Variants of the diagram are grouped according to the position of the + stability field and its boundary mineral equilibria regardless of the reaction volume. Some of the diagrams (Korikovsky, 1969; Fed kin, 1970) imply negative slopes of univariant reaction lines. The shaded areas correspond to the stability of the + assemblage. grades is muscovite, which gives way to potassic feldspar at higher temperatures. Hoschek (1967) proposed to identify such rocks based on the following proportions of major components: (FeO + MgO)/(FeO + MgO + Al 2 O 3 ) < 0.63 and FeO/(FeO + Al 2 O 3 ) < 0.58. In the petrogenetic diagram (Symmes and Ferry, 1992), these rocks are richer in Fe [molar X Fe = FeO/(FeO + MgO + MnO) = 0.65 0.85] and Al (X Al = (Al 2 O 3 3K 2 O)/(Al 2 O 3 3K 2 O + FeO + MgO + MnO) = 0.3 0.6] than the average normal metapelites, which typically have X Fe = 0.52 and X Al = 0.13 (Shaw, 1956; Ague, 1991). In an AFM triangular diagram (Thompson, 1957), these chemical compositions plot above the garnet chlorite tie line (Fig. 2). Most researchers believe that these rocks are redeposited and metamorphosed products of ancient lateritic weathering crusts (Yudovich and Ketris, 2000; Franceschelli et al., 2003). CONSTRUCTING PRINCIPLES OF THE PETROGENETIC GRID The petrogenetic grid for Fe Al metapelites was constructed using a complex approach on the basis of natural observations, experimental data, and thermodynamic calculations. A principal P T diagram for rocks of this type was calculated in the KFMASH system with two negative degrees of freedom and six components: K 2 O FeO MgO Al 2 O 3 SiO 2 H 2 O (Korzhinskii, 1973). This diagram cannot display all possible complications caused by elevated amounts of MnO, CaO, Na 2 O, Fe 2 O 3, and ZnO in the rocks. We assumed that these components are nearly completely concentrated in certain phases pervasively present in metapelites: Na 2 O and CaO are almost completely incorporated into plagioclase, Fe 2 O 3 is contained in ore minerals, and MnO and ZnO are predominantly concentrated in garnet and staurolite. The list of minerals occurring in metapelites under low and moderate pressures is practically limited by the following ten phases: Grt (garnet), St (staurolite), (chloritoid), (biotite), Chl (chlorite), Crd (cordierite), Als (andalusite, sillimanite, or kyanite), Ms (muscovite), Qtz (quartz), and H 2 O (pore fluid). The symbols of minerals are given according to (Kretz, 1983). The experimental data on the KFMASH system are fairly contradictory, particularly in what concerns the position of the Al 2 SiO 5 triple point. For example, according to (Holdaway, 1971) and (Richardson et al., 1969), the intersection of the equilibrium lines Ms + Qtz = Sil + Kfs + H 2 O and And = Sil occurs at temperatures of 615 and 675 C and pressures of 2.1 and 4.1 kbar, respectively. Judging from the data in (Holdaway, 1971), the mineral assemblage And + + Crd ± Chl should be stable only at very low pressures (<2.5 kbar), which is in conflict with the wide spread of this mineral assemblage in regionally metamorphosed rocks. At the same time, Fe-rich cordierite in the Crd + Kfs + Sil + + Ms + Qtz assemblage should be stable only at P H2 O < 2.6 kbar (Holdaway and Lee, 1977), which is in conflict with experimental data on its stability limits (Skippen and Gunter, 1996; Carey, 1995). Because of this, the equilibrium lines and the coordinates of the Al 2 SiO 5 triple point were plotted according

MINERAL EQUILIBRIA AND P T DIAGRAM 75 P, kbar A Als + Ms Grt + + Als 14 St Crd + Chl + Ms [Crd, Grt] Als + Ms + [Crd, Chl] Grt 10 F Chl M + Chl + Ms + Ms + Grt St + Ms Grt+ + Als 6 + Als St + Chl St + Als + Grt + Grt+ Chl + Ms Grt+ Chl + Ms St + Chl + Ms [Crd, Als] Crd + St + Chl + Ms [, Grt] St + Crd + Ms Crd + Als + Chl + Ms Ky Sil 2 Ky And And + Chl + Grt And + Chl Grt + Crd [St, ] [Grt, ] And + Chl + Crd + St + And Grt + Crd + And Chl + St + Crd Als + Chl St + Crd [Ghl, ] [Als, ] Grt + Crd + St St Chl + St Grt + Crd Grt + Crd + And + Chl Grt + Crd + St Chl + Ms + Grt Crd + [, Als] Grt+ Crd + Ms Ms + Qtz Als + Kfs + H 2 O [, Chl] Grt + Crd + Ms 0 400 500 600 T, C Sil And Fig. 2. Schematic P T diagram proposed for Fe Al metapelites in the KFMASH system. Heavy dotted lines correspond to Al 2 SiO 5 polymorph equilibria, according to calculations (Holland and Powell, 1985, 1990; Bohlen et al., 1991; Pattison, 1992) and experimental data (Kerrick and Heninger, 1984); the thin dotted line corresponds to the Ms + Qtz = Als + Kfs + H 2 O equilibrium, according to experimental data (Chatterjee and Johannes, 1974). Muscovite, quartz, and water are the excess phases constantly present in the system (not shown in the diagram, except when muscovite participates in univariant reactions with biotite). The number of univariant lines emanating from invariant points is constrained to five. The invariant point without cordierite and chlorite [Crd, Chl] can be metastable, because it occurs in the melting region at P H2 O = P tot. The inset in the upper right-hand corner presents an AFM triangular plot showing the field of the chemical compositions of Fe Al metapelites (ellipse) and the compositions of the coexisting minerals; the star shows the average composition of typical metapelite. to other calculated (Holland and Powell, 1985, 1990; Bohlen et al., 1991; Pattison, 1992) and experimental (Kerrick and Heninger, 1984) data. Consequently, we assumed that the Al 2 SiO 5 triple point is located at T = 550 ± 35 C and P = 4.5 ± 0.5 kbar. These P T parameters lie between the experimental estimates reported in (Holdaway, 1971; Richardson et al., 1969). Literature data on the chemistry of minerals of variable composition (Likhanov, 1988; Likhanov et al., 2001; Okuyama-Kusunose, 1993) suggest the following relations between the values of X Fe = FeO/(FeO + MgO) of minerals: Grt 95 92 > St 92 88 > 90 85 > 75 70 > Chl 70 65 > Crd 65 50. The compositions of the minerals utilized to calculate chemical reaction equations are

76 LIKHANOV et al. Table 1. Matrix of the stoichiometric coefficients of minerals for calculating reaction equations Component Grt St Chl Crd Ms Als Qtz Si 2.99 7.76 2.03 2.64 2.53 5.00 3.09 1.00 1.00 Al 1.95 19.02 3.93 1.82 3.01 3.97 2.83 2.00 0 Fe 1.98 3.36 1.76 1.69 3.09 1.19 0.04 0 0 Mg 0.10 0.31 0.19 0.55 1.33 0.78 0.04 0 0 K 0 0 0 0.89 0 0 0.82 0 0 H 2 O 0 2 2 1 4 0.5 1 0 0 X Fe 0.95 0.92 0.90 0.75 0.70 0.60 Note: The stoichiometric coefficients of cation proportions for minerals were calculated for the following numbers of oxygen atoms: St 48, Crd 18, Chl 14, Grt and 12, and Ms 11, Als 5, Qtz 2; X Fe = FeO/(FeO + MgO) is the Fe mole fraction of minerals. listed in Table 1. According to these data, the AFM ternary chemographic plot for these phases has the configuration presented in the inset of Fig. 2. The stoichiometric coefficients of the chemical reactions, which were calculated using the matrix algebra apparatus incorporated into the MATHEMATICA 4.2 program package and the NullSpace procedure (Likhanov and Reverdatto, 2002), are presented in Table 2. The multisystem nets and the slopes of reaction lines were calculated following the rules of the Schreinemakers (1948) analysis with regard for the heat and volume values of the dehydration reactions according to the Gibbs method. The mathematical expressions and the apparatus of thermodynamic calculations were described in (Spear et al., 1982). Minerals of variable composition were considered to be mixtures of the respective solidsolution end members. Thermodynamic data for minerals were compiled from the databases (Berman et al., 1985; Berman and Brown, 1985; Berman, 1988) (Table 3). The values for intermediate compositions were calculated for the ideal mixing of the end members interpolated with regard for the mixing entropy. The choice of this database was caused by the fact that it included thermodynamic parameters for phases whose chemical compositions most closely approximate the chemistry of natural mineral assemblages in Fe Al metapelites. For example, chlorites in this rock type are characterized by elevated Al 2 O 3 concentrations (2.9 3.1 f.u.) compared with the average compositions of chlorite (2.6 2.7 f.u.) in typical metapelites (Kepezhinskas, 1977). The thermodynamic constants for water at different T and P were calculated by the equation of state from (Haar et al., 1979). The P tot was assumed to be equal to (Dobretsov et al., 1970). P H2 O One of the key problems in construction a petrogenetic grid was determining the position of univariant equilibrium lines emanating from invariant points [Als, Crd], [Als, ], and [, Chl], which determined the character of mineral reactions in Fe Al metapelites with increasing temperature at low and moderate pressures. Calculations indicate that the stability field of the + Ms assemblage is constrained by three reactions: 0.32 + 0.26 + 0.77Qtz = 0.32Grt + 0.12Chl + 0.28Ms + 0.16H 2 O on the low-temperature side, 0.19Grt + 0.14Chl + 0.39Ms = 0.07St + 0.36 + 0.68Qtz + 0.45H 2 O at higher temperatures and moderate pressures, and 0.05Grt + 0.21Chl + 0.28Ms + 0.43Qtz = 0.26Crd + 0.25 + 0.75H 2 O at low pressures. The stability of the Grt + Crd + Ms assemblage is constrained by two univariant reactions: 0.06St + 0.20 + 0.92Qtz = 0.19Grt + 0.13Crd + 0.22Ms + 0.04H 2 O at moderate temperatures and 0.12Grt + 0.15Crd + 0.27Ms = 0.57Als + 0.25 + 0.70Qtz + 0.10H 2 O at high temperatures. Figures 2 and 3 demonstrate the configuration of the diagram with an approximate P T positioning and the stability field of the principal mineral assemblages, which were constructed with the aforementioned assumptions. The stability fields of staurolite, chloritoid, and cordierite and the compatibility relations within these limits are in good agreement with most experimental data on the KFMASH and purely Fe system (Ganguly, 1968, 1969; Helferdahl, 1961; Hoschek, 1969; Hess, 1969; Richardson, 1967, 1968; Seifert, 1970). DISCUSSION The analysis of the diagrams led us to the following petrologically important conclusions concerning Fe Al metapelites (Figs. 2, 3): (1) The stability field of the + Crd assemblage is characterized by the lowest pressures and temperatures (T = 430 530 C, P = 1 2 kbar). (2) The Grt + Crd + Ms assemblage is also stable under relatively low pressures (P 3 kbar) but higher temperatures (T = 570 680 C) than those of the + Crd assemblage; the Grt + Crd + Ms assemblage gives way to the + Als assemblage with increasing temperature. (3) The + assemblage is stable within a narrow temperature range (~50 C) in complexes of both low and elevated pressures; this assemblage is replaced by the + Ms assemblage with a temperature

MINERAL EQUILIBRIA AND P T DIAGRAM 77 Table 2. Calculated stoichiometric coefficients of mineral equilibria in the KFMASH system Reaction Grt St Chl Crd Als Ms Qtz H 2 O (Crd, Als, St) 0.32 0 0.34 0.26 0.12 0 0 0.28 0.77 0.16 (Crd, Als, ) 0.19 0.07 0 0.36 0.14 0 0 0.39 0.68 0.45 (Crd, Als, ) 0.27 0.07 0.49 0 0.04 0 0 0 0.42 0.71 (Crd, Als, Grt) 0 0.09 0.29 0.29 0.09 0 0 0.32 0.29 0.79 (Crd, Als, Chl) 0.21 0.08 0.48 0.09 0 0 0 0.10 0.24 0.80 (Crd, Grt, ) 0 0.05 0 0.27 0.09 0 0.77 0.27 0.04 0.49 (Crd, Grt, St) 0 0 0.15 0.30 0.08 0 0.60 0.33 0.02 0.64 (Crd, Grt, Chl) 0 0.16 0.33 0.03 0 0 0.86 0.03 0.04 0.35 (Crd, Grt,, Ms) 0 0.15 0.30 0 0.01 0 0.87 0 0.26 0.26 (Crd, Chl, St) 0.25 0 0.36 0.09 0 0 0.52 0.10 0.07 0.72 (Crd, Chl,, Ms) 0.07 0.14 0.19 0 0 0 0.88 0 0.39 0.11 (Crd, Chl, ) 0.16 0.11 0 0.04 0 0 0.94 0.04 0.07 0.23 (, Grt, St) 0 0 0 0.17 0.24 0.37 0.27 0.18 0.12 0.81 (, Grt,, Ms) 0 0.03 0 0 0.14 0.27 0.61 0 0.62 0.38 (, Grt, Als) 0 0.01 0 0.25 0.24 0.28 0 0.28 0.08 0.85 (, Grt, Chl) 0 0.06 0 0.20 0 0.11 0.92 0.22 0.03 0.20 (, Chl, St) 0.12 0 0 0.25 0 0.15 0.57 0.27 0.70 0.10 (, Chl, Als) 0.19 0.06 0 0.20 0 0.13 0 0.22 0.92 0.04 (, Chl,, Ms) 0.17 0.10 0 0 0 0.02 0.78 0 0.57 0.19 (Als,, Ms, Grt) 0 0.08 0.23 0 0.16 0.29 0 0 0.89 0.20 (Als,, Ms, Chl) 0.25 0.05 0.41 0 0 0.05 0 0 0.54 0.69 (Chl,, Ms, Grt) 0 0.14 0.29 0 0 0.01 0.87 0 0.24 0.28 (Chl,, Ms, St) 0.25 0 0.31 0 0 0.05 0.28 0 0.62 0.61 (Grt,, Ms, St) 0 0 0.06 0 0.16 0.29 0.48 0 0.73 0.35 (Als,, Ms, St) 0.18 0 0.20 0 0.07 0.16 0 0 0.75 0.58 (St,, Ms, ) 0.05 0 0 0 0.14 0.27 0.38 0 0.76 0.42 (St,, Ms, Crd) 0.27 0 0.35 0 0.03 0 0.38 0 0.55 0.60 (, Als,, Ms) 0.10 0.04 0 0 0.12 0.23 0 0 0.86 0.43 (, Als, St) 0.05 0 0 0.25 0.21 0.26 0 0.28 0.43 0.75 Note: Equilibria are labeled with the parenthetical symbols of minerals that are absent from the reactions. Reactants and reaction products have positive and negative stoichiometric coefficients, respectively. See Table 1 for the mineral compositions. increase. The latter assemblage is stable within a narrow temperature range under elevated pressures and over a broader temperature interval at low pressures. (4) The stability field of the + Ms assemblage is constrained by the appearance of the Crd + assemblage on the higher temperatures side at low pressures and by the assemblage at moderate pressures. The Crd + assemblage is stable within broad temperature and pressure ranges. (5) The stability fields of the and + Als assemblages are characterized by narrow temperature intervals. The assemblage gives way to + Als with increasing temperature. The latter assemblage is, in turn, replaced by the Grt + + Als association, which remains stable within broad pressure limits. (6) With a temperature increase at low and moderate pressures in biotite-bearing rocks, staurolite appears before kyanite. However, as the pressure increases, the respective temperature interval narrows, which results in a decrease in the temperature interval between the staurolite and kyanite isogrades at high pressures. Analysis of Phase Relations in Different Petrogenetic Grids (1) The stability of the + Crd assemblage in Figs. 2 and 3 at the lowest P T metamorphic parameters is consistent with the stability fields of this assemblage in petrogenetic grids of Korikovsky (1969), Kepezhinskas and Khlestov (1971), and Glebovitskii

78 LIKHANOV et al. Table 3. Thermodynamic data utilized in constructing the petrogenetic grid Mineral Formula V 0, J/bar S 0, J/mol K k 0, J/mol k 1 10 2, J/mol k 2 10 5, J/mol k 3 10 7, J/mol Quartz SiO 2 2.27 41.3 80.011 2.403 35.467 49.157 Andalusite Al 2 SiO 5 5.16 91.5 236.478 11.029 75.268 93.644 Kyanite Al 2 SiO 5 4.40 82.2 246.625 13.940 64.082 75.224 Sillimanite Al 2 SiO 5 4.99 95.8 268.930 22.723 12.713 25.096 Muscovite KAl 3 Si 3 O 10 (OH) 2 14.07 288.3 651.489 38.732 185.232 274.247 Garnet Mg 3 Al 2 Si 3 O 12 11.32 267.3 640.719 45.421 47.019 0.0 Fe 3 Al 2 Si 3 O 12 11.51 340.0 573.961 14.831 292.920 502.208 Biotite KMg 3 AlSi 3 O 10 (OH) 2 14.99 335.6 610.379 20.838 215.330 284.104 KFe 3 AlSi 3 O 10 (OH) 2 15.35 418.3 731.046 52.199 99.313 151.847 Staurolite Mg 4 Al 18 Si 7.5 O 44 (OH) 4 44.19 876.0 2502.311 145.710 668.042 912.051 Fe 4 Al 18 Si 7.5 O 44 (OH) 4 44.67 984.1 2577.674 162.654 611.859 869.205 Chlorite Mg 4.5 Al 3 Si 2.5 O 10 (OH) 8 20.98 429.8 1139.740 82.329 196.789 273.893 Fe 4.5 Al 3 Si 2.5 O 10 (OH) 8 21.52 554.0 1224.520 101.390 133.583 225.692 Chloritoid MgAl 2 SiO 5 (OH) 2 6.84 130.0 407.697 27.592 86.244 120.709 FeAl 2 SiO 5 (OH) 2 6.96 158.0 444.369 38.803 48.693 91.633 Cordierite Mg 2 Al 4 Si 5 O 18 + 0.5 H 2 O 23.30 459.8 927.409 50.705 248.346 332.775 Fe 2 Al 4 Si 5 O 18 + 0.5 H 2 O 23.54 514.9 965.089 59.177 220.254 311.352 Note: V 0 molar volume, S 0 molar entropy, k 0, k 1, k 2, and k 3 coefficients for calculating the heat capacity C p = k 0 + k 1 T 0.5 + k 2 T 2 + k 3 T 3 (Berman et al., 1985; Berman and Brown, 1985; Berman, 1988). (1973), as well as with the occurrence of this mineral assemblage in the outer portions of contact metamorphic aureoles (Halfendahl, 1961) and in low-pressure regionally metamorphosed complexes (Nel, 1927; Budanova, 1991) outside the staurolite stability field. (2) The Grt + Crd + Ms mineral assemblage is usually omitted from petrogenetic grids for typical metapelites represented by equilibria in the KFMASH system (Pattison and Tracy, 1991; Harte and Hudson, 1979), because the assumption of the stability of this assemblage leads to either the complete disappearance of the stability field of the alternative + And assemblage at P 3 4 kbar (Glebovitskii, 1973), or this field is shifted toward higher temperatures than those of the Grt + Crd + Ms assemblage (Lepezin, 1972, 1978). The petrogenetic grid proposed in (Spear and Cheney, 1989) allows for the stability of this assemblage only in the presence of potassic feldspar and only if the garnet is manganoan and ferrous. Analyzing the genesis of this assemblage, Korikovsky (1979) has arrived at the conclusion that the Grt + Crd + Ms assemblage can be formed only if it contains manganoan garnet (X Sps X Prp ), while metapelites with low bulk-rock Mn concentrations (X Sps X Prp ) contain the stable + Als assemblage. In Fe Al metapelites these assemblages do not exclude each other, and the stability field of the Grt + Crd + Ms assemblage gives way to the + Als field with increasing temperature (Fig. 2). This is consistent with data of Powell and Holland (1990), who have demonstrated that the Grt + Crd + Ms assemblage is stable in Fe Al rocks within a broad temperature range T = 520 680 C at P = 2 4 kbar. (3 4) The stability of + assemblage within a narrow temperature range at low and moderate pressures is consistent with all variants of the petrogenetic diagrams (Fig. 1), except only for the petrogenetic grid of Spear and Cheney (1989), in which this assemblage is shown as stable over a broad P T interval. However, thermodynamic analysis demonstrates that the relations between the sizes of the stability fields of the boundary + and + Ms assemblages can significantly vary because of the effect of MnO (Spear and Cheney, 1989; Droop and Harte, 1995; Mahar et al., 1997). For example, at Fe/(Fe + Mg) = 0.95 and Mn/(Mn + Mg + Fe) = 0.2 for garnet in the Ms + Qtz + + Chl + Grt + assemblage, the petrogenetic grid in (Spear and Cheney, 1989) admits the narrowing of the stability temperature interval for the + assemblage to ~50 C within the andalusite stability field because of the widening of the narrow field of the + Ms limiting assemblage. The petrogenetic grid of Powell and Holland (1990) does not permit the appearance of the + assemblage in the normal KFMASH system, but the introduction of minor amounts of Fe 3+ into the octahedral sites of biotite and chloritoid makes it possible the origin of this assemblage at the expense of + Ms and its stability at T = 460 550 C and P 3 kbar. Notable differences

MINERAL EQUILIBRIA AND P T DIAGRAM 79 P, kbar A Als + Ms Grt + + Als St + Chl + Ms Als 14 Crd St Grt Chl Als 10 F Grt M Chl + Chl + Ms St + Chl + Ms 8 St + Ms Grt+ + Als Als + Chl + Ms Crd + Grt Crd 6 + + Ms + Ms 7 Crd + St + Chl + Ms St + Crd + Ms 6 Ky Sil 5 4 3 2 Ky And Crd And + Chl + Crd Grt + Crd + And Chl + St + Crd + Ms Crd + Ms + Qtz Grt + Crd + Ms Als + Kfs + H 2 O 2 1 Sil And Grt + Crd + Ms Grt Crd 0 400 500 600 T, C Fig. 3. Stability fields of the principal mineral assemblages of Fe Al metapelites in the KFMASH system. The shaded (gray and black) areas correspond to the stability fields of eight mineral assemblages specified in the triangular AFM plot. Arrows indicate the evolution of the mineral assemblages in Fe Al rocks under low and moderate pressures: (1) Tono hornfels, Kitakama Mountains, Japan (Okuyama-Kusunose, 1993); (2) hornfels of the Ayakhta aureole, Yenisei Range, Russia (Likhanov et al., 2001); (3) Aspromonte metapelites, S. Calabria, Italy (Graesner and Schenk, 1999); (4) Syi metapelites, Kuznetsk Alatau, Russia (Likhanov, 1988); (5) metapelites in the Selimiye nappe, Turkey (Regnier et al., 2003); (6) Dutchess County metapelites, Taconic Mountains, New York, United States (Whitney et al., 1996); (7) metapelites in the three-state area, Taconic Mountains, United States (Wang and Spear, 1991); and (8) Variscan metapelites, S. Alps, Italy (Spalla et al., 1999). in the topology of the diagrams exist at low and intermediate metamorphic grades at T 570 C, when Fe Al metapelites usually show the following sequence of mineral reactions with increasing temperature: + + Ms at elevated pressures or + + Ms Crd + at low pressures. This conclusion is consistent with data in (Spear and Cheney, 1989), in which the P T diagram was developed on the basis of the thermodynamic dataset from (Berman, 1988). Phase equilibria in classic metapelite complexes of the Buchan and Barrovian types are principally different. In most petrogenetic diagrams (Figs. 1a, 1b), the + mineral assemblage can be formed only in the staurolite facies (in the And and St- zones under low and moderate pressures, respectively) via the decomposition of the Grt + Chl + Ms assemblage. (5 6) The conclusions concerning the temperature stability ranges of the, + Als, and Grt + + Als

80 LIKHANOV et al. assemblages within the framework of the + Als Grt + + Als evolutionary sequence are consistent with virtually all configurations of petrogenetic grids for typical metapelites in the KFMASH system (Glebovitskii, 1973; Korikovsky, 1979; Fed kin, 1970; Albee, 1965; Harte and Hudson, 1979; Kepezhinskas and Khlestov, 1977; Powell and Holland, 1990; Spear and Cheney, 1989) (Figs. 2, 3). Comparison of the Petrogenetic Grid and Natural Observations In order to assay the applicability of petrogenetic grid to Fe Al metapelites (Fig. 2), the P T parameters of characteristic mineral assemblages and their persistent combinations were correlated with data on thoroughly studied zonal metamorphic complexes. As reference rocks for the comparative analysis, we selected Fe- and Al-rich metapelites affected by regional metamorphism within a wide range of temperatures and pressures (Graesner and Schenk, 1999; Likhanov, 1988; Likhanov et al., 2001; Okuyama-Kusunose, 1993; Regnier et al., 2003; Spalla et al., 1999; Whitney et al., 1996; Wang and Spear, 1991). The P T evolutionary paths of these metamorphic complexes (1 8) are numbered in Fig. 3 in order of increasing pressure. The following prograde (with increasing temperature) evolutionary sequence of mineral assemblages was established for most of the aureoles: Chl + Ms + + Ms or Crd +, which is generally in good agreement with the petrogenetic P T diagram (Fig. 2). It is important to stress the following features of the evolution of these complexes that are consistent with the thermodynamic calculations. The + mineral assemblage appears at a temperature of 450 500 C and is stable within a narrow temperature range (~50 C) in complexes of both low and elevated pressure. Exception are the low-depth and Mn-rich hornfels of the Ayakhta aureole (Likhanov et al., 2001; Likhanov, 2003) and the Dutchess County metapelites (Whitney et al., 1996), in which the lower stability boundary of this assemblage is shifted to lower temperatures. Wang and Spear (1991) undertook a detailed petrological examination of the stability of the + assemblage in metapelite schists of the Barrovian type and determined that the appearance of this assemblage is controlled by the lithologies of the rocks, in particular, their Fe mole fractions (which is >60 for the bulk rocks). If this parameter is lower (<0.5), the stable assemblage is + Ms, with the rocks of intermediate composition exhibiting the following prograde succession of mineral assemblages: + Ms + + Ms. According to Wang and Spear (1991), the breakdown of the Grt Mn Fe Mg + Chl + Ms association in high-mn rocks results in the + assemblage in the low-temperature part of the zone, which gives way to the Grt Fe Mg + Chl + Ms with increasing temperature (this garnet has a different composition). In the Mn-free system, this assemblage is produced by the reaction Chl + Ms + + Qtz + H 2 O. These data are at variance with the phase relations documented in Barrovian (Korikovsky and Fedorovsky, 1980; Albee, 1972; Grambling, 1983) and Buchan (Labotka, 1981) metamorphic complexes of typical metapelites, in which the + assemblage appears no earlier than the St zone, first, through the decomposition of the earlier Grt Fe Mg + Chl + Ms assemblage and, later, due to net-transfer reactions like Chl Fe Mg + Ms + + Chl Mg Fe + Qtz + H 2 O. The + Ms assemblage appears at temperatures of 500 550 C, depending on the depth of the aureole, and remains stable within a narrow temperature interval ( 50 C), which tapers with increasing pressure. The stability field of this assemblage is constrained at high temperatures (T = 560 580 C) by the appearance of the Crd + assemblage at low pressures and at moderate pressures. Some discrepancies between the sizes of the stability fields of these assemblages, namely, the wider field of the + Ms assemblage at T = 480 620 C and P ~ 3 kbar and the absence of mineral reactions participated by staurolite, are characteristic of the hornfels of the Ayakhta aureole (Likhanov et al., 2001; Likhanov, 2003). These relations can be explained by the participation of Mn-bearing garnet (X Sps X Prp ) and, consequently, the widening of the stability temperature range of garnet-bearing assemblages with the narrowing of the stability fields of boundary mineral assemblages (Spear and Cheney, 1989). In Fe Al metapelites, in which less manganoan garnet (X Sps X Prp ) is stable, staurolite-bearing assemblages appear at T = 560 580 C, which is consistent with the petrogenetic grid and the mineral reactions described in Fe Al rocks of low and moderate pressures elsewhere (Graesner and Schenk, 1999; Likhanov, 1988; Spalla et al., 1999; Whitney et al., 1996). The stability of the Grt + Crd + Ms assemblage, which appears locally in the Crd zone of the Ayakhta aureole at T = 580 620 C and P ~ 3 kbar (Likhanov et al., 2001; Likhanov, 2003), corresponds to the P T parameters of the stability of this assemblage in the petrogenetic grid (Figs. 2, 3). CONCLUSIONS A new variant of a petrogenetic grid was developed for Fe- and Al-rich metapelites in the KFMASH system. The P T diagram was constructed and the P T coordinates of its elements were refined on the basis of natural observations, experimental data, and thermodynamic calculations. The calculations were conducted using the thermodynamic databases (Berman et al., 1985; Berman and Brown, 1985; Berman, 1988). The comparative analysis of petrogenetic grids for typical and Fe Al metapelites indicates that the medium- and high-temperature regions of most diagrams (Glebovitskii, 1973; Korikovsky, 1979; Fed kin, 1970; Albee, 1965; Harte and Hudson, 1979; Kepezhinskas and

MINERAL EQUILIBRIA AND P T DIAGRAM 81 Khlestov, 1977; Powell and Holland, 1990; Spear and Cheney, 1989) are practically identical, except only the appearance of the stability field of the Grt + Crd + Ms assemblage in low-pressure Fe Al metapelites. Notable differences between the topologies of the diagrams occur at low and intermediate metamorphic grades, at T 570 C, at which Fe Al metapelites are characterized by the following succession of prograde (with increasing temperature) mineral reactions: + + Ms Crd + at low pressures or St + at moderate pressures. The results of thermodynamic modeling are in agreement with data on the metamorphic evolution of low- and moderate-pressure Fe Al metapelites worldwide. ACKNOWLEDGMENTS The authors thank G.G. Lepezin and V.V. Khlestov (Institute of Mineralogy and Petrography, Siberian Division, Russian Academy of Sciences) for discussion of the results of this research. 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