1. metamorphic zyxwvutsrqpon Geol., zyxwvutsr 1994, zyxwvutsrq 12, 99-119 A calculated petrogenetic grid for the system K20-FeO- Mg0-AI2O3-SiO2-H20, with particular reference to contact-metamorphosed pelites GUOWEI XU, T. M. WILL* AND zyxwvu R. POWELL School of Earth Sciences, University zyxwvut of Melbourne, Parkville, Vic. 3052, Australia ABSTRACT zyxwvutsrqp A petrogenetic grid is presented for the system KFMASH zyxwv (K,O-FeO-MgO-A1,0,-SiO,-H,O), including biotite, muscovite, K-feldspar, chlorite, chloritoid, staurolite, cordierite, garnet, ortho- amphibole, orthopyroxene, spinel, andalusite, sillimanite, kyanite, quartz and corundum with H,O in excess, which was calculated using the computer program THERMOCALC and the Powell and Holland internally consistent thermodynamic dataset. By removing the normal constraint of having quartz in excess, both quartz-bearing and quartz-absent equilibria are shown. Quartz-absent equilibria are particularly relevant at high-T and low-P conditions, because of their common occurrence at these conditions. The calculated mineral assemblage and mineral compositional variations in terms of FeMg-, and (Fe, Mg)SiAlL, exchange vectors are broadly compatible with observations on natural rocks, particularly when non-KFMASH components are taken into account. Key words: contact metamorphism; KFMASH; metapelitic rocks; petrogenetic grid. INTRODUCTION Petrogenetic grids are powerful tools in summarizing P-T-X information for metamorphic rocks, because they can be used to document the mineral assemblage changes through the metamorphic peak. Many such grids for metapelitic rocks have been constructed, using a variety of methods (e.g. Korzhinskii, 1959; Albee, 1965; Hess, 1969; Kepezhinskas & Khlestov, 1977; Harte & Hudson, 1979; Pattison & Harte, 1985; Spear & Cheney, 1989; Powell & Holland, 1990). Most of the existing grids for metapelitic rocks are constructed for muscovite, quartz and HzO in excess (+mu + q + H,O), which is often applicable to regionally metamorphosed terranes. However, these grids cannot show corundum-bearing assemblages or muscovite- breakdown reactions. In addition, few grids contain enough information in 1ow-P regions below 2 kbar, where many univariant equilibria are extremely Fe-rich (Albee, 1965; Harte & Hudson, 1979). Low-P metamorphic rocks (e.g. in contact aureoles) are commonly very fine grained (especially low-grade rocks). Furthermore, as a result of either bulk compositional or metamorphic grade change, both quartz- and corundum-bearing assemblages are found in these rocks and muscovite-absent assemblages are common at high grade. Despite these complexities, qualitative petrogenetic grids for low-P metamorphic conditions have been constructed from petrological observations and limited experimental calibration (Riverd- atto, 1973; Pattison & Harte, 1985). A calculated petrogenetic P-T grid for the model * zyxwvutsrqponmlk Present address: Institut fur Mineralogie, Universitat Wiirzburg, Am Hubland, 8700 Wurzburg, Germany. system KFMASH is presented. A number of calculated petrogenetic grids based solely on thermodynamic data of mineral end-members have been constructed (Spear & Cheney, 1989; Will et al., 1989; Guiraud et al., 1990; Powell & Holland, 1990). Such grids have two main purposes: (1) to establish quantitative relationships between mineral composition and various intensive variables, and (2) to test the applicability of so-called ‘internally consistent’ thermodynamic datasets (e.g. Ber- man, 1988; Holland & Powell, 1990) to rocks. By comparing such calculated grids with grids constructed from observation on naturally occurring assemblages, the thermodynamic datasets can be assessed for their petrological consistency. A calculated grid is a ‘complete’ grid, in contrast with a petrologically derived one, as it contains all the invariant and univariant information of every possible bulk composition in the model system. On the other hand, a petrologically derived grid can only provide equilibria information for a usually small range of bulk compositions that correspond to the rocks studied. THE PETROGENETIC GRID The petrogenetic P-T grid presented here is calculated for the model system K,0-Fe0-Mg0-A1,03-Si0,-H,0. A temperature range of 480-800°C and pressure range of 0.1-7 kbar are selected as the P-T window. Minerals included are: muscovite (mu), biotite (bi), K-feldspar (ksp), chlorite (chl), chloritoid (ctd), cordierite (cd), staurolite (st), garnet (g), orthoamphibole (oa), spinel (sp), olivine (ol), orthopyroxene (opx), andalusite (and), sillimanite (sill), kyanite (ky), quartz (q), corundum (cor) and HzO. The compositional parameters of these minerals 99