The natural reaction muscovite+chlorite+chloritoid=andalusite+biotite+ quartz+H20 and a new petrogenetic grid

The univariant KFMASH reaction, chlorite+chloritoid+muscovite=andalusite+biotite+quartz+H2O, occurs in a low pressure (1–2 kbar) thermal aureole, defining a new biotite isograd. The Mg/Fe+Mg ratios of chlorite, chloritoid and biotite in the reaction are 0.14, 0.04, and 0.12, respectively. The univariant equilibrium is generated from a low pressure invariant point involving the phase chlorite, chloritoid, biotite, aluminosilicate, and staurolite that are added to the petrogenetic grid of Harte and Hudson (1979). This addition eliminates the unrealistic stability field of cordierite+staurolite in Harte and Hudson grid. Two other univariant reactions, terminal reactions of chlorite and muscovite, are also observed in the aureole. The three univariant reactions separate the aureole into four zones, chlorite-chloritoid zone, andalusite-biotite-chlorite zone, andalusite-biotite-cordierite zone, and K-feldspar zone.