CONTACT-METAMORPHISM AROUND THE STAWELL GRANITE, VICTORIA, AUSTRALIA

The contact metamorphosed metapelitic and metapsammitic rocks surround ing the Stawell granite, western Victoria, Australia, are divided into three zones: the low-grade zone, the medium-grade zone and the high-g rade zone. Detailed petrological study shows consistency of element di stributions, implying that equilibrium was widely attained in the rock s, although equilibrium volumes are generally small (millimetre scale) and considerable mineral chemical variations exist between adjacent d omains. The metamorphic mineral assemblages are generally of high vari ance (KFMASH variance greater-than-or-equal-to 2). Consequently, the c hemical evolution of assemblages is controlled largely by bulk composi tion and metamorphic temperature, the former factor being more importa nt in most rocks. The chemographic relations of mineral assemblages in low- and medium- to high-grade zones are presented in compatibility d iagrams projected from biotite, quartz and H2O, and biotite, K-feldspa r and H2O, respectively. These compatibility diagrams have the advanta ge of showing both quartz-bearing and quartz-absent assemblages. The m etamorphic reactions are modelled successfully by a calculated petroge netic grid that combines both KFASH and KMASH equilibria. Based on pet rographic observations and with constraints from the calculated petrog enetic grid, the following KFMASH reactions, in the order of increasin g metamorphic grade, are responsible for producing the various mineral assemblages in the Stawell rocks: chl + mu + q = bi + cd + V, chl + q + cd = g + V, mu + bi + q = ksp + cd + V, mu + q = ksp + and + cd + V (or KASH mu + q = ksp + and + V), mu + cd = ksp + and + bi + V, mu bi + and = ksp + sp + V, and + bi ksp + sp + cd + V, mu + bi ksp + cor + sp + V, mu = ksp + cor + and + sp + V (or KASH mu = ksp + cor + V), bi + cd + q = g + ksp + V. The combined KFASH and KMASH grid provides constraints on reaction coefficients in the above sequence of reactio ns and on temperature and pressure of metamorphism.