Contact metamorphism around the Stawell granite, Victoria, Australia

Abstract

ABSTRACTThe contact metamorphosed metapelitic and metapsammitic rocks surrounding the Stawell granite, western Victoria, Australia, are divided into three zones: the low‐grade zone, the medium‐grade zone and the high‐grade zone. Detailed petrological study shows consistency of element distributions, implying that equilibrium was widely attained in the rocks, although equilibrium volumes are generally small (millimetre scale) and considerable mineral chemical variations exist between adjacent domains. The metamorphic mineral assemblages are generally of high variance (KFMASH variance ≤ 2). Consequently, the chemical evolution of assemblages is controlled largely by bulk composition and metamorphic temperature, the former factor being more important in most rocks. The chemographic relations of mineral assemblages in low‐ and medium‐ to high‐grade zones are presented in compatibility diagrams projected from biotite, quartz and H2O, and biotite, K‐feldspar and H2O, respectively. These compatibility diagrams have the advantage of showing both quartz‐bearing and quartz‐absent assemblages. The metamorphic reactions are modelled successfully by a calculated petrogenetic grid that combines both KFASH and KMASH equilibria. Based on petrographic observations and with constraints from the calculated petrogenetic grid, the following KFMASH reactions, in the order of increasing 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 reactions and on temperature and pressure of metamorphism.