METAMORPHIC TEXTURES IN THE BUGABOO AUREOLE AND THEIR PETROLOGICAL IMPLICATIONS

Metamorphic textures in the Bugaboo aureole preserve a subtle record of reaction processes. Two minerals are focused on: andalusite and K-feldspar. Andalusite preserves microtextural evidence for four episodes of growth going upgrade: (1) initial, modally minor growth, accompanied by plagioclase porphyroblasts, from a paragonite-bearing precursor phyllite assemblage, marked by small porphyroblasts with a very fine, dense inclusion pattern; (2) volumetric growth, accompanied cordierite and biotite and at the expense of chlorite, marked by a dense but not quite as fine grained network of inclusions as the earlier andalusite; (3) a range of textures indicating growth following and, in some cases at the expense of, cordierite in chlorite-free samples, including development of clear rims on the earlier-formed inclusion-rich andalusite porphyroblasts, interstitial andalusite between cordierite and inclusion-rich andalusite porphyoblasts, and overgrowth of porphyroblast-enveloping matrix fabrics of a coarser grain size than at lower grade; and (4) growth accompanying the development of K-feldspar at the expense of primary muscovite, characterized by inclusion-poor rims of a greater width than seen downgrade, sometimes visible in hand sample as pale rims on darker cores. K-feldspar show two distinct textures: (1) inclusion-filled cryptoperthitic porphyroblasts, accompanied by the fourth andalusite texture, marking the subsolidus reaction of muscovite and quartz to andalusite and K-feldspar; and (2) in a few samples at the highest grade, coarser-grained, irregularly-shaped grains intergrown with quartz and plagioclase in leucocratic segregations, suggestive of crystallization from a melt, implying localized, fluid-fluxed migmatisation.