A TEXTURAL STUDY OF THE DISTRIBUTION OF CORDIERITE IN METAPELITIC HORNFELSES FROM THE BUGABOO CONTACT AUREOLE (SE BRITISH COLUMBIA): IMPLICATIONS FOR THE MECHANISMS OF METAMORPHIC CRYSTALLIZATION

Hornfelses from the 1 km Bugaboo contact aureole in south eastern British Columbia contain porphyroblasts of biotite, andalusite and cordierite in a weakly foliated matrix of quartz, plagioclase, white mica and, within the outer aureole, chlorite. Cordierite growth dominantly post-dates biotite and andalusite crystallization, and is syn- to post-kinematic to the deformation of the rock matrix. Based on phase equilibria modelling, a metamorphic P and T of ca. 1.8-2.2 kbar and 500-525°C is inferred for the conditions of earliest cordierite growth. Preliminary results obtained from X-ray micro-computed tomography (XR-µCT) and statistical analyses of the three-dimensional spatial distribution of cordierite from the Bt-And-Crd-Chl zone in the outer aureole indicate a random distribution of cordierite crystals at length scales greater than the mean nearest-neighbour distance. An ordered distribution at the mean diameter distance is present due to the inability of one crystal to be located inside another, also known as the hard core effect. Cordierite growth is therefore interpreted to have been mainly limited by the attachment and detachment of chemical species at their crystal surfaces. The spatial distribution of biotite in the same sample was also obtained and shows a high degree of clustering because its initial distribution was overwritten by the subsequent growth of cordierite. In addition, the chemical composition of cordierite and biotite seem to be independent of metamorphic grade, suggesting a considerable departure from equilibrium upon their crystallization. Further research will link overstepping to texture formation of cordierite and other porphyroblastic phases at varying metamorphic grades and peak temperatures within the Bugaboo contact aureole.