The Nature of Anorthosite - Country Rock Interaction During Granulite Facies Metamorphism: An Example From the Whitestone Anorthosite

Abstract

<p> The Whitestone Anorthosite is a relatively small anorthosite body (160 km2 ) located within the Parry Sound structural domain, Western Grenville Province, Ontario. Both the anorthosite and the surrounding gneisses have been affected by a granulite grade metamorphic event which predates the Grenville Orogeny. The outer margin of the anorthosite body has been strongly deformed and recrystallized and is characterized by a pervasive metasomatic alteration consisting of garnet, scapolite, hornblende, apatite, biotite, sphene, carbonate and opaques. The country rock gneisses exhibit a corresponding discontinuous, and highly variable, reaction aureole. Pre-existing mafic gneisses are particularily affected, being characterized by the breakdown of orthopyroxene and hornblende, an increase in garnet, clinopyroxene, apatite and opaques, and enrichment in Fe, Ti and P. </p> <p> The metasomatic alteration exhibited by the Whitestone Anorthosite is thought to be due to a combination of two processes: 1. Mechanical mixing at the anorthosite/country rock contact during intense deformation (tectonic assimilation), and 2. Widespread absorption of mobile components (predominantly volatiles) from both included material and the surrounding gneisses. </p> <p>The formation of the country rock reaction aureole is a continuous solid state metamorphic process, whereby mobile components are preferentially leached from the rock leaving a mafic restite. Post-deformation cooling of the anorthosite, combined with an increased volatile flux, has imparted a polygonal mosaic texture suggestive of contact metamorphism. The typical garnet-clinopyroxene assemblage exhibited by mafic gneisses within the reaction aureole, is a consequence of the increased Fe/Mg ratio which stabilizes this assemblage at lower P, T conditions. The temperature of final equilibration and recrystallization is estimated to be 750 ± 70 °C, based on clinopyroxene-garnet geotherrnometry. </p> <p> A similar metasomatic interaction, to the one outlined in this thesis, is to be expected at all anorthosite/country rock contacts which have been overprinted by granulite metamorphism. </p>