THE ROLE OF LATE STAGE MAGMATIC FLUIDS DURING METASOMATIC REACTION OF GABBRO TO GRANOPHYRE

In the Duluth Layered Series (DLS) of the Duluth Complex, a layer of granophyre is situated above the Upper Contact Zone (UCZ) and below the Anorthositic series (AS). This layer shows irregular contacts and complex intrusive relationships which challenge the commonly accepted hypothesis that the granophyre was emplaced as silicic magma. Evidence presented in this study from field relationships, compositional analyses, and textural analyses suggests that the granophyric material was a product of metasomatic reactions of a magmatic fluid (containing sodium carbonate, silica, and water) with pre-existing gabbro, rather than reflecting magmatic emplacement of silicic melt. The reactions include albitization of calcic plagioclase, formation of abundant actinolite, deposition of calcite within plagioclase crystals and along veins, and conversion of ilmenite to titanite. Major element mapping by SEM-EDS indicates reaction zones whereby elements such as Mn become concentrated in the remaining ilmenite because titanite is a poor host for this element. LA-Q-ICP-MS analyses of trace elements in ilmenite-titanite alteration pairs are used to show crystallization history based on fractionation patterns and chemical relationships. Veins of granophyre along what appears to be a heavily altered fault plane in the DLS provides further evidence of fluid transport from lower in the intrusion, pointing to the fluids originating from later stages of igneous differentiation occurring at greater depth during a top down emplacement sequence.