Terra Nova, 25, 48–56, 2013


The Palaeoproterozoic Vallecito conglomerate of the Needle Mountains, Colorado, experienced deformation and metamorphism during emplacement of the ∼1.4 Ga Eolus batholith, producing a contact aureole defined by sillimanite near the pluton, andalusite and sillimanite together ∼2 km from the pluton, and andalusite alone at greater distance. In the andalusite–sillimanite zone, sillimanite-rich layers, poor in andalusite, define shear bands. In contrast, regions between shear bands contain abundant andalusite and sillimanite. A deformed sample is enriched in immobile elements, with an estimated major element mass loss of 43% relative to a weakly deformed sample. Modal and compositional variation between deformed and undeformed regions indicates that mass transfer was central to shear band development. This process changed the composition of the deformed sample, facilitated the localization of deformation, catalyzed metamorphism and allowed the assemblage to more rapidly approach equilibrium than weakly deformed assemblages.