Investigating the Composition of Magmas Produced during Partial Melting of Arclogite

At thick-crusted arcs, arclogites (garnet clinopyroxenites) form in the lower crust as cumulates and restites. These rocks are significant in that they have densities up to 10% greater than the underlying mantle, and as such, frequently founder into the asthenosphere. Because they often contain amphibole, foundering arclogites may partially melt by dehydration melting as they descend into the sub-arc mantle. Despite the ubiquity of lithospheric removal among geologic models used to explain the evolution of Cordilleran arcs, few studies have acknowledged the potentially significant role of arclogite partial melting in contributing to arc magmatism. To evaluate the contribution of arclogite melting to arc magmatism, we perform partial melting experiments on a hydrous arclogitic composition at 2 GPa from 1150-1230°C. The starting material is composed of an average arclogite (silica depleted with 45 wt.% SiO2) with 15 wt.% amphibole (0.3 wt.% H2O) and equal proportions of garnet and clinopyroxene. Partial melting at 1150°C yields 6 wt.% basanitic melt, 63 wt.% augite, and 31 wt.% garnet. At 1190°C and 15% melting, garnet has been entirely consumed, leaving a residue composed of augite with trace amounts of spinel. Melt at this temperature is trachybasaltic. At 1230°C, the partially melted arclogite consists of 37 wt.% basaltic melt, 50 wt.% augite, and 13 wt.% enstatite. As the degree of melting increases, the melt composition becomes increasingly rich in SiO2, consistent with the early consumption of garnet. Arclogitic melts are further characterized by high alkali and TiO2 (>3.5 wt.%) contents. In this presentation, we will compare our results with natural arc magmas as well as with melt and residue compositions calculated from thermodynamic modeling programs.