ARCLOGITES IN THE SUBARC LOWER CRUST: EFFECTS OF CRYSTALLIZATION, PARTIAL MELTING, AND RETAINED MELT ON THE FOUNDERING ABILITY OF RESIDUAL ROOTS

Thick-crusted (> 45 km) Cordilleran arcs exhibit cyclic processes including pulses of high-flux magmatism interspersed with periods of magmatic quiescence. Most models assume that during high-flux events, fractional crystallization and partial melting within the deep crustal hot zone generate a dense (> 3.4 g/cm^³) arclogitic subarc root that readily founders into the mantle. Yet these models do not consider that (1) retention of low-density melt within the subarc root and (2) protolith lithology of the restitic portion of the subarc root may greatly impact the density of the root and its susceptibility to foundering. To address the effect of retained melt on the foundering ability of the subarc root, we calculate the density and time for foundering of melt-bearing arclogitic residue at 1.5, 2, and 2.5 GPa. We find that melt fractions > 10-18% are required to stabilize the root within the lower crust; melt fractions below this threshold lower the viscosity of the residue and decrease the time for foundering by an order of magnitude. We then constrain through phase equilibria modeling the effect of partial melting of different lower crustal protoliths on the density of the restitic subarc root. To do this, we model the density evolution of restites in equilibrium with their derivative melts during open-system, isobaric partial melting of typical crustal assemblages (basalt, metapelite, and metagraywacke) from 600-1100°C at 1.5 and 2 GPa. We find that melt-depleted restites derived from basaltic compositions are amphibole-bearing arclogites with densities conducive to foundering, which at 2 GPa can occur even in the presence of co-existing hydrous residual melt. Foundering of the amphibole ± melt-bearing root may refertilize the mantle wedge and induce melting of the surrounding asthenosphere and/or the arclogitic mass. In comparison, partial melting of metasedimentary country rock produces alkali feldspar-rich residues that never achieve densities conducive to foundering. Thus, if high-flux events are driven by the influx of melt-fertile lithosphere beneath the arc as envisioned by the Cordilleran cycle model, then partial melting of the metasedimentary portion will generate low-density residues that remain in the lower crust and contribute to the thickness, geochemistry, and seismic structure of the bulk arc.