Arclogites in the Subarc Lower Crust: Effects of Crystallization, Partial Melting, and Retained Melt on the Foundering Ability of Residual Roots

Abstract

Thick-crusted (>45 km) Cordilleran arcs exhibit cyclic processes including periods of magmatic quiescence interspersed with pulses of high-flux magmatism. 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 can readily founder into the mantle. Yet these models do not consider that: (1) the retention of low-density melt within the subarc root, and (2) the protolith lithology of the restitic portion of the subarc root may greatly impact the density evolution of the root and its susceptibility to foundering. In this article, we first address the effect of retained melt on the foundering ability of the subarc root by calculating the density and time for foundering of melt-bearing arclogitic residue at 1.5, 2, and 2.5 GPa. We find that melt volumes >10-18% are required to stabilize the root within the lower crust; melt volumes below this threshold lower the viscosity of the residue so much so as to 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 and mineralogical evolution of restites in equilibrium with their derivative melts during open-system, isobaric partial melting of typical crustal assemblages from 600-1100 • C at 1.5 (∼50 km) and 2 GPa (∼65 km). In our models, typical end-member assemblages in a lower crustal hot zone include basalt, metapelite, and metagraywacke. 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 the coexisting hydrous felsic residual melt. Foundering of the amphibole ± melt-bearing root may refertilize the mantle wedge and induce melting of the surrounding asthenosphere as well as the arclogitic mass. However, if temperatures in a 50-km deep subarc hot zone are not sufficient to drive melt depletion of basaltic restites, these dense (3.1-3.3 g/cm 3 ) residues are gravitationally stable, increasing the density of the lower crust and lowering the elevation of the arc. 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.

Chemical Mohometry: Assessing Crustal Thickness of Ancient Orogens Using Geochemical and Isotopic Data

“…(2002) showed that Moho can range from 35 to over 80 km deep in nearby areas of the Central Andes although in detail several surfaces can be identified as potential Moho discontinuities (Assumpção et al., 2013). The petrology of lower crustal domains under magmatic arcs is such that sizable masses of residual/cumulate pyroxenites are both required by models (Bowman et al., 2021) and observed in the few areas where old lower crustal sub‐arc domains are exposed at the surface (Kidder et al., 2003; Walker et al., 2015) as well as in the xenolith record (Ducea & Saleeby, 1998). These pyroxenites, with or without amphibole, garnet, magnetite/ilmenite, or plagioclase, cover a range of seismic velocities that can straddle the boundary between lower crust and upper mantle from granulite to arclogite (Ducea et al., 2021 and references therein).…”