Arclogites and their role in continental evolution; part 2: Relationship to batholiths and volcanoes, density and foundering, remelting and long-term storage in the mantle

Arclogites are eclogite-like rocks formed magmatically as ultramafic residues and cumulates in the roots of thick arcs (Ducea et al., 2021 companion paper). They are inferred to be volumetrically important assemblages to complement subduction-related magmatic rocks at depth, in areas where the upper plate crust is thick. At the surface and in the shallow crust, these arcs form stratovolcanoes found at many sites around the Pacific and batholiths, which are exposed in extinct arcs, such as the western North American Cordillera. Arclogites complement these shallower manifestations of magmatism in subduction zones in that they represent the ultramafic residues left behind following the extraction of intermediate melts. Mass balance calculations constrained by the low silica contents of garnet, amphibole, and iron-titanium oxides dictate that the silica content of a given arc tracks with the volume of arclogitic residues beneath it. We show that melt extraction takes place in punctuated events of hot-zone evacuation of the lower crust which effectively make up the magmatic flare-ups documented in the mid- to upper-crust. These cyclic events lead to the densification of arclogitic residues and trigger their foundering. We show that in addition to garnet, FeTi oxides play an important role in the densification of roots and can trigger foundering even in garnet-free arcs. Recent models show that foundering may be small scale and lateral shearing may have an important role in removing sub arc residues. Consequently, previously postulated large magnitude uplift and large-scale mantle-derived magmatism may not accompany foundering episodes. Partial melting of descending arclogitic bodies is expected to produce small amounts of nepheline (and/or leucite) normative magmas similar in composition to alkaline massifs found in continental interiors, not basaltic or intermediate melts. The foundering rates of arclogites are calculated to be around 20-40 km(3)/km/Myr in Phanerozoic arcs. The fate of foundered arclogite may include stalling at the 660 km discontinuity, accumulation and mixing with subducted oceanic material at slab graveyards along the core-mantle boundary, and/or disaggregation via ductile flow in the mantle. Regardless, there has to be a reservoir in the mantle representing these lower crustal recycled bodies over time. We show that the most likely such reservoir is EM1, one of the well-known and commonly identified recycled material in ocean island basalts, and that 1-3% of the volume of the mantle may be made of this reservoir.

Automated summary

Arclogites are rock formations resembling eclogites, formed at the roots of thick arcs as ultramafic residues. They play an important complementing role in subduction zones by representing the volatile mineral residues left behind after magma extraction.