Partial melting of arclogite and petrogenesis of alkaline-silicate complexes

Abstract

Magmatic processing in the lower crust of thick-crusted (>40 km) arcs generates garnet-rich clinopyroxenite residues (arclogites) that are denser than the underlying mantle and therefore susceptible to foundering. Arclogites, which can contain substantial amphibole, may undergo partial melting as they sink into the subarc mantle. To constrain the geochemical composition of arclogite-derived melts and the role of arclogite in magmagenesis, we conducted partial melting experiments on average amphibole-bearing arclogite ARC15 at 2 GPa from 1040 to 1300 °C. At subsolidus conditions, garnet and clinopyroxene coexist with lesser amounts of amphibole, ilmenite, and biotite. Hydrous ARC15 begins melting between 1040 and 1080 °C. Once amphibole and biotite are exhausted by 1140 and 1190 °C, respectively, garnet becomes the primary melting phase according to the reaction 0.8 Garnet + 0.2 Clinopyroxene = 1 Melt. At 1300 °C, ARC15 yields 18 wt.% melt, indicating an anomalously low melt productivity of 0.12%/°C and rendering it the least melt-productive of all experimental pyroxenites investigated at 2 GPa. Geochemically, melts sourced from ARC15 are predominantly basanitic with low SiO2 (43–52 wt.%) and high FeO (10–17 wt.%), TiO2 (2.5–3.7 wt.%), and alkali (Na2O + K2O = 3–6.5 wt.%) contents. Such compositions do not resemble magmas erupted at thick-crusted arcs, indicating that arclogites are not a dominant source of magmatism in these settings. Rather, the compositions of ARC15 melts more closely match those of magmas emplaced at post-collisional alkaline-silicate complexes, which form as a result of orogenic collapse and subsequent continental rifting and are common hosts of economic rare-earth element (REE) deposits. Trace element modeling further indicates derivation of alkaline-silicate magmas from variably metasomatized, light REE-enriched arclogite. Our results suggest a geodynamic model in which subduction beneath thickened crust leads to arclogite formation, foundering, and metasomatism as the material descends through the subduction-influenced mantle. During this stage, partial melts sourced from arclogite are likely modified by subduction-related secondary processes which dilute their diagnostic geochemical signatures. Subsequent collision and orogenic collapse induce extensional stresses that facilitate asthenospheric upwelling, leading to partial melting of the arclogite-bearing mantle and efficient extraction of primitive arclogite melts to the surface to form alkaline-silicate complexes and associated REE deposits.