Arclogites and their role in continental evolution; part 1: Background, locations, petrography, geochemistry, chronology and thermobarometry

Arclogites, or clinopyroxene-, garnet-, amphibole-, and Fe[sbnd]Ti oxide-bearing cumulates and restites (collectively representing residues) to andesitic continental arc magmas, are reviewed here and in a companion paper (Ducea et al., 2020). Experimental petrology and petrologic observations suggest that these eclogite facies rocks form magmatically in deep crustal hot zones beneath arcs with crustal thicknesses exceeding 35–40 km. Volcanic and plutonic products of thinner arcs may instead be entirely extracted from amphibolite to granulite facies and garnet-free pyroxenite residues. Arclogites are perhaps best known as xenoliths, with notable examples from young (Sierra Nevada and Central Arizona) and modern (Colombia) sub-arc environments. We suspect that arclogite occurs more commonly than currently recognized in the xenolith record from orogenic and cratonic domains. Arclogite is also found as discrete intervals in the deepest exposures of the Kohistan arc and as small volume inclusions in tectonically exposed peridotite massifs (e.g., Beni Bousera, Morocco). Geochemically, these rocks are low silica (SiO₂ < 50%) assemblages with low Nb/Ta and Sr/Y ratios and enrichments in heavy REEs such that they represent the complement to the andesitic-dacitic liquids that make up the surface volcanics and batholiths of most arcs. Virtually all rock-forming minerals in arclogites are of similar or greater density than the underlying mantle, making them ideal candidates for foundering. Arclogites are formed in the lowermost crust of arcs at 35–70 km depth and record high temperatures (~800–1000 °C) at the time of formation which then cool and metamorphose at ~650–750 °C if they remain attached to the crust for an extended period of time. Ages of these rocks are obtainable by Sm[sbnd]Nd and Lu[sbnd]Hf garnet isochron geochronology as well as titanite or rutile U[sbnd]Pb geochronology, although these ages can be reset through long-term storage in hot lower crustal environments. Recent discovery of zircon accessory minerals in arclogites makes these rocks datable with greater precision and greater chance of preserving crystallization ages by U[sbnd]Pb chronology.