Subduction-zone metamorphism, calc-alkaline magmatism, and convergent-margin crustal evolution

At the dawn of the age of plate tectonics, Akiho Miyashiro published a seminal paper in which he drew attention to significant differences in regional metamorphic phase assemblages, described contrasting geologic occurrences, and inferred their characteristic ranges of physical conditions. He advanced the paired metamorphic belt concept, involving an oceanward, narrow, high-P/low-T blueschist zone intimately intermixed with ophiolites, and a landward, broad, low-P/high-T realm associated with arc volcanic–plutonic rocks. More recent studies have illuminated the following: (1) Two main types of convergent plate junction can be distinguished, Pacific underflow of thousands of km of oceanic lithosphere, and Alpine closure of an intervening oceanic basin leading to continental collision. (2) Plate descent carries mafic + felsic lithologies to depths of ~ 35–120 km or more at relatively low temperatures, producing high-pressure (HP) Pacific-type and ultrahigh-pressure (UHP) Alpine-type metamorphic terranes, respectively. (3) Exhumed HP–UHP complexes display low-aggregate bulk densities, reflecting chiefly buoyancy-driven ascent of quartzofeldspathic allochthons. (4) At a depth of ~ 35 km in a warm-lithosphere subduction zone, sialic crust should melt if an aqueous fluid were abundant; it does not, hence the activity of H2O must be low at such depths. (5) Global observations indicate that volcanic–plutonic arcs are sited 100 ± 20 km above subducting oceanic plates; devolatilization of sinking, hydrated oceanic crust ± mantle promotes the generation of calc-alkaline arc melts in this magmagenic zone. (6) Pacific-type underflow of basaltic crust-capped plates produces new and recycled continental crust, whereas Alpine-type convergence reshuffles collided terranes but does not generate juvenile sialic crust.