Subduction zone metamorphism – pioneering contributions from the Alps

Abstract

Circum-Pacific-type high-pressure (HP) metamorphic belts occupy contractional sutures marked by intensely deformed, imbricated nappes that verge seaward. Mineral assemblages reflect recovered depths of ∼25–50 km for mafic and felsic crustal units carried down on an underlying oceanic plate; such units recrystallized prior to disengagement and exhumation of slices of chiefly low-density rock. Low-grade parts of Alpine-type subduction complexes are similar to regurgitated circum-Pacific-type HP belts, but coesite ± microdiamond inclusions in tough, unreactive host minerals show that lithospheric underflow and continental collision involve partial recovery of ultra-high-pressure (UHP) rocks from depths of 100–140 km. Associated garnet peridotites from the Central Alps and several other collision sutures display exsolution lamellae and intergrowths suggesting the former existence of majoritic garnet and additional phases requiring formation at >300 km, attesting to earlier, unrelated mantle upwelling, or to the resurrection of profoundly subducted and back-reacted complexes. HP and UHP terranes return to mid-crustal levels mainly driven by buoyancy. Times of deep-seated storage and rates of exhumation are short; mineral parageneses, intervals of HP–UHP recrystallization and nappe structures reflect the two-way, relatively rapid transport of now-exposed subduction complexes. More fully than any other areas, integrated mineralogic-geochemical-structural studies of the Alps have illuminated relationships between metamorphic petrology and convergent plate tectonics. HP–UHP transformations of mafic and quartzofeldspathic rocks initially formed at, or near, the Earth's surface, imbricate structures of subduction complexes including their prograde metamorphic zonations + retrograde parageneses, association with deep-seated UHP mantle rocks and speedy ascent were first documented in the Alps.