High- and Ultrahigh-Pressure Metamorphism—a View from the Western and Central Alps

Circumpacific blueschists and eclogites occupy convergent suture zones consisting of penetratively deformed nappes overturned seaward; mineral assemblages indicate maximum recovered depths of 30-50 km for lithologic units carried downward on an underlying oceanic plate during high-pressure (HP) metamorphism, prior to disengagement and partial exhumation of the dominantly low-density crustal material. Although low-grade portions of Alpine subduction zones are similar to Circumpacific HP belts, neoblastic coesite ± microdiamond inclusions in tough, rigid host minerals show that subduction-induced underflow and continental collision involves fragmentary recovery of ultrahigh-pressure (UHP) rocks from depths of 100-130 km, far deeper than regurgitated Circumpacific HP belts. Moreover, garnet peridotites from the central Alps, and several other continental collision sutures display intergrowths and exsolution lamellae reflecting the former existence of majoritic garnet, and other phases requiring depths of formation >300 km, attesting to earlier mantle upwelling or the resurrection of profoundly subducted complexes. Most recovered HP and UHP terranes have returned to upper crustal levels propelled by their overall buoyancy. The duration of deep-seated storage and rates of exhumation are currently debated, but mineral parageneses, times of HP-UHP recrystallization, and nappe structures reflect the two-way, relatively rapid transport of now-exposed subduction complexes. Fluid-rock and lithosphere-asthenosphere interactions have recycled volatiles to the deep Earth through subduction of both hydrous and nominally anhydrous phases. More fully than any other region, integrated mineralogic-geochemical-structural studies of the Alps have illuminated interrelationships among metamorphic petrology and plate tectonics.