Metamorphism in Subduction Zones

Abstract

Metamorphism in subduction zones transforms crustal rocks into given mineral assemblages under given pressure (P)-temperature (T) conditions. This gives rise to metamorphic rocks with different peak thermobaric ratios, which can be translated into metamorphic thermal gradients. As a consequence, regional metamorphism along convergent plate boundaries is classified into three types. The first is low T/P Alpine type at low thermal gradients, resulting in blueschist to eclogites facies series at high to ultrahigh pressure. The second is moderate T/P Barrovian type at moderate thermal gradients, leading to amphibolite to granulite facies series at medium to high pressures. The third is high T/P Buchan type at high thermal gradients, giving rise to amphibolite to granulite facies series at medium through high to ultrahigh temperatures. As such, three metamorphic facies series can be produced within different P/T fields, providing a genetic link to dynamic regimes at convergent plate boundaries. Whereas compressional heating is responsible for prograde Alpine and Barrovian type metamorphism at lower thermal gradients during subduction, extensional heating is responsible for Buchan type metamorphism at elevated thermal gradients during rifting. This yields contrasting thermal gradients for bimodal metamorphism in ongoing and fossil subduction zones. Consequently, their products show different relationships in space to different types of orogen. Whereas paired metamorphic belts are separated from each other along accretionary orogens, polymetamorphic products are superimposed on each other along collisional orogens. Therefore, the change of thermal gradients with time is a key to the understanding of regional metamorphism at convergent plate boundaries.