Exhumed Blueschists and Eclogites: Hotter Than the Average Model

Abstract:

The maximum-pressure P-T conditions (P_max-T) of exhumed subduction-related metamorphic rocks are compared to estimates of P-T conditions predicted by computational thermal models of subduction systems. While the range of proposed models encompasses most P_max-T, most models are 200-400°C too cold. In general, discrepancies are greatest for P_max < 2 GPa where only a few of the highest-T modeled paths overlap petrologic observations. Comparison among published models suggests several possible explanations for these differences. Variables that affect temperatures within the subduction zone include the timing of subduction initiation relative to metamorphism, age of the subducting oceanic crust, rate of convergence, and the dip angle of the subducting plate. An additional factor is whether subducted material is constrained to move coherently with the incoming plate or whether it convects within the plate interface. Higher temperatures are predicted for relatively young subducting crust, slow convergence rates, and shallow subduction dips. Simulations in which material from the subducted slab decouples from the slab and rises buoyantly into an overlying weak layer (e.g. hydrated mantle) also result in higher temperatures for exhumed oceanic crust. Our compilation and comparison suggest either that most models are missing one or more important controls on heat sources and heat transfer or that exhumed blueschists and eclogites are more buoyant than typical subducted rocks.