Evidence of hydration of the mantle wedge and its role in the exhumation of eclogites

Serpentinite samples from the Indus suture zone, representing a shallower p art of a paleo-subduction zone, show low-grade metamorphic recrystallizatio n (chrysotile+magnetite +/- magnesite +/- talc). They are cumulates of melt s formed in the uppermost mantle or the base of the Nidar intra-oceanic arc . Serpentinite samples associated with the Tso Morari eclogitic unit, repre senting the more deeply subducted portion of a paleo-subduction zone. exhib it high-grade metamorphic recrystallization (antigorite+magnetite +/- forst erite +/- talc) and the trace element chemistry of these samples suggests a strongly depleted mantle wedge origin. Nd concentrations and epsilon Nd va lues show that fluids responsible for hydration of the mantle wedge were de rived from subducting clastic sediments overlying Tethyan oceanic crust. Th e exhumation of eclogites requires a mechanically weak zone at the interfac e between the subducting plate and the mantle wedge. We suggest that serpen tinites associated with the Tso Morari eclogites acted as a lubricant for t he exhumation of the eclogitic unit. Geophysical data suggest common occurr ences of hydrated ultramafic rocks about 10 km thick along the interface be tween the mantle wedge and the subducting plate. We propose that such a low -viscosity zone played an important role for the exhumation of eclogitic ro cks. (C) 2001 Elsevier Science B.V. All rights reserved.