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 part of a paleo-subduction zone, show low-grade metamorphic recrystallization (chrysotile+magnetite +/- magnesite +/- talc). They are cumulates of melts formed in the uppermost mantle or the base of the Nidar intra-oceanic arc. Serpentinite samples associated with the Tso Morari eclogitic unit, representing the more deeply subducted portion of a paleo-subduction zone. exhibit high-grade metamorphic recrystallization (antigorite+magnetite +/- forsterite +/- talc) and the trace element chemistry of these samples suggests a strongly depleted mantle wedge origin. Nd concentrations and epsilon Nd values show that fluids responsible for hydration of the mantle wedge were derived from subducting clastic sediments overlying Tethyan oceanic crust. The exhumation of eclogites requires a mechanically weak zone at the interface between the subducting plate and the mantle wedge. We suggest that serpentinites associated with the Tso Morari eclogites acted as a lubricant for the exhumation of the eclogitic unit. Geophysical data suggest common occurrences of hydrated ultramafic rocks about 10 km thick along the interface between the mantle wedge and the subducting plate. We propose that such a low-viscosity zone played an important role for the exhumation of eclogitic rocks. (C) 2001 Elsevier Science B.V. All rights reserved.