Slow subduction and buoyant exhumation of the Sanbagawa eclogite

Abstract

The Western Iratsu body of the Sanbagawa belt (SW Japan) represents a <2km thick garnet–epidote amphibolite slice with minor intercalated occurrences of eclogite. Equilibrium phase assemblage diagrams (pseudosections) for eclogite and garnet–epidote amphibolite from this body can be successfully used to reconstruct the pressure (P)–temperature (T) path and the evolution of the associated mineral assemblages. Microstructural observations combined with calculated pseudosections reveal two distinct stages of subduction-related metamorphism referred to as M1 and M2. P–T conditions of M1 evolved from 0.9GPa, 570°C to 1.2GPa, 660°C, implying relatively high thermal gradients possibly developed shortly after the initiation of subduction. The available age data and the characteristics of the P–T path suggest that the Western Iratsu body was detached from the slab at 1.2GPa (~40km depth) and the subsequent subduction occurred at a very slow rate due to weak mechanical coupling to the downgoing slab. The prograde metamorphism associated with the second subduction phase (M2) reached the eclogite facies (~1.8GPa, 510–560°C), but mafic rock compositions recrystallized into either eclogite or garnet–epidote amphibolite assemblages, depending on the bulk rock (CaO+Na2O)/(FeO+MgO) ratio. The thermal peak of M2 (~620°C at 1.6GPa) was attained during the early phase of exhumation. Further decompression and re-equilibration (M3) took place in the epidote-amphibolite facies. The M2 to M3 P–T evolution is associated with a progressive increase in thermal gradient (T/P ratio), which can be explained by thermal modeling as the result of the inflow of a very young hot section of the slab. The bulk density of the whole Western Iratsu body at the point of onset of exhumation—the return point—is estimated to be ~3180kgm−3, which is less than the density of mantle, implying that buoyant rise is a viable mechanism for its initial exhumation. Buoyant rise is in accordance with the widespread early-exhumation related deformation that has a dip-parallel stretching lineation. Exhumation of the buoyant discrete mafic slice along the decoupled slab–mantle interface could have been triggered by the heating that lead to fluid liberation and a resultant reduction of the rock strength.