Metamorphic evolution of garnet-bearing ultramafic rocks from the Gongen area, Sanbagawa belt, Japan

Garnet-bearing ultramafic rocks including clinopyroxenite, wehrlite and websterite locally crop out in the Higashi-akaishi peridotite of the Besshi region in the Cretaceous Sanbagawa metamorphic belt. These rock types occur within dunite as lenses, boudins or layers with a thickness ranging from a few centimetres to 1 metre. The wide and systematic variation of bulk-rock composition and the overall layered structure imply that the ultramafic complex originated as a cumulate sequence. Garnet and other major silicates contain rare inclusions of edenitic amphibole, chlorite and magnetite, implying equilibrium at relatively low P-T conditions during prograde metamorphism. Orthopyroxene coexisting with garnet shows bell-shaped Al zoning with a continuous decrease of Al from the core towards the rim, consistent with rims recording peak metamorphic conditions. Estimated P-T conditions using core and rim compositions of orthopyroxene are 1.5-2.4 GPa/700-800degreesC and 2.9-3.8 GPa/700-810degreesC, respectively, implying a high P/T gradient (>3.1 GPa/100degreesC) during prograde metamorphism. The presence of relatively low P-T conditions at an early stage of metamorphism and the steep P/T gradient together trace a concave upwards P-T path that shows increasing P/T with higher T, similar to P-T paths reported from other UHP metamorphic terranes. These results suggest either (1) down dragging of hydrated mantle cumulate parallel to the slab-wedge interface in the subduction zone by mechanical coupling with the subducting slab or (2) ocean floor metamorphism and/or serpentinization at early stage of subduction of oceanic lithosphere and ensuing HP-UHP prograde metamorphism.