2004 Volume 113 Issue 5 Pages 617-632
Garnet-bearing ultramafic rocks including clinopyroxenite, wehrlite, and websterite locally crop out in the Higashi-akaishi peridotite of the Besshi region in the Sanbagawa metamorphic belt. These rock types occur within dunite as lenses, boudins, or layers with a thickness ranging from a few centimeters to 1 meter. The wide and systematic variations of bulk-rock composition and 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 T and hydrous 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, and its rim records peak metamorphic conditions. Estimated P-T conditions imply a high P/T gradient (> 3.1 GPa/100 °C) from 1.5-2.4 GPa/700-800 °C to 2.9-3.8 GPa/700-810°C during prograde metamorphism. Olivines recrystallized at the high P/T prograde metamorphic stage show a B-type lattice preferred orientation (LPO) with a-axis concentrations normal to the stretching lineation. The presence of B-type LPO indicates that deformation of the prograde metamorphic stage possibly progressed under hydrous and high-stress conditions at the wedge mantle adjacent to the subducted slab. The Higashi-akaishi peridotite is a unique example that well records the prograde evolution of subducted ultramafic rocks.
