Hot and deep: Rock record of subduction initiation and exhumation of high-temperature, high-pressure metamorphic rocks, Feather River ultramafic belt, California

A thin (<300 m) unit of high-grade metamorphic rocks found at the base of the Feather River ultramafic belt (FRB) interpreted to be a metamorphic sole yields insight into subduction initiation and ophiolite emplacement. Peak metamorphic conditions were determined by grt-cpx-phengite thermobarometry, which yielded results of 650--760°C at 1.4--2.2 GPa for the unit. Amphibole thermobarometry yielded results of 650--720°C at 1.6--1.9 GPa for the structurally high part of the unit and 550--650°C at 1.3--1.7 GPa for the structurally low part of the unit. These results are consistent with the sole having formed during the inception of subduction.;P-T conditions were also determined using amphibole thermobarometry for an amphibolite lens interleaved within the FRB (550--700°C at 1.2--1.8 GPa) and a sheeted dike from the structurally higher Devil's Gate ophiolite (705°C at 0.3--0.7 GPa). The pressure contrasts indicate that exhumation of the high-grade rocks occurred on at least two major zones.