Hot and Deep: Rock Record of Subduction Initiation, Feather River Ultramafic Belt, California

The Feather River ultramafic belt of Northern California represents a polygenetic suture zone that likely formed from multiple subduction zones. It provides an opportunity to study the ancient rock record of complex plate interactions similar to Cenozoic tectonics in the southwest Pacific. On its western boundary is a thin (<500 m) sheet of amphibolite. Field mapping in the North Fork Feather River area reveals that this unit is in direct contact with the base of the ultramafic unit wherever it is exposed. The amphibolite is bounded on the west by the pumpellyite grade rocks of the Calaveras complex, and is cut by a hornblende gabbro dike. High temperature shear-sense indicators and the east-dipping foliation of the amphibolite are indicative of hot underthrusting beneath the ultramafic unit.

Protoliths of the amphibolite are metachert and metabasalt. Assemblages range from epidote amphibolite to garnet amphibolite to amphibolite-granulite transition. Grt-Cpx-Phengite thermobarometry and yielded a temperature of 727-740ºC and pressure of 14-17 kbar. Amphibole thermobarometry yielded a temperature of 680 ºC and a pressure of 19 kbar. Based on the field relationship and P-T conditions we interpret the amphibolite to represent a metamorphic sole that formed during the hot initiation of subduction. This implies that a previous subduction zone was present to form the oceanic lithosphere the amphibolite was thrust under. The Feather River ultramafic belt formed under a west-dipping subduction prior to 240 Ma. After blockage of the subduction zone an east-dipping subduction zone formed around 240 Ma with the amphibolite recording the initiation of subduction within young oceanic lithosphere. The peak pressure conditions are much higher than those based on the thickness of the overlying ophiolite indicating that the amphibolite has been exhumed relative to its Feather River ultramafic belt.