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

Studies of a 10 to 300-m-thick unit of high-grade metamorphic rock (“external schists”) that crops out along the western border of the Feather River ultramafic belt (FRB), northern California, yield new insights into subduction initiation and ophiolite emplacement processes. The high-temperature (T) foliation of the external schists dip moderately to steeply eastward beneath the ultramafic rocks of the FRB, a 150-km-long slab of suboceanic upper mantle and the high-T fabric shows a tops-to-the-west (FRB-side-up) sense of shear. The structurally highest external schists record peak metamorphic conditions of 650–760 °C at 1.3–2.2 GPa. In contrast, sheeted dikes of the Devil's Gate ophiolite that overlie the ultramafic rocks yield metamorphic conditions of 710–730 °C at about 0.3–0.7 GPa. A km-scale lens of amphibolite within ultramafic rocks yields somewhat lower pressures than the structurally highest external schist, as do the structurally lower rocks within the external schists. Significant exhumation of the external schists relative to the structurally overlying ophiolitic rocks occurred along at least two major zones and the most significant exhumation was accommodated at least 1.5 km structurally above the ultramafic-external schist contact. Based on available geochronology, intraoceanic subduction may have initiated at approximately 240 Ma, and exposure of the external schist occurred prior to the deposition of rocks in the structurally highest part of the Calaveras Complex (minimum 177 Ma), a subduction complex that structurally underlies the external schists. High-T metamorphism of the Devil's Gate ophiolite may have resulted from partial (failed) ridge subduction.