Abstract
The Feather River Belt (FRB) of the northern Sierra Nevada is a 150-km long strip of variably serpentinized ultramafic rock, with little to no crustal section, underlain by amphibolite-facies metabasalt and metaclastic rocks interpreted to be a metamorphic sole. To date, this sole has yielded an unusually wide range of K/Ar and Ar/Ar hornblende ages spanning from 395 Ma to 236 Ma. Many of these dates are difficult to interpret due to lack of published supporting data. Two of the best exposures of the sole are near Goodyear's Bar, along the North Fork Yuba River, and near Caribou, along the North Fork Feather River. The Goodyear's Bar exposure consists of interlayered garnet-clinopyroxene amphibolite and garnet-biotite metaclastite while that near Caribou consists of rutile-bearing garnet amphibolite and garnet-biotite metaclastite. Zircons were separated from metaclastic rocks of the metamorphic sole at both locations, imaged with cathodoluminescence (CL), then dated with U-Pb geochronology by LA-ICP-MS. CL imagery shows both samples consist of convolute zircons and detrital cores surrounded by convolute rims. These zircons yield both detrital and metamorphic ages. Cores of zircon from Caribou yield a detrital maximum depositional age of ca. 420 Ma. In contrast, the detrital cores of zircon from Goodyear's Bar are all discordant. Two separate metamorphic events are preserved the zircons of the two localities. Concordant U-Pb ages from CL-convolute rims and zircons from Goodyear's Bar record an older event at ca. 350 Ma. The second event is shown by strongly discordant CL-convolute rims and zircon from Caribou spanning a range of ages with a youngest population at ca. 255 Ma, which we take as the age of the metamorphic event. Overall, these data indicate that two metamorphic events sufficient to at least partially reset zircon at ca. 350 Ma and 255 Ma are preserved in different localities along the FRB sole. If interpreted as the age of subduction initiation, the younger event appears to coincide with the regional initiation of arc magmatism to the east whereas the older event does not correspond to known coeval arc rocks.