Mafic igneous member
Mafic igneous member
Triassic(?) and(or) Paleozoic
Chiefly Sheep Ranch metabasite and Devil’s Gate ophiolite that consists of amphibolite facies metabasite derived from basalt, andesite, and trachyandesite. Forms southern extension of the Feather River ultramafic belt (Jackson, 2012). Unit occurs in a narrow klippen thrust over the Calaveras Complex - marine sedimentary rocks (unit MRCS).
A large mafic amphibolite body within the ultramafic rocks (unit FRU) from the northernmost part of the unit in the Feather River area has yielded a hornblende K-Ar age of ~241 Ma (Hacker and Peacock, 1990). In the Devil’s Gate area, this unit has yielded a hornblende K-Ar age of ~248 Ma (Hietanen, 1981), and hornblende Ar-Ar ages of ~272 and ~395 Ma (Standlee, 1978). In the Yuba River area, this unit has yielded hornblende K-Ar ages of ~322 and ~345 Ma, and muscovite K-Ar age of ~273 Ma (Bohlke and McKee, 1984). Saleeby and others (1989b) report a zircon U-Pb age of ~388 Ma for a small granitic intrusive nearby, and they suggest that it represents the timing of amphibolite-facies metamorphism. They further note that this age is coeval with the Bowman Lake batholith (part of unit Gsb) intruded into the Northern Sierra terrane to the east.