Ultramafic member
Ultramafic member
Devonian or older
Occurs in the northern Sierra Nevada along branches of the Calaveras-Shoo Fly thrust between the Northern Sierra terrane to east and Calaveras Complex to the west. Chiefly variably serpentinized peridotite, mainly harzburgite, lesser dunite, and minor lhehrzolite. Unit includes massive and layered metagabbro, sheeted dikes, pillow lava, and chert. Sparse metagabbro crosscutting dikes. Unit also includes local gabbro and diorite, amphibolite derived from metabasalt, and plagiogranite. Unit intensely faulted and sheared.
About half of unit consists of unserpentinized peridotite bodies ranging from 2-6 km long. Bodies generally elongate in a north-south direction and have gradational boundaries with surrounding massive serpentinite. Ultramafic rocks display both cumulate and tectonite fabrics.
A plagiogranite from the northernmost part of the unit in the Feather River area has yielded a zircon U-Pb age of ~314 Ma (Saleeby and others, 1989b). Northwest of Downieville, a relatively small body of metagabbro within this unit has yielded a hornblende K-Ar age of ~285 Ma (Hietanen, 1981).