Granitic units
Granitic units
Late to Middle Jurassic, ~158-169 Ma
Chiefly diorite and granodiorite. The southern part of this unit forms a large, composite pluton (the Emigrant Gap composite pluton) to the north and northeast of Emigrant Gap mafic units (unit Gegm), comprised of several diorite and granodiorite plutons (French Lake pluton, Downey Lake pluton, Grouse Ridge pluton) with zircon U-Pb ages of ~158-164 Ma and plagioclase K-Ar ages of ~156-179 Ma, surrounding a quartz monzonite (Black Buttes quartz monzonite) with a zircon U-Pb age of ~168 Ma (Drake and others, 1975; Snoke and others, 1982; Saleeby and others, 1989b; Girty and others, 1995; Howell, 1998; Wooden and others, 1999). The southern part also includes a body of biotite granite along the Middle Fork American River below French Meadow Reservoir (Harwood and others, 2014), which has yielded a zircon U-Pb age of ~165 Ma (Langenheim and others, 2025). The northern part of this unit, the Haypress Creek composite pluton includes the Yuba Pass granodiorite and quartz diorite, Bald Ridge granodiorite, Berry Creek quartz diorite and tonalite, Bear Valley quartz monzonite to gabbro, and Dead Horse Canyon gabbro and quartz monzonite (Phillipson, 1995). This part of the unit has zircon U-Pb ages of ~165-169 Ma (Girty and others, 1995; Philipson, 1995; Langenheim and others, 2025), as well as biotite K-Ar ages of ~97 and ~99 Ma and hornblende K-Ar age of ~125 Ma (Evernden and Kistler, 1970) and Rb-Sr isochron ages of ~153-157 Ma (John and others, 1994). A locality yielding a zircon U-Pb age of ~167 Ma (Langenheim and others, 2025), from a body too small to map in Cenozoic cover just north of the pluton east of the Mohawk Valley fault, shows that the unit extends across that fault. Younger ages of 128 to 101 Ma in northeastern part may represent unmapped plutons or dikes. Unit intrudes Early to Middle Jurassic Sailor Canyon Formation (unit NSTS). The younger part of the unit intrudes the Tuttle Lake Formation (unit NSTT), postdating the deformation, whereas the older part is coeval with and has been interpreted as source of the parent magma for volcanic rocks in the Tuttle Lake Formation (Howell, 1998).