Rocklin-Penryn plutons
Rocklin-Penryn plutons
Early Cretaceous, ~142 Ma
Originally mapped as two separate plutons northwest of Folsom Lake, the older Penryn and younger Rocklin. The Penryn pluton is mainly medium- to coarse-grained quartz diorite, the Rocklin pluton is mainly fine- to medium-grained trondhjemite. This unit has a zircon U-Pb age of ~142 Ma (Irwin and Wooden, 2001), along with biotite K-Ar ages of ~128-140 Ma, two muscovite K-Ar ages of ~128 Ma, and a hornblende K-Ar age of ~131 Ma (Curtis and others, 1958; Swanson, 1978, recalculated in Wagner and others, 1981).