Jack Main Canyon intrusive suite
Jack Main Canyon intrusive suite
Late Cretaceous, ~91-110 Ma
Occurs west of the crest of central Sierra Nevada, in northwest Yosemite Park. Chiefly granodiorite and diorite. Feldspar Pb-Pb and zircon U-Pb ages from this unit range from ~91-110 Ma (Wooden and others, 1999; Leopold, 2016; Ardill and others, 2018). This unit also has biotite K-Ar ages from ~87-94 Ma and hornblende K-Ar ages from ~90-98 Ma, as well as one plagioclase K-Ar age of ~91 Ma, one K-feldspar K-Ar age of ~84 Ma, and one quartz K-Ar of ~108 Ma (Evernden and Kistler, 1970). In addition, two localities in the northwest part of this unit have yielded much older K-Ar ages (biotite ~125 and ~145 Ma, hornblende two ages of ~148; Evernden and Kistler, 1970). These ages are considerably older than the emplacement age of the unit indicated by feldspar Pb-Pb ages, and suggest that the unit as mapped contains some rocks that belong to an older unit, probably the adjacent Don Pedro intrusive suite (unit Gdp). Major plutons: granodiorites of Boundary Lake, Lake Vernon, Bearup Lake, Rancheria Mountain, and Bummers Flat, and quartz diorite of Mount Gibson. East of Calaveras Big Trees State Park, includes: