Bald Rock plutonic complex
Bald Rock plutonic complex
Early Cretaceous, ~133-134 and ~140-142 Ma
Trondhjemite and hornblende-biotite quartz diorite east of Lake Oroville. Zircon U-Pb ages suggest two periods of emplacement, with two ages obtained for each: ~133-134 Ma (Rousseau, 2016; Langenheim and others, 2021), and ~140-142 Ma (Saleeby and others, 1989b; Langenheim and others, 2021). Note the similarity with the two age groupings of the nearby Merrimac plutonic complex (unit Gmm). Aluminum-in-hornblende geobarometry indicates pressures of 3 kbar for the Bald Rock pluton (McLeod, 1990).