Merrimac plutonic complex
Merrimac plutonic complex
Early Cretaceous, ~134 and ~140-142 Ma
Three overlapping plutons north of Lake Oroville. 1) An eastern pluton, predominantly tonalite with lesser diorite and quartz diorite in its southern part; undated but oldest based on cross-cutting relations. 2) A northern pluton, composed of concentrically zoned granodiorite and tonalite, with zones of quartz diorite within the tonalite, yielding zircon U-Pb dates ~140-142 Ma (Irwin and Wooden, 2001; Langenheim and others, 2021). 3) A southern pluton composed of a central granodiorite with minor tonalite and granite surrounded by tonalite with minor granodiorite and quartz diorite, yielding zircon U-Pb ages of ~134 Ma (Irwin and Wooden, 2001; recalculated in Langenheim and others, 2021). Al-in-hornblende geobarometry indicates emplacement of these plutons at ~2-3 kbar (McLeod, 1990).