Bucks Lake plutonic complex
Bucks Lake plutonic complex
Early Cretaceous, ~139-140 Ma
Three plutons, two overlapping and a small separate pluton on the northwest of Quincy. An eastern pluton, pyroxene diorite core surrounded by quartz diorite (Bucks Lake Pluton); a western normally zoned pluton transitioning from strongly foliated hornblende-biotite quartz diorite at the margin to unfoliated, coarse-grained monzotonalite in the central part (Grizzly Pluton); and the much smaller northern pluton, monzotonalite and hornblende diorite (Oliver Lake Pluton). This unit also includes a small unnamed and undated pluton of biotite tonalite along the Dogwood Peak fault to the southeast. The Grizzly Pluton, and perhaps the very small southeastern pluton, cross the Dogwood Peak fault (Foothills suture) between the western metamorphic belt and the remainder of the Sierra Nevada. This unit has zircon U-Pb ages of 139-140 Ma (Saleeby and others, 1989b; Langenheim and others, 2021; Langenheim and others, 2025), as well as hornblende K-Ar ages of ~136, 142, and 143 Ma, and a biotite K-Ar age of ~132 Ma (the two older hornblende K-Ar ages are a bit older than the zircon U-Pb ages, so the actual age of those samples is probably at the low end of their error bar). Al-in-hornblende geobarometry indicates emplacement at ~3 kbar for the Grizzly pluton (McLeod, 1990). This complex may be in part correlative with coeval plutons or parts of plutons to the south in the western metamorphic belt such as the Merrimac plutonic complex (unit Gmm) and Bald Rock plutonic complex (unit Gbr).