NEW EMPLACEMENT AGES FOR PLUTONS OF THE PORTOLA 30’X60’ QUADRANGLE IN THE NORTHERN SIERRA NEVADA, CALIFORNIA, AND IMPLICATIONS FOR THE REGIONAL GEOLOGIC FRAMEWORK

Zircons collected from several plutons in the Portola 30’x60’ quadrangle were dated using SHRIMP ion microprobe U/Pb geochronology. Some of these plutons have existing K/Ar and U/Pb ages while others were undated. Existing geochronology of the Indian Valley pluton, along the North Fork Yuba River, includes a hornblende K/Ar cooling date of 143 ± 5 Ma (Bohlke and McKee, 1984) and a U/Pb date of 151 ± 2 Ma (Day and Bickford, 2004); zircons from this study yield a ²⁰⁶Pb/²³⁸U date of 155 ± 3 Ma. This pluton lacks foliation, unlike the western part of the Scales pluton (~165 Ma), ~5 km to the north. The lack of deformation indicates the Indian Valley pluton was emplaced after Middle and Late Jurassic deformation documented in various parts of the Sierra, and the new date provides constraint for the culmination of that deformation in this region.

A small granodiorite pluton located along the Middle Fork Feather River and mapped by Grose (2000) as part of the larger Middle Jurassic Haypress Creek pluton (HCP) yields a new U/Pb date of 114 ± 1 Ma. Existing U/Pb zircon geochronology for the HCP indicates emplacement at 168-164 Ma, during Middle Jurassic deformation (Hanson et al., 1996). Most of the HCP resides southwest of the Mohawk Valley Fault, and though exposures of it are mapped across the fault, our new Cretaceous age for the granodiorite shows that it is unrelated to the HCP. The pluton may be related to a large mass of granodiorite mapped nearly 4 km to the east, with biotite and hornblende K/Ar dates of ~108 Ma (Saucedo and Wagner, 1992). Samples from a highly magnetic dioritic pluton ~8km to the northwest have not yielded zircons for U/Pb analysis to determine if it is also Cretaceous.

In the eastern part of the quadrangle, a mass of quartz monzodiorite was mapped by Grose (2000) as Cretaceous, though he speculated the pluton could be cogenetic with the adjacent Jurassic metamorphic rocks. Our new U/Pb date of 163 ± 3 Ma places the pluton as definitively Middle Jurassic. The monzodiorite is mapped as in contact with granodiorite that may be related to a large Cretaceous pluton to the east that yielded U/Pb dates of 94 ± 2 and 95 ± 1 Ma. However, a fault is mapped along the eastern margin of the monzodiorite and adjacent granodiorite, adding to the possibility that both are Jurassic plutons that have been uplifted by extensive Cenozoic faulting in the region.