CAN LOW-VOLUME MAGMATISM GENERATE LARGE-SCALE ERUPTIONS? PETROLOGIC INVESTIGATIONS OF JURASSIC PLUTONS FROM THE WESTERN AND CENTRAL SIERRA NEVADA BATHOLITH, CA

Plutons are valuable tools for studying volcanic processes in the absence of volcanic materials. In the Sierra Nevada batholith (SNB) in California, finding a connection between the Jurassic plutonic and volcanic rocks has proven challenging. This is either because 1) a large Jurassic record is missing due to overprinting by the Cretaceous magmatic flare-up, or 2) the Jurassic low-volume magmatism never allowed for plutons to grow large enough magma reservoirs to feed volcanic eruptions and instead they were fed directly from the mantle without storage in an established magma plumbing system.

This study focuses on the Jurassic King Creek and Fish Creek as well as the Standard, Granite Creek, and Cobb Creek plutons in the central and western SNB, respectively, to determine their petrogenetic and timing relationships, and to examine if they produced volcanic eruptions. Evidence that these granodioritic, dioritic, and gabbroic plutons are crystal cumulates (i.e. have lost melt, possibly to a volcanic eruption) is seen via the accumulation of plagioclase crystals, as well as in the whole rock geochemistry. U-Pb zircon ages of 168.39±1.66 Ma and 159±1.1 Ma were obtained for the King Creek and Fish Creek plutons, respectively. These ages imply that these plutons are either unrelated or represent different pulses of magma within a larger (ca. 70 km²), longer-lived system. Sr isotope ratios for both plutons are primitive, with some evolved signatures observed in the Fish Creek pluton, which indicates minor crustal input. The U-Pb zircon ages for the Standard (162.8±1.1 Ma), Granite Creek (164.01±1.1 Ma), and Cobb Creek (163.73±0.95 Ma) plutons all overlap within error, which suggests that these plutons are coeval and may represent a ca. 700 km² center of magma focusing. All have primitive Sr isotope ratios, indicating a similar source. Chemical modeling using whole-rock and plagioclase and hornblende element compositions will be applied to assess the crystallinity of the magma chamber vs melt and an estimate of magma temperatures and compositions when melt loss occurred, as well as volume of melt lost. Evidence of cumulate properties and sufficient melt loss will provide support for the hypothesis that low-volume magmatism in the Jurassic formed an established magma plumbing system that processed magmas and fed volcanic eruptions, whether the volcanic products are preserved or not.