INVESTIGATING LOW-VOLUME MAGMATISM IN THE JURASSIC WESTERN AND CENTRAL SIERRA NEVADA BATHOLITH AND IMPLICATIONS FOR VOLCANISM

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 Cretaceous magmatism, 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 magma plumbing system.

This study focuses on the Jurassic King Creek and Fish Creek plutons 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 quartz monzonitic, 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 and the discrepancy between whole rock compositions and calculated melt compositions from amphibole compositions, which yielded dacitic to rhyolitic compositions. U-Pb zircon ages of 168.39±1.66 Ma and 159±1.1 Ma for the King Creek and Fish Creek plutons, respectively, suggest these plutons are either unrelated or represent different pulses of magma within a larger, longer-lived system. Al-in-hornblende thermobarometry suggests these plutons were emplaced at depths of ca. 3-10 km.

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. Pluton emplacement depths averaging ca. 4-15 km were calculated using Al-in-hornblende thermobarometry. Chemical modeling using whole-rock and mineral compositions will be applied to determine volume of melt lost and assess the crystallinity of the magma chamber vs melt when melt loss occurred. Evidence of cumulate properties and sufficient melt loss provides support for the hypothesis that low-volume magmatism in the Jurassic formed an established magma plumbing system that processed magmas and fed volcanic eruptions.