ARE THE JURASSIC KING CREEK AND STANDARD PLUTONS CRYSTAL CUMULATES THAT HAVE LOST MELT?

A source for volcanic eruptions, plutons serve as valuable tools for studying volcanic processes in the absence of volcanic materials. In the Sierra Nevada batholith (SNB) in California, the voluminous Cretaceous magmatic flare-up event overprinted much of the Jurassic plutonic and volcanic rocks produced during the relatively lower-volume Jurassic flare-up event. As such, finding a connection between the currently existing Jurassic plutonic and volcanic rocks in the SNB has proven challenging. Furthermore, field relationships and isotopic dating on Sierran Jurassic plutons and small occurrences of volcanic rocks have not provided strong support that the plutons fed the volcanic eruptions. The main question is whether this is the case because 1) a large Jurassic record of volcanism that was fed by plutons is missing due to the overprinting by the younger and more voluminous Cretaceous magmatic flare-up and later erosional events, or 2) whether the Jurassic low-volume magmatism never allowed for plutons to grow large enough magma chambers to feed volcanic eruptions and instead they were fed directly from the mantle without storage in a plutonic magma plumbing system.

This study uses detailed petrologic and geochemical analyses of the Jurassic King Creek and Standard plutons in the central and western SNB, respectively, to determine if they could have produced large or even just small eruptions. Both pluton lithologies range from diorite to granodiorite and have Sr and Nd isotopes with more depleted mantle signatures. The Standard pluton has a U-Pb zircon age of 161.87±0.38 Ma, and the King Creek pluton is dated at ~168 Ma. Evidence that both plutons are crystal cumulates (i.e. have lost melt) is evident texturally via the accumulation of plagioclase crystals. Potential evidence for crystal accumulation is also found in the whole rock element geochemistry of the plutons. Variations in abundances of Al₂O₃, CaO and K₂O as well as Ba, Sr, Rb, and Zr support the microscopic evidence that variable amounts of at least plagioclase, biotite, and zircon accumulation has occurred. Cathodoluminescence imaging underway will better show cumulate structures and possibly identify different populations of plagioclase, indicating magma mixing prior to fractional crystallization (and melt loss) and thus prolonged magmatic activity.