Leucogranites of the Sierra Nevada Batholith, California: Source of Mega-eruptions? (2009)

Undergraduate: Michael Pardue

Faculty Advisor: Drew Coleman
Department: Geology

---

This study investigates the trace and major element concentrations of samples from 11 leucocratic plutons and 1 aplite dike in the Sierra Nevada batholith of California. The rare earth element (REE) signatures of the leucogranites are strikingly similar to the respective signatures of high-silica rhyolites (HSRs), thus contradicting a direct genetic connection to them. REE patterns of HSRs are characterized by a deep, negative anomaly in Europium (Eu) and enrichment of other REEs. Thus, if HSRs are in fact material extracted from magma that in time cools to form large, granodiorite plutons, it is expected that the removal of this material would impart a positive Eu anomaly on its residual cumulate, a signature that is absent in the leucograntites analyzed in this project. The depth of the Eu anomaly, represented by Eu/Eu*, varies between the leucogranites and HSRs. I interpret these geochemical similarities between volcanic (HSRs) and plutonic (leucogranites) rock to mean that the leucogranites are not geochemically complementary to HSRs, but rather fully crystallized equivalents of volcanic rocks. In addition to the geochemical analysis, various petrological analyses were performed on samples from each respective pluton in order to better understand processes occurring during crystallization of this largely understudied classification of rocks.