INVESTIGATING THE VOLCANIC-PLUTONIC CONNECTIONS USING GEOCHEMISTRY IN THE MINARETS CALDERA, EASTERN-CENTRAL SIERRA NEVADA MOUNTAIN RANGE, CALIFORNIA

Understanding how magma plumbing systems operate at different levels of the crust helps us better understand volcanic eruptions and ultimately their prediction. Investigating how exactly volcanic materials and deeper magma chambers (preserved as plutons) are geochemically linked can provide insights into the magma processes that produce the compositional variations observed at different levels of the magma plumbing and their vertical interconnectivity. Vertical cross sections of contemporaneous volcanic and plutonic rocks, however, are rare.

One such location is the Minarets caldera in the eastern central Sierra Nevada Mountains, where both the surficial, volcanic and the deeper, plutonic parts are exposed. The rock record in the Minarets caldera shows a well-preserved intracaldera ignimbrite, an intercalated caldera-collapse breccia, and a granitic to granodioritic intrusion known as the Shellenbarger Lake pluton located at the center of the volcanic complex. The volcanic and plutonic rocks have been determined to be contemporaneous at ca. 100 Ma (Tomek et al., 2015). Fiske and Tobisch (1994) hypothesized that the Shellenbarger Lake pluton might represent the magma source for the volcanic deposits. If true and the Minarets caldera represents an upper crustal magma plumbing system, we can test the following hypothesis: Are the volcanic and plutonic rocks of the exact same composition or are they complementary to one another? The former will imply that the volcanics are partial extractions of a homogeneous magma chamber, whereas the latter will imply that the erupted materials may have fractionated from the magma reservoir and the pluton represents the restitic crystal-rich magma.

To test this hypothesis, volcanic and plutonic samples from the Minarets caldera were analyzed for petrography and XRF major and minor trace elements. The XRF results were furthermore used for geochemical modeling to test a fractionation model. Preliminary results indicate that the volcanic rocks include both equivalent and complementary compositions. Major oxide and trace element data of complementary rocks indicate that plagioclase was likely fractionated, which is variably abundant as a phenocryst in the plutonic rocks. Some volcanic units plot with or near plutonic compositions, indicating equivalent compositions.