THE IMPORTANCE OF FRACTIONAL CRYSTALLIZATION IN FORMING THE COMPOSITIONAL VARIATION IN THE NORTHEASTERN CATHEDRAL PEAK LOBE IN YOSEMITE NATIONAL PARK

Determining how magma plumbing systems operate in magmatic arcs is crucial for better understanding volcanic eruptions and crustal growth, yet the nature of active intrusive magma bodies at depth and how they grow and evolve through time remains debated. Magma processes that define the compositional variation in exposed intrusions are typically manifold: determining which processes dominated and the scale at which they operated is critical in understanding the size and interconnectivity of former magma mush bodies at final emplacement depths and in magmatic arcs as a whole.

The focus of this study is to determine the length scales of magma processes responsible in forming the compositional variation in the northeastern lobe of the Cathedral Peak (CP) unit in the 95-85 Ma Tuolumne intrusive complex (TIC), Yosemite National Park, Sierra Nevada, CA. This area is of particular interest because it contains the youngest parts (ca. 86 to 84Ma) of the TIC and thus reveals the location of the final magma mush body before TIC magmatism shut down. The increasingly leucocratic compositions towards the NE lobe, including aplitic dikes extending into the host rock, led to the hypothesis that this area underwent extensive fractionation from the interior of the northern CP granodiorite. To test this hypothesis, we have conducted field mapping, petrography and XRF analysis across the northern CP unit and into the NE lobe.

The northern CP lobe is generally composed of megacrystic granodiorites and granites and finer grained, equigranular leucogranites. The majority of the samples contain K-feldspars that are typically 1-2 cm and locally up to 6-12 cm in size. The rocks of the NE lobe at Twin Lakes are primarily finer grained granodiorites-granites and leucogranites with K-feldspars usually ≤1 cm long and rare larger grains. Our preliminary petrography and XRF geochemistry results indicate that the NE CP lobe underwent fractionation of hornblende, biotite, plagioclase, zircon and apatite. K-feldspar is smaller and does not appear to have been fractionated. This is consistent with major oxide and trace element behavior in NE CP lobe rocks. Further testing of this hypothesis is in progress. However, our initial data suggest that the NE CP lobe hosted the youngest moderately sized, fractionating magma mush body in the TIC.