THE CASE FOR A LARGE CATHEDRAL PEAK MAGMA CHAMBER, TUOLUMNE INTRUSIVE COMPLEX, SIERRA NEVADA

Incremental magma emplacement is a widely accepted mechanism for pluton growth, however, it is debated whether magma increments ever amalgamate to form larger, longer-lived magma bodies. The 95-85 Ma, 1,100 km² Tuolumne intrusive complex, Sierra Nevada, is composed of four major granodiorite units and gradational transition zones between them that are interpreted to record magma mixing. Gradational zones are particularly prominent in the TIC interior where the ca. 400 km² porphyritic Half Dome (pHD) and Cathedral Peak (CP) units are exposed.

Careful field observations and geologic map distributions of the pHD and CP granodiorite units and transitions indicate a consistent pattern of concentrically arranged compositional variations that grade into one another and are remarkably identical in succession along this boundary across the TIC. The 1) outer pHD is characterized by 3-4 cm K-feldspar and euhedral hornblende, biotite, and titanite; 2) the next inner unit is a pHD-CP hybrid that contains less euhedral minerals but <10-12cm K-feldspar megacrysts; 3) inward the CP transition still contains <10-12 cm K-feldspar megacrysts, but loses euhedral hornblende, biotite, and titanite; 4) it grades inwards to a CP biotite granodiorite unit with 2-4 cm K-feldspar phenocrysts; 5) which gives way to leucogranite bodies in the CP interior.

These zones resemble “bathtub rings” of crystallization and their consistent compositional similarities along strike and changes across strike likely represent crystallization fronts of a large, interconnected magma body that underwent gradual mixing of CP into pHD. The following support this notion: 1) Whole rock U-Pb zircon ages are consistent along strike around each of the pHD-CP zones and young inward across zones; 2) cognate inclusions and minerals from the pHD are found in pHD-CP transitions and lessen into inner CP units, supporting the idea that pHD (and subsequently formed units) was interconnected across the TIC core during pHD growth; 3) biotite cooling temperatures in regions around the CP were reset during CP emplacement at least 5 km away from the pluton indicating a large CP magma body. We conclude that regardless how many increments built the inner TIC, the pHD and CP units crystallized as an entity that was interconnected across the inner TIC and episodically re-intruded by CP.