AS OPEN-SYSTEM AS IT GETS: LONG-LIVED AND SPATIALLY EXTENSIVE MAGMA MIXING, FRACTIONAL CRYSTALLIZATION, AND MELT LOSS DURING MAGMA FOCUSING IN THE TUOLUMNE INTRUSION, SIERRA NEVADA BATHOLITH, CA (Invited Presentation)
Plagioclase, a liquidus phase, shows compositions compatible with incremental and gradual in-mixing of CP-type magmas during fractionation of eHD to form pHD granodiorite. Evidence for eHD-CP mixing is also seen in (1) distinct eHD apatite cores found in pHD apatite, a high T phase, (2) Kfs megacrysts in the pHD-CP transition with 0.5 myr older, pHD zircon ages from Kfs cores compared to CP-age zircons in the rims and groundmass, and (3) the spread of bulk-rock single zircon ages of eHD to CP age, which agree with broad overlap in bulk-rock Sr and Nd isotopes. Hornblende, which crystallized at <850°C, preserves evidence of magma mixing (incl. in schlieren from all units), however, the extent to which eHD hornblende is recycled into pHD is unclear. Small K-feldspar (low T phase) indicates minor local mixing. Titanite, which crystallized <760°C is distinct from unit to unit, indicating that it typically did not survive high temperature mixing events. Mineral analyses show evidence for fractionation to rhyolitic melts. Bulk-rock–mineral Fe-Mg Kd values indicate that up to 40% of these rhyolitic melts leaked from the evolving eHD-CP magma, potentially to erupt or form late leucogranite bodies (Memeti et al., session T24).
With the intrusion of the eHD into fractionated KC magma at the KC lobe at ~93 Ma, the Tuolumne intrusion entered a period of spatiotemporal magma focusing dominated by rapid build-up of magma mush and across-unit magma mixing at >>750˚C, separated by episodes of <750˚C dominated by fractional crystallization until 84.5 Ma.