GSA Connects 2021 in Portland, Oregon

Paper No. 37-3
Presentation Time: 2:05 PM

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)


MEMETI, Vali1, BARNES, Calvin2, WERTS, Kevin2, OPPENHEIM, Louis1, CHAMBERS, Melissa1 and ARDILL, Katie3, (1)Department of Geological Sciences, California State University Fullerton, 800 N State College Blvd, Fullerton, CA 92831, (2)Department of Geosciences, Texas Tech University, Box 41053, Lubbock, TX 79409-1053, (3)Geology Department, California State University, Sacramento, 6000 J St, Sacramento, CA 95819

Open-system magmatic behavior is thought to occur in upper- to mid-crustal arc plutons. However, major element bulk-rock and textural data often yield equivocal evidence for such processes in intermediate to felsic magmas. We employed mineral scale U-Pb zircon dating on inclusions in K-feldspar megacrysts, and major and trace element geochemistry on rock-forming and accessory minerals to determine the magma processes and their time and length scales in the 95-84.5 Ma, 1100 km2 Tuolumne intrusion, Sierra Nevada, CA. The partially nested units young inward and are separated by gradational to sharp contacts: Kuna Crest diorite-granodiorite (KC), equigranular and porphyritic Half Dome granodiorites (eHD, pHD), and Cathedral Peak K-feldspar (Kfs)-megacrystic granodiorite-granite (CP).

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.