T37 The How, When, Where, and Why of Open-System Magma Processes

Sunday, 10 October 2021
1:30 PM - 5:30 PM
Portland Ballroom 252 (Hybrid Room)

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

Paper number: T37-4
Program: Technical Sessions
Time: 2:05 PM – 2:25 PM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 53, No. 6

doi: 10.1130/abs/2021AM-371273

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