GROWTH RATES OF K-FELDPSAR MEGACRYSTS AND THE LIQUID-MUSH LOCKUP IN SHALLOW FELSIC RESERVOIRS GIVEN BY U/PB TIMS-TEA ON ZIRCON

K-feldspar megacrysts are common in granitic to granodioritic plutons, though their origin and evolution are still poorly understood. Experiments show that while K-feldspar is one of the last minerals to saturate in felsic magma, there may still be 60-70% liquid available for developing large crystals. Alternatively, recent models state that they might form at temperatures as low as 400°C by textural coarsening in a nearly solidified crystal mush. In the case of large upper crustal mushes that feed volcanic systems, melt extraction is influenced by magma influx and melt fraction, and therefore constraining the timing of K-feldspar megacryst formation can help address eruptibility.

The ca. 7 Ma Mt. Capanne pluton and associated porphyries (Elba Island, Italy) are shallow (0.1-0.2 GPa), granitic intrusions that host both orthoclase and sanidine megacrysts. U/Pb TIMS-TEA on zircon extracted from the core and the rim of single megacrysts shows that the rims are 10 to 40 ky younger than the cores and have distinct zircon trace element signatures. The total time of megacryst growth in the melt is at least 42 ky, with a growth rate of 1 to 6 mm³/year. Dates from zircons included in the megacrysts span about 0.4 Ma, a much longer duration than the actual megacryst growth. This could reflect zircon saturation fluctuations linked to mixing/rejuvenation within the magmatic reservoir or antecrystic zircons recycled from previous magma pulses. Megacrsyts with poikolitic rims may record the liquid-mush lockup. We have integrated the growth rates calculated from U-Pb geochronology with petrographically constrained thermodynamic modeling of phase equilibria with MELTS. Our results provide robust temporal constraints on the timing of the liquid-mush lockup and how the viscosity and melt fraction might have evolved through the transition.