Dating zircon inclusions in sanidine-megacrysts from core to rim documents growth over hundreds of thousand years at super-solidus conditions

We present in-situ U-Pb and U-Th ages of zircon inclusions from core to rim in a single 8 cm-large K-Feldspar megacryst from the 32.9 ka dacite dome eruption of Taápaca volcanic complex (Central Andes, Chile). These large sanidine crystals formed in a rhyodacite magma and erupted in a hybrid dacite after magma mixing just prior to ascent. Zircon in the core (n = 13) are in U-Th secular equilibrium with U-Pb dates of 530±35 to 950±35 ka. Zircons in the mantle (n = 10) are again in U- Th secular equilibrium with U-Pb ages of 333±13 to 480±43 ka. Only four zircons yielded U-Th isochron ages of 302 (+55/-36) to 339 (+70/-42) ka (n = 4). Rim-hosted zircons (n = 16) are all significantly younger with U-Th ages of 58 (+12/-11) to 251 (+112/-54) ka. Thus, zircon continuously crystallized and became successively trapped during sanidine growth in the rhyodacite host magma over several hundreds of thousands of years (from 920 ky to 25 ky before eruption), with a maximum individual zircon growth time of 30–50 ky. These results agree with Ba-diffusion timescales for the host K-feldspar and other megacrysts from the same dome eruption. Our combined results indicate at least 500 ky of megacrystic growth at magmatic conditions of 740 to 820 °C and 1 to 2 kbar (thermobarometry on amphibole inclusions in the sanidine) and protracted “hot” magma storage at upper crustal levels in continental arc settings