ZIRCON U–TH GEOCHRONOLOGY OF MODERN PLUTONS BENEATH TWO ARC VOLCANOES

Plutonic clasts in pyroclastic deposits and xenoliths in lava flows commonly sample young intrusive complexes whose zircons can be dated by ion microprobe. Mount Mazama, Oregon, is a medium-K calc-alkaline Cascade arc volcano whose 7.7-ka climactic eruption ejected granodiorite and related plutonic blocks during formation of Crater Lake caldera. Plutonic blocks from the Mazama eruption have chemical and radiogenic isotopic compositions similar to erupted magmas, but typically have low δ18O values owing to late-Pleistocene subsolidus exchange with meteoric hydrothermal fluids. Zircons from the blocks typically have many 10's to 100's of ppm U and Th, and grew relatively late in high-crystallinity magmas. SHRIMP-RG ²³⁸U–²³⁰Th zircon model ages from four granodiorites range from ~20 ka to ≥300 ka, with concentrations near 50–70, ~110, and ~200 ka that correspond to periods of dacitic volcanism (Bacon and Lowenstern, 2005, EPSL 233:277–293). The youngest zircon model ages for blocks from different locations may help map the distribution of intrusions within a composite pluton. Mount Veniaminof, Alaska, is a medium-K tholeiitic Aleutian arc volcano that has erupted basaltic to rhyodacitic magmas over the past ~250 kyr. Gabbro, diorite, and miarolitic granodiorite blocks ejected from Veniaminof in its 3.7-ka caldera-enlarging eruption represent cumulate mush and complementary vapor-saturated residual melt segregations from a shallow intrusive complex that from time-to-time released eruptible volumes of nearly aphyric differentiated melt (Bacon, Sisson, and Mazdab, 2007, Geology 35:491–494). Zircons from granodiorite and diorite commonly have spongy texture, elevated REE patterns, exceptionally high U and Th contents (up to 2 and 5 wt.%, respectively), and high Th/U ratios which may be explained by crystallization in the presence of oxidizing late-stage magmatic aqueous fluid. Zircon data form single populations with ²³⁸U–²³⁰Th isochron ages of 17.6 ± 2.7 ka and 10.2 ± 4.0 ka for granodiorite and diorite, respectively (±2σ). Sparse zircons from two gabbros give ²³⁸U–²³⁰Th model ages of 36 ± 8 ka and 26 ± 7 ka. Zircons in Veniaminof plutonic blocks record brief unidirectional crystallization episodes, whereas zircon populations in Mazama blocks include crystals ("antecrysts") that have been recycled.