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Young cumulate complex beneath Veniaminof caldera, Aleutian arc, dated by zircon in erupted plutonic blocks

¹ U.S. Geological Survey Volcano Hazards Team, 345 Middlefield Road, MS 910, Menlo Park, California 94025, USA

Mount Veniaminof volcano, Alaska Peninsula, provides an opportunity to relate Quaternary volcanic rocks to a coeval intrusive complex. Veniaminof erupted tholeiitic basalt through dacite in the past 260 k.y. Gabbro, diorite, and miarolitic granodiorite blocks, ejected 3700 ¹⁴C yr B.P. in the most recent caldera-forming eruption, are fragments of a shallow intrusive complex of cumulate mush and segregated vapor-saturated residual melts. Sensitive high-resolution ion microprobe (SHRIMP) analyses define ²³⁸U-²³⁰Th isochron ages of 17.6 ± 2.7 ka, 5 +11/–10 ka, and 10.2 ± 4.0 ka (2) for zircon in two granodiorites and a diorite, respectively. Sparse zircons from two gabbros give ²³⁸U-²³⁰Th model ages of 36 ± 8 ka and 26 ± 7 ka. Zircons from granodiorite and diorite crystallized in the presence of late magmatic aqueous fluid. Although historic eruptions have been weakly explosive Strombolian fountaining and small lava effusions, the young ages of plutonic blocks, as well as late Holocene dacite pumice, are evidence that the intrusive complex remains active and that evolved magmas can segregate at shallow levels to fuel explosive eruptions.

Key Words: Mount Veniaminof • caldera • geochronology • zircon • ion probe • granodiorite • gabbro • Aleutian arc • Alaska • eruption • magma differentiation

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