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

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 similar to 260 k.y. Gabbro, diorite, and miarolitic granodiorite blocks, ejected 3700 C-14 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 U-238-Th-230 isochron ages of 17.6 +/- 2.7 ka, 5 + 11/-10 ka, and 10.2 +/- 4.0 ka (2 sigma) for zircon in two granodiorites and a diorite, respectively. Sparse zircons from two gabbros give U-238-Th-230 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.