ZIRCON U–TH AGES OF CUMULATES AND SHALLOW GRANODIORITE, AND THE POTENTIAL FOR EXPLOSIVE ERUPTIONS OF THE THOLEIITIC MOUNT VENIAMINOF VOLCANO, ALASKA PENINSULA

The Aleutian arc Mount Veniaminof volcano provides an unusual opportunity to directly relate Quaternary volcanic rocks to a coeval pluton. Veniaminof erupted medium-K basaltic through dacitic magmas in the past ~250 kyr that define an arc tholeiitic trend. Crystal-poor intermediate and evolved lavas have compositions that are readily explained by crystallization differentiation. Gabbro, diorite, and miarolitic granodiorite fragments ejected in the 3,700 14C yr B.P. most recent caldera-forming eruption of Mount Veniaminof are direct evidence of shallow assembly of a pluton consisting of cumulate mush and vapor-saturated residual melt segregations. SHRIMP-RG analyses define 238U–230Th isochron ages of 17.5 ± 1.8 ka and 11.7 +4.7/–4.5 ka for zircon crystallization in granodiorite and diorite, respectively. Sparse zircons from two gabbros give 238U–230Th model ages of 36.6 +7.1/–6.7 ka and 26.4 +6.7/–6.4 ka. Zircons from granodiorite and diorite, commonly bearing ~0.2–5 mm fluid inclusions, have exceptionally high U and Th contents (up to 2 and 5 wt.%, respectively) that may be explained by crystallization in the presence of late-magmatic aqueous fluid during passive degassing in the shallow subsurface. Deformation monitoring might detect (1) inflation owing to segregation of shallow sill-like masses of differentiated melt in the mush column or growth of miarolitic cavities in solidifying granodiorite, or (2) deflation by collapse of cavities during passive degassing. Veniaminof historically has produced small strombolian eruptions and effusion of basaltic andesite lava rich in crystal-mush debris. However, existence of a substantial mush column in the recent past and a late Holocene explosive eruption of dacite pumice compositionally similar to granodiorite are evidence that evolved magmas can form readily at shallow levels and that hazardous explosive eruptions remain possible.