MOBILE, “DRY” DACITES BY SHALLOW, GABBROIC DIFFERENTIATION OF ARC BASALTS AT MOUNT VENIAMINOF VOLCANO, ALASKA PENINSULA

At 350 km³, glacier- and caldera-capped Mt. Veniaminof, Alaska Peninsula, is among the largest Aleutian volcanoes. It is arc tholeiitic (THI: 1.09) consisting mostly of rapidly accumulated basalts and basaltic andesites, joined subsequently by andesites, then dacites over 300 kyr activity (Calvert et al., this meeting). Besides volume and temporal evolution, Veniaminof is distinguished by: (1) near absence of amphibole despite common dacites; (2) phenocryst-poor dacite lava flows as thin as a few m, effused up to 10 km, indicating fluid, mobile magmas—domes are absent; (3) sparse phenocrysts in dacites (plag­ 2-pxns FeTi-ox) with simple textures and compositions; (4) nearly aphyric andesite pyroclastic flows, demonstrating eruption of andesitic liquids; (5) basalts and basaltic andesites with texturally complex phenocrysts (plag oliv cpx spin/magnt). Whole-rock SiO₂ concentrations increase modestly from 49 to 53–54 wt.% as Mg# falls from to 68 to 40–50 (MgO from 9.5 to 4 wt.%); then SiO₂ concentrations increase markedly to 69.5 wt.%, a change matched by maxima in FeO*, TiO₂, and V concentrations signalling Ti-magnt. joining the fractionating assemblage. Incompatible element enrichments indicate strong SiO₂-increase commencing at residual melt fractions of ~0.7, and that the most evolved dacites are melt fractions of ~0.2, although Sr, Nd, Pb, Hf isotopes reveal modest (to ~6 wt.%) progressive assimilation of crust accompanies differentiation. Evolved magmas appear to form at shallow levels—even if parent magmas are relatively dry (1.7–2 wt.% H₂O based on THI), differentiation would enrich dacitic melts in H₂O sufficient to stabilize amphibole unless pressures were too low to hold adequate H₂O in the melt. Persistent eruptions of basalt and basaltic andesite show that mafic magmas reach shallow levels readily at Veniaminof. Miarolitic gabbro to granodiorite blocks ejected in the latest (3700 ¹⁴C yr BP) caldera-forming eruption evince a shallow intrusive complex. Causes of Veniaminof’s temporal evolution may include: basalt flux declined, slowing recharge and mixing, allowing andesitic and dacitic liquids to ooze from the top of the intrusive complex; or (and) the intrusive complex thickened, storing heat so shallow mafic injections remain partly molten sufficiently long to segregate evolved melts.