Rejuvenation of shallow-crustal silicic magma bodies at Augustine and Hayes volcanoes, Alaska

Abstract:

Rejuvenation of crystal-rich magma bodies leading to eruption can occur on a variety of scales and in varied tectonic settings. Two examples from the Aleutian arc highlight 1) segregation of silicic melt from an intermediate mush, and 2) “defrosting” of a shallowly emplaced intrusion. Augustine Volcano erupted a late Pleistocene rhyolite pumice fall that we link through zircon geochronology to cumulate dioritic blocks, ripped from Augustine’s shallow magmatic plumbing system and ejected during the 2006 eruption. Unpolished zircon rims from the rhyolite yield a U-Th age of ~25 ka, and interiors yield a dominant age population of ~26 ka. Zircons from diorites have interior ages and compositions indistinguishable from those of the rhyolite. The diorites, rhyolite, and early Holocene dacites define whole-rock linear unmixing trends consistent with melt (rhyolite) extraction from a mush (dacites), leaving behind a cumulate residue (diorites). A volatile-rich basalt erupted just prior to the rhyolite likely facilitated melt extraction from the mush. The rhyolitic Hayes River ignimbrite, erupted from Hayes volcano, contains dense porphyry blocks that match pumices in composition and phenocryst content and are samples of a shallow intrusion. Autocrystic monazite accommodated up to several weight % Th and significantly affected the U-Th ratio of the magma during differentiation. An isochron for early melt and low-U monazites yields an age of ~67 ka, whereas one for late melt and high-U monazites yields ~42 ka. This younger age is indistinguishable from the laser single crystal Ar-Ar age for sanidine of 41±2 ka (1 sigma). We interpret the apparent ~25 k.y. crystallization interval to represent the assembly and differentiation timescale associated with the Hayes magma body. Sharp reverse zoning in sanidine from pumice (but not porphyry) records a thermal pulse not seen in the more slowly reacting phases, suggesting that a rejuvenation event occurred just prior to eruption.