HOLOCENE CALDERA-FORMING AND POSTCALDERA ERUPTIONS OF SEMISOPOCHNOI VOLCANO, WESTERN ALEUTIAN ISLANDS, ALASKA

Semisopochnoi volcano, located in the Rat Islands of the western Aleutian island arc, experienced a caldera-forming eruption (CFE) in the early Holocene that produced a 7 × 6 km caldera and an ignimbrite that blankets much of 20 ×17 km Semisopochnoi Island. Along coastal bluffs and in gullies on the volcano’s lower flanks, the CFE ignimbrite is generally massive, unwelded, and in some places lithic-rich near the top. Along the southern and eastern caldera margins, zones within the ignimbrite are welded. The deposit includes both black, dense, obsidian-like clasts, as well as tan, variably oxidized, highly-inflated pumice clasts. The base of the ignimbrite is not exposed thus obscuring whether deposition was preceded by a fall deposit. The CFE produced whole-rock compositions that cluster in three groups. A majority of analyzed clasts, including those both pumiceous and dense, forms a tight cluster at 62.9 to 63.4 wt% SiO₂, has high P₂O₅ (0.3 to 0.4 wt%), and lacks amphibole. A minor component with similar silica content, but lower P₂O₅, is amphibole bearing. Finally, a small number of analyzed scoria clasts sampled from high in the deposit have 58–60 wt% SiO₂. The Semisopochnoi CFE is older than 5000 ¹⁴C yr BP as determined by dating of soils within a tephra-soil complex that overlies the ignimbrite. A distinctive ashfall on nearby Amchitka Island with an underlying soil ¹⁴C date of 6520 yr BP may be correlative with the Semisopochnoi CFE, however verification will require additional work.

Semisopochnoi has had an active postcaldera eruptive history. At least six monogenetic postcaldera vents are recognized both inside and outside the caldera. In addition, Cerberus is a basaltic andesite compound stratocone (~3.5 km³) with three main summit craters that have produced numerous tephras and lava flows that cover much of the caldera floor. Sugarloaf Peak on the southern tip of the island is a basaltic cone, ~3.5 km³ in volume, and is the likely source of the only known historical activity on the island in 1987. Semisopochnoi eruptive products, as a whole, range from basalt to dacite, are tholeiitic, low- to medium-K, and fall within the middle of arrays formed by other Aleutian arc volcanic rocks on many variation diagrams, though the Semisopochnoi array is at the high end for many incompatible elements.