Continued Magmatic Unrest: Geochemical Evolution of Recent Eruptions from Mt. Cleveland, Aleutian arc, AK

Mt. Cleveland, a previously unstudied composite volcano (1730 m) in the central Aleutians, last erupted in 1994 and 2001. Mt. Cleveland occupies an unusual tectonic setting, near the transition from sub-arc continental crust to oceanic crust further west. It and the other Islands of Four Mountains are closer to the forearc trench than any other Aleutian volcanoes, and the close configuration of five Quaternary volcanoes is unusual and not understood. To constrain the magmatic evolution and hazard potential of Cleveland and to better understand this unstudied region of the arc, we present new mapping and geochemical results related to field seasons in 2001-2003. The edifice is mainly constructed of lava flows, lahars and isolated domes. Collapse of the summit sometime between September 2002 and August 2003 indicates active magma withdrawal, perhaps temporary, at the agitated volcano. Lava and tephra from both recent eruptions are basaltic andesite (56.7-58.3 wt. % SiO₂; 3.9-4.3 wt. % MgO), and these more mafic compositions compared to most, older Cleveland lavas (58-70.7 % SiO₂, 1.8-3.7 wt. % MgO, n=20 samples) substantiates that influx of new magma preceded the recent eruptions. Disequilibrium mineral textures and zoned plagioclase compositions (An_90-58) confirm that open system processes, including recharge and mixing, have been a dominant characteristic of the magmatic system. Correlation between major elements and trace elements (e.g., Sr, Cr) indicate that some lavas were influenced by fractionation of olivine and plagioclase. Mantle-normalized REE concentrations are not strongly light-element enriched (La_N/Lu_N=1.9-2.3), and the relatively flat, similar patterns corroborate limited olivine and plagioclase fractionation (Eu_N/Eu_N*= 0.8-1.0). New Sr, Nd, Pb and Hf isotopic analyses will demonstrate the extent of source variability and allow comparison to the regionally radiogenic Sr and Pb isotopic compositions found at Yunaska and Seguam Islands ∼300 km to the west.