Mid‐Pleistocene basalt from the Seguam Volcanic Center, central Aleutian Arc, Alaska: Local lithospheric structures and source variability in the Aleutian Arc

Abstract

Recent geophysical and tectonic studies make it possible to examine the relation between local structural and tectonic evolution of the modern Aleutian Arc and the sources and processes that produce the broad spectrum of Aleutian basaltic lavas. Mid‐Pleistocene basalt from the Seguam Island volcanic center in the central Aleutian Arc erupted through strongly extended arc crust, contains 4–8% MgO, is low in K2O (<0.5%), high in Al2O3 (>18%), and is distinguished from Aleutian basalt erupted through less extended crust by a low abundance of incompatible and rare earth elements (REE) and low Sr and Pb content. The Ba/La (47–72) and Pb/La (0.7–1.5) ratios are high, incompatible element ratios are variable, and REE patterns cross one another. The 87Sr/86Sr ratio (0.70365 ± 0.00003) is the highest value obtained from the Aleutian islands, also 206Pb/204Pb (18.91 ±0.02), 207Pb/204Pb (15.60 ±0.02), and 208Pb/204Pb (38.51 ±0.04) ratios are among the most radiogenic values known, whereas the 143Nd/144Nd ratio (0.51297 ±0.00003) is among the lowest measured. Low abundances of incompatible elements, high Ba/La and Pb/La ratios, uniform Sr, Nd, and Pb isotope ratios, and high eruptive temperatures (∼1160° C) suggest that basalt erupted at Seguam ascends through extended subarc lithosphere without extensive chemical exchange with the upper mantle or lower crust. Partial melting of geochemically distinct portions of an isotopically uniform source region best explains observed geochemical characteristics. Trace element abundances rule out an eclogite source and suggest instead a peridotite‐dominated source with ∼5–10% of a subducted sediment component. This peridotite may reside in the mantle wedge or in the Amlia Fracture Zone (AFZ) of the Pacific plate that was subducted beneath Seguam ∼1 Ma. In addition, the AFZ possibly enhanced the subduction of terrigenous sediment into the source region beneath Seguam and this extra terrigenous component may explain the Ba/La, Pb/La and Sr, Nd, and Pb isotope ratios. The distinctive composition of Seguam basalt reflects a complex set of physical conditions, including fracture zone subduction and intraarc extension, that affected both the source region and thermo‐chemical history of magma ascent. Local structural and tectonic factors impact profoundly the nature of island arc magma sources and processes. Models that propose a common parental arc magma type or common processes on a global or arcwide scale are too generalized to explain the diversity actually observed in the Aleutian Arc.