Magnesian Andesites and the Subduction Component in a Strongly Calc-Alkaline Series at Piip Volcano, Far Western Aleutians

Piip Volcano is a hydrothermally active seamount located in the strike-slip regime immediately north of the far Western Aleutian Ridge. Fractionation of hydrous and oxidized magnesian andesites (MA) produced an igneous rock series at Piip Volcano with a lower average FeO*/MgO (more strongly calc-alkaline) than any in the Central or Eastern Aleutian arc. Basaltic rocks in the Piip Volcano area are rare, and those that do occur have characteristics transitional toward MA (high SiO2 and Na2O; low CaO/Al2O3). The compositions of the MA and their predominance as parental magmas throughout the Western Aleutians since Middle Miocene time suggest that transpressional tectonics causes primitive basaltic melts of the mantle wedge to pool immediately below the arc crust, where they interact with warm, ambient peridotite to produce highly silica-oversaturated lavas of mantle origin. Proposed consequences of a long melting column in the mantle wedge (e.g., high percentage melting and tholeiitic volcanism) are not observed at Piip Volcano, despite that fact that it is built on very thin crust. Arc-related incompatible element signatures in volcanic rocks of the Piip Volcano area (e.g., high Ba/La, La/Sm, and Th/Ta) are broadly transitional between mid-ocean ridge basalt (MORB) and basalts of the Central and Eastern Aleutians. Interelement and isotopic ratios are, however, MORB-like (206Pb/204Pb<18.2, εNd>10, 87Sr/86Sr<0.7028). Mixtures of hypothetical slab melts (Western Aleutian adakite) and depleted MORB mantle produce an enriched peridotite source with incompatible element and isotopic characteristics of the Western Alcutian rocks. Components from recycled marine sediment (e.g., radiogenic Pb) are absent, possibly because they have been stripped out at shallow levels by the long, oblique subduction path beneath the forearc. The incompatible element signatures of the Western Aleutian rocks (including Ta depletion) are largely inherited from small percentage melts of the subducting slab, which enrich the mantle wedge source. Fluid-dominated processes of mass transport are not required to explain the arc-type incompatible element signature of the Western Aleutian rocks.