The Paleogene Basement Series on Attu Island in the Western Aleutians is dominated by tholeiitic basalts chemically similar to mid-ocean ridge basalts (MORB; La/Yb~2, &egr;Nd>+10.0, 206Pb/204Pb<18.4). These basalts evolved in ''open'' magma systems. Contemporaneous rhyolites and albite granites are chemically analogous to silicic volcanic rocks of modern ocean ridges (e.g., Iceland, Galapagos Spreading Center). The occurrence of chemically arc-type tholeiitic basalts, with high Th/La and La/Ta relative to MORB, suggests that a depleted MORB-like mantle source was variably modified by a subduction component. The Attu Basement Series is not allochthonous. Geological and tectonic constraints imply formation in arc-adjacent, transtensional rifts that developed in a strike-slip regime that was established ~43 m.y. ago. Rifting of this kind may have produced much of the Western Aleutian crust between ~43 and 15 Ma. Subsequently, small volumes of crystal-rich andesite and dacite were erupted on Attu Island as well as throughout the Western Aleutians. These strongly calcalkaline rocks (FeO*/MgO~1.1, CaO/Al2O3~0.35 at 65% SiO2) are chemically akin to magnesian andesites of Piip Volcano, a hydrothermally active seamount that overlies small dilational structures within the broadly transpressional regime of the modern western arc. This transpressional setting is inferred to have been established throughout the Western Aleutians approximately 15 m.y. ago. The switch from tholeiitic magnetism in a transtensional regime to strongly calcalkaline magmatism in a transpressional regime may have resulted from clogging of the Aleutian-Kamchatka junction with buoyant, subduction-related terranes. These terranes probably originated to the east and were transported by strike-slip motion along the western arc. ¿ American Geophysical Union 1993