From birth to death of arc magmatism: The igneous evolution of Komandorsky Islands recorded tectonic changes during 50 Ma of westernmost Aleutian history

The Komandorsky Islands form the westernmost end of the Aleutian Island Arc. Four igneous complexes, spanning almost 50 Ma of magmatism, have previously been identified (Ivaschenko et al., 1984: Far East Scientific Centre, Vladivostok, 192 pp.). The petrogenesis of this protracted magmatic record and accurate absolute ages of events, however, remain poorly constrained. Our study investigates the relationship between magma composition and tectonic setting. The Komandorsky igneous basement formed in subduction zone setting. It hosts some of the oldest igneous rocks of the entire Aleutian Arc with the onset of magmatism occurring at 47 Ma. This early stage was characterized by classic fluid-dominated arc volcanism, which produced two coeval but likely genetically unrelated magmatic series of tholeiitic mafic and tholeiitic to calc-alkaline felsic rocks. To date, no boninites have been found and therefore arc initiation is different at the Aleutians than at Izu-Bonin-Marianas or the oldest rocks in the Aleutians have yet to be discovered. The prolonged production of the contrasting basalt-rhyolite association on Komandorsky Islands had lasted ~25 Ma and ceased around the Oligocene-Miocene boundary. Concurrently to this long-lasting activity, a gradual transition to a different mode of arc magmatism took place reflected by newly discovered Sr-enriched, HREE-depleted calc-alkaline basaltic andesitic lavas of mid-upper Eocene age spanning a time of at least ~7 Ma. This so-called Transition Series displays a moderate garnet signature marking the increased contribution of a slab-melt component to the magma sources of the Komandorsky Islands. Slab-melt contribution increased with decreasing age leading to strongly adakitic magmatism as early as ~33 Ma (Lower Oligocene), reflected by eruption of high-Sr (up to 2,500 ppm), highly HREE-depleted Adak-type magnesian basaltic andesites and andesites. These remarkable magmas became predominant during the Lower Miocene. They were followed at ~17 Ma by extremely HREE-depleted calc-alkaline intrusives. Over time there is a clear decrease in Pb isotopic ratios from radiogenic sediment-affected Central Aleutian to unradiogenic Pacific MORB-type compositions similar to Miocene Komandorsky Basin basalt. The geochemical evolution reflects a dramatic change in convergence direction from roughly orthogonal to highly oblique (Duncan and Keller, 2004: G-cubed, v. 5, Q08L03). Increasing oblique subduction led to intense stripping of sediment and enhanced heating of the flat-plunging Pacific lithosphere. This facilitated partial slab melting. However, a significantly increasing amount of slab melt in mid-late Miocene times required an additional heat source, which was probably provided by a slab tear or even slab break-off (Levin et al., 2005: Geology, v. 33, p. 253-256).