Isotope and Trace Element Geochemistry of Augustine Volcano, Alaska: Implications for Magmatic Evolution

Abstract

Abstract Augustine Volcano, a Quaternary volcanic centre of the eastern Aleutian Arc, produces predominantly andesites and dacites of low- to medium-K calc-alkaline composition. Mineralogical and major element characteristics of representative lavas suggest that magmatic evolution has been influenced by both crystal fractionation and magma-mixing processes. However, incompatible trace element variations (e.g. K/Rb) indicate that these evolved lavas have been contaminated by the mafic arc crust of the underlying Talkeetna accreted terrane. The limited range of isotope compositions also supports the assimilation of non-radiogenic mafic crust (e.g. 87Sr/86Sr = 0.7032−0.7034; 143Nd/144 Nd = 0.51301−0.5130). In addition, Pb-isotope compositions parallel the North Pacific mean oceanic trend (206Pb/204 Pb = 18.3−18.8; 207Pb/204Pb = 15.5−15.6; 208Pb/204Pb = 38.2−38.3) and do not require a subducted sediment component in the source. Relatively high (Ba/La) N (0.79−18.10) and B/Be (14.5) ratios do, however, suggest a metasomatic fluid component derived from the dehydration of the subducting plate. The thickened continental crust (∼35 km) of the eastern Aleutian Arc prevents the ascent of basaltic melts, which fractionate and assimilate at various depths to produce andesitic magmas. These andesites evolve towards more silicic compositions by fractional crystallization. The absence of evidence for a large high-level crustal magma chamber implies that the magmatic system beneath the volcano is young and at an immature stage of evolution.