ISOTOPE AND TRACE-ELEMENT GEOCHEMISTRY OF AUGUSTINE-VOLCANO, ALASKA -IMPLICATIONS FOR MAGMATIC EVOLUTION

Augustine Volcano, a Quaternary volcanic centre of the eastern Aleutia n Are, Produces predominantly andesites and dacites of low- to medium- K calc-alkaline composition. Mineralogical and major element character istics of representative lavas suggest that magmatic evolution has bee n influenced by both crystal fractionation and magma-mixing processes. However, incompatible trace element variations (e.g. K/Rb) indicate t hat these evolved lavas have been contaminated by the mafic are crust of the underlying Talkeetna accreted terrane. The limited range of iso tope compositions also supports the assimilation of non-radiogenic maf ic crust (e.g. Sr-87/Sr-86=0.7032-0.7034; Nd-143/Nd-144=0.51301-0.5130 ). In addition, Pb-isotope mean oceanic trend (Pb-206/Pb-204=18.3-18.8 ; Pb-207/Pb-204=15.5-15.6; Pb-208/Pb-204=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 metasom atic fluid component derived from the dehydration of the subducting pl ate. The thickened continental crust (similar to 35 km) of the eastern Aleutian Are prevents the ascent of basaltic melts, which fractionate and assimilate at various depths to produce andesitic magmas. These a ndesites evolve towards more silicic compositions by fractional crysta llization. The absence of evidence for a large high-level crustal magm a chamber implies that tile magmatic system beneath the volcano is you ng and at an immature stage of evolution.