DISTINCTLY DIFFERENT PARENTAL MAGMAS FOR PLUTONS AND LAVAS IN THE CENTRAL ALEUTIAN ARC

Parental magmas for calc-alkaline plutons in the central Aleutians have a distinctly different source compared to Holocene tholeiitic lavas in the same region. As a result, the Holocene lavas are not representative of the net magmatic transfer from the mantle into the arc crust, as is commonly assumed. Eocene to Miocene (9-40 Ma) intermediate to felsic plutonic rocks from the central Aleutian arc show higher SiO₂ at a given Mg#, higher εNd- and εHf-values, and lower Pb isotope ratios than Holocene volcanic rocks from the same region. Instead, the plutonic rocks resemble volcanics from the western Aleutians isotopically, and have compositions similar to the bulk continental crust. These data could reflect temporal variation of Aleutian magma source compositions, from Paleogene-Neogene “isotopically depleted” and predominantly calc-alkaline to Holocene “isotopically enriched” and tholeiitic. Alternatively, the data may reflect different processes of transport and emplacement for the magmas that form plutons and lavas: calc-alkaline magmas with higher Si content and high viscosity – perhaps after extensive mid-crustal degassing of initially high water contents – may preferentially form plutons. This implies that the upper and middle arc crust is more similar to the calc-alkaline bulk composition of the continental crust than the lavas alone.