SLAB-WINDOW PROCESSES IN THE KODIAK ISLAND REGION, ALASKA

Southern Alaska contains a belt of early Tertiary near trench intrusives that have been interpreted to be the result of the subduction of an active spreading-ridge. As the ridge was subducted, a slab-window formed. The primary observable effect of the passage of a slab window beneath the Kodiak Island region was the formation and emplacement of the >800 km^2 Kodiak Batholith. The main body of the elongate batholith was emplaced either as a single magmatic pulse or as multiple batches of magma that were compositionally and temporally very similar. A U/Pb TIMS age of 58.4 ± 0.2 Ma, based on five concordant zircon fractions, has been obtained. Field evidence for both brittle (diking and stoping) and ductile (folding and wall-rock flow) processes exist, which indicate that the emplacement of the main batholith occurred via multiple material transfer processes. This also suggests that the batholith ascended from the site of magma generation to the current level of exposure as a visco-elastic diapir. On Kodiak Island, slab window metamorphism can almost exclusively be attributed to the batholith. Away from the batholith the ambient metamorphic grade (temperatures of 200-250° C) is what would be expected in an accretionary prism. Within the contact aureole, peak metamorphic assemblages of cordierite, garnet and biotite suggest temperatures of >650° C at 2-3 kbar. However, the aureole is less than five kilometers wide where the batholith is widest (10km) and usually the aureole is much narrower. The lack of substantial regional metamorphism can be reconciled with the passage of a slab window if the slab window was substantially below the current level of exposure. Evidence for this exists in the form of large kyanite xenocrysts that are present within the Kodiak batholith. This indicates that the magma generation site had to occur at depths of at least 15-20 km, which is 5-10 km below the current level of exhumation. Five to ten kilometers of rock is sufficient to insulate an area for several 100,000’s of years without substantial regional metamorphism from the slab window and to allow its passage. In such a case, the only evidence of the slab window would be the resultant magmatic bodies. This is what we observe in the Kodiak Island region.