GENESIS AND CONTAMINATION OF THE KODIAK BATHOLITH, KODIAK ISLAND, ALASKA: USING d18O TO QUANTIFY THE ASSIMILATED COMPONENT OF THE BATHOLITH

Reconstructions of the regional plate tectonics and radiometric dating suggest that the genesis of the Kodiak batholith, Kodiak Island, Alaska, and correlative near-trench intrusions from Sanak to Baranof islands in southern Alaska, resulted from the subduction of the active Kula-Farallon mid-ocean ridge. The subduction of an active spreading center creates a slab window, allowing the juxtaposition of upper mantle magma with the accretionary prism. A siliceous melt was generated as greywacke and argillite trench sediments melted during the passage of the slab window. Using field evidence, stable oxygen isotope analysis, and geochemistry of major and trace elements, this study characterized the Kodiak batholith as a highly peraluminous S-type granite-tonalite. This characterization is consistent with the assimilation of pelitic and metapelitic sediments into the melt, as assumed in the purported tectonic model. Pelitic and metapelitic stope blocks, biotite and quartz clots, and aluminosilicate minerals were observed assimilation components in the batholith. Sampling locations with abundant assimilation components yielded higher d¹⁸O values. Elevated d¹⁸O values of quartz separates ranging from 11.12‰ to 15.05‰ correlate well with a 1-5 field scale referencing the magnitude of observed assimilation components in the sampling locations. The elevated d¹⁸O values suggest the incorporation of sedimentary flysch in the melt, and the strong correlation between the field scale and the d¹⁸O values suggests that assimilation across the batholith is perhaps spatially controlled. The highly peraluminous character and the 65-70% silica content of the batholith are also consistent with the incorporation of pelitic sediments into the melt. The Kodiak batholith has an S-type character, as opposed to the I-type character observed in other Sanak-Baranof intrusions, because its genesis incorporated more evolved, pelitic sediments into the melt instead of volcanoclastic sediments.