GEOCHEMICAL, AND TECTONIC EVOLUTION OF THE KODIAK BATHOLITH AND THE TRENCHWARD BELT, KODIAK ISLAND, AK: JUXTAPOSITION ALONG THE CONTACT FAULT

Kodiak Island, AK contains two Paleocene magmatic belts, which have been explained as resulting from the single passage of a migrating (19 cm/yr) T-R-T triple junction. In this hypothesis, basaltic and gabbroic rocks of the trenchward belt (62.6±0.6 - 60.15±0.86 Ma, K-Ar /Ar-Ar) formed as the spreading-ridge entered the toe of the accretionary prism, and the larger granitic Kodiak batholith (59.2 - 58.4±0.2 Ma, U-Pb zircon) resulted from a slab-window at depth. However, radiometric dates indicate that the two belts differ in age by 1-3 Ma. Therefore, the Kodiak batholith and the trenchward belt cannot have formed from the single passage of a triple junction because at 60-62 Ma the triple junction should have been several hundred km west of Kodiak Island.

The Kodiak batholith and the trenchward belt also differ geochemically. Modeling of trace, REE and oxygen isotope data indicate that the Kodiak batholith formed by equilibrium crystallization of high melt fractions (F=1.0 - 0.5) of argillite and graywacke, had garnet as a residual phase, and was derived from >80% sedimentary sources with only a small basaltic component. In comparison, the trenchward belt rocks formed by assimilation fractional crystallization of a MORB source with an argillite assimilant, and contained no garnet. This suggests the trenchward belt rocks crystallized near the surface (<5 km), whereas Kodiak batholith magmas were generated at greater depths (>15 km).

One interpretation for the petro-chronologic differences between the two Kodiak magmatic belts is displacement along the Contact fault. The Contact fault extends throughout southern AK, and runs between the Kodiak batholith and the trenchward belt, but its displacement is unknown. The fault is highly localized in the central part of the island, but on its NE and SW ends splits into numerous parallel strands. Near Eagle Harbor, the fault core is exposed as a sharp, NW dipping, 1-2 cm thick black microgouge surrounded by 10's of meters of shattered rock. Slickenside data indicate north side up oblique-thrust motion with right and left-lateral movement, but left-lateral slickensides are dominant and yield an average slip vector of 57 and 50 deg. (T/P). Left-lateral motion of several hundred km, would align the trenchward belt with other forearc intrusions of similar age (e.g. the Shumagin batholith).