THE KODIAK BATHOLITH AND THE TRENCHWARD GHOST ROCKS FM. MAGMATIC BELT, KODIAK ISLAND, AK: TECTONIC JUXTAPOSITION OR TWO DISCRETE EVENTS?

The Paleocene intrusive rocks on Kodiak Island are part of the 2100 km long Sanak-Baranof near trench plutonic belt. Within this belt plutonic rocks decrease in age from 61-50 Ma from west to east. These plutons have been attributed to the progressive subduction of a spreading ridge and resulting slab-window. The average migration rate of the triple junction based on the above ages is 19 cm/yr. A similar rate can also be calculated from internal Kodiak batholith ages, and the Kodiak batholith ages align with the rest of the Sanak-Baranof belt plutons.

On Kodiak Island there are two Paleocene magmatic belts, which traditionally have been treated as having formed form a single passage of the R-T-R triple junction. In this hypothesis the basaltic and gabbroic rocks of the trenchward belt formed as the spreading-ridge entered the toe of the accretionary prism and the larger granitic Kodiak batholith resulted from a slab-window at depth. However, newly determined radiometric dates suggest that they differ in age by 1-3 Ma. U/Pb zircon dates from the Kodiak batholith yield ages that range from 59.2-58.4±0.2 Ma in the SW and NE, respectively. Ages in the trenchward belt range from 62.6±0.6 (K-Ar) on the southwest side of the island to 60.15±0.86 (Ar-Ar whole rock) in the northeast at Pasagshak Bay. Therefore, the Kodiak batholith and the trenchward belt in the Ghost Rocks Formation 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 to the west of Kodiak Island.

There are several explanations for the age disparity between the two belts. These include: tectonic juxtaposition by the Contact fault, multiple passages of the triple junction or off-axis ridge magmatism. Geochemical data such as REE curves and Zr/Nb >30 indicate that the trenchward belt rocks are MORB or E-MORB suggesting affinity to a spreading center. Geochemical data also indicate that the basaltic trenchward belt rocks are similar to the basaltic component in the Kodiak batholith. Therefore the two Kodiak Paleocene magmatic belts likely share a genetic relationship. We suggest that the most likely explanation for the age differential of the Kodiak batholith and the trenchward belt is displacement of two different sections of the Sanak-Baranof belt along the Contact fault.