CONSTRUCTION OF AN ISLAND ARC: A COMPARISON BETWEEN THE KODIAK AND THE TONSINA-NELCHINA TALKEETNA ARC CRUSTAL SECTIONS

The Kodiak Border ranges ultramafic complex, the Afognak batholith and the Shuyak Formation on Kodiak and Afognak Islands together form the respective lower, middle and upper portions of a Jurassic/Triassic island arc crustal section. The Kodiak section exhibits structural and geochemical trends similar, but not identical to, the Tonsina-Nelchina segment of the Talkeetna island arc located >500 km to the northeast. Exposed at the base of the Kodiak section 20-35% MgO cumulate ultramafic rocks (clinopyroxenite with some dunite and wehrlite) and layered gabbros. In the mid to upper crust, Afognak batholith tonalite and quartz diorite intrude the basalt flows, pillows and interbedded volcaniclastic sediments of the Shuyak Formation.

Despite the fault bounded nature of the ultramafic lower crustal and mantle rocks, continuous chemical trends in elements such as MgO, Ni, Cr, and REE's exist across all three units indicating that they are genetically related. Such trends are similar to those observed in the Tonsina-Nelchina arc section, with the following exceptions: 1) the ultramafic rocks have somewhat shallower pressures (7 vs. 10 kbar), and lack garnet bearing gabbros, and 2) the Shuyak Formation volcanic rocks are more mafic than the underlying Afognak batholith. In the Tonsina-Nelchina section, magma chemistries continuously evolve upwards. Overall these data, suggest that the Kodiak portion of the Talkeetna arc had a thinner crust (20-25 vs 30-35 km) and had a more oceanic and immature character than the Tonsina-Nelchina section.

In the Talkeetna arc, at multiple along strike locations, a blueschist unit occupies the same structural position as the Border Ranges ultra mafic complex. Kodiak and Afognak Islands the Raspberry schist and the ultramafic complexes have a similar pressure, age, cooling history and are both fault bounded to the south against the Uyak Formation. A similar structural relationship exists in the Tonsina-Nelchina section with the Liberty Creek schist. One explanation for such relationships is that during an episode of subduction erosion the blueschist units locally replaced part of the arc root.