Tectonic and petrologic evolution of the Kodiak batholith and the trenchward belt, Kodiak Island, AK: Contact fault juxtaposition?
نویسنده
چکیده
[1] Two fore‐arc magmatic belts exist in Kodiak Island, the granitic Kodiak batholith and the basaltic trenchward belt. Both are part of the 2100 km long Sanak‐Baranof belt, interpreted to result from trench‐ridge‐trench (T‐R‐T) triple‐junction migration. In this hypothesis, trenchward belt basaltic rocks formed as the spreading ridge first entered the accretionary prism, and the larger Kodiak batholith resulted from a slab window at depth. However, radiometric dates between the two belts differ in age by up to 1–3 Ma. Kodiak batholith U/Pb zircon ages range from 59.2 to 58.4 ± 0.2 Ma (SW‐NE), whereas trenchward belt ages range from 62.6 ± 0.6 (K‐Ar) to 60.15 ± 0.86 (Ar/Ar whole rock) SW‐NE. This suggests that both fore‐arc magmatic belts did not form from a single triple‐ junction passage, because at 60–62 Ma the triple junction was several hundred kilometers west of Kodiak Island. Multiple explanations exist for the age disparity between the two belts, including oblique ridge subduction, Contact fault tectonic juxtaposition, slab window thermal lag, multiple triple‐junction passages, or off‐axis ridge magmatism. Geochemical data such as rare earth metal (REE) curves and Zr/Nb of >30 indicate the trenchward belt rocks have a mid‐ocean ridge basalt (MORB)–like character. Major and trace element modeling indicate that trenchward belt rocks resulted from assimilation‐ fractional‐crystallization of MORB with Ghost Rocks Formation argillite, whereas the Kodiak batholith formed via partial melting and en masse mobilization of Kodiak Formation metasedimentary rocks with a small basaltic component and subsequent equilibrium‐crystallization and restite separation. The petrologic origin of both belts is compatible with spreading ridge subduction. Fault zone structural data suggest the Kodiak batholith/trenchward belt age differential resulted from oblique ridge subduction and Contact fault displacement.
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