THE EFFECTS OF RIDGE SUBDUCTION ON CHEMICAL AND ISOTOPIC ZONING OF THE KODIAK BATHOLITH, SOUTHERN ALASKA

The Kodiak batholith (>700 km2), part of the Sanak-Baranof Tertiary belt of near-trench intrusive rocks in southern Alaska, forms an elongate body (>100 km long) transecting Kodiak Island from SW to NE. The batholith predominantly consists of biotite tonalite and biotite granodiorite that contain aluminosilicates (andalusite±sillimanite) and intruded an accretionary prism locally mineralized by gold-bearing hydrothermal systems. Small satellites of diorite, quartz gabbro and leucogranite flank the batholith (plutons to the south of the batholith are slightly older). Country rock inclusions (Kodiak Fm.) vary in size, abundance, and degree of disaggregation and reaction. Along the SW to NE axis of the batholith the age decreases from ~59.2±0.2 to 58.4±0.2 Ma (Farris and others, Geol. Soc. America Bull., in review). The batholith is calc-alkalic (K2O/Na2O ~0.5-1), has an aluminum saturation index >1.1, has FeOt/(FeOt+MgO) ~0.65 (at SiO2 = 65 wt. %), and from SW to NE, increases in SiO2 (~61 wt. %-73 wt. %), and decreases in TiO2 (~0.9 wt. %-0.3 wt. %). The granitic rocks have light REE-enriched chondrite-normalized patterns (La/Yb ~6-10, small or no negative Eu anomalies), Sr/Y <17, and mantle-normalized negative anomalies for Nb and Ti, and positive anomalies for Pb. In contrast, the quartz gabbros and diorites are generally characterized by flat chondrite-normalized REE patterns (La/Yb ~1, no Eu anomalies), and mantle-normalized negative anomalies for Nb, P and Ti. Initial Pb isotope compositions on acid-leached feldspars (²⁰⁶Pb/²⁰⁴Pb ~18.85-18.95; ²⁰⁷Pb/²⁰⁴Pb ~15.56-15.61; ²⁰⁸Pb/²⁰⁴Pb ~38.45-38.65) are intermediate between contemporaneous mantle and average crust. Within the main body of the batholith, from SW to NE, ²⁰⁶Pb/²⁰⁴Pb increases slightly. Nd isotopic compositions are also intermediate between the mantle and crust (¹⁴³Nd/¹⁴⁴Nd < 0.51280). Intracrustal melting of compositionally diverse flysch in the accretionary prism likely controlled the chemical and isotopic character of the batholith. This is consistent with the hypothesis that the granitic rocks are associated with migration of a slab-window beneath the accretionary prism.