The role of ridge subduction in determining the geochemistry and Nd–Sr–Pb isotopic evolution of the Kodiak batholith in southern Alaska

The Paleocene Kodiak batholith, part of the Sanak–Baranof belt of Tertiary near-trench intrusive rocks, forms an elongate body (~ 150 km long) that transects Kodiak Island from SW to NE. The batholith consists of three zones (Southern, Central, and Northern) of kyanite-, muscovite-, and garnet-bearing biotite tonalite and granodiorite and less abundant granite that intruded an accretionary prism (Kodiak Formation, and Ghost Rocks Formation). Small and likely coeval bodies (Northern, Western, and Eastern satellite groups) of quartz gabbro, diorite, tonalite, granodiorite, and leucogranite flank the batholith. The batholith is calc-alkalic, has an aluminum saturation index of > 1.1, FeO_t/(FeO_t + MgO) ~ 0.65 (at SiO₂ = 65 wt.%), and increases in SiO₂ (~ 61 wt.%–73 wt.%) and decreases in TiO₂ (~ 0.9 wt.%–0.3 wt.%) from SW to NE. As a group, the granitic rocks have light REE-enriched chondrite-normalized patterns with small or no negative Eu anomalies, primitive mantle-normalized negative anomalies for Nb and Ti, and positive anomalies for Pb. Small to large negative anomalies for Th are also distinctive. The quartz gabbros and diorites are generally characterized by generally flat to light REE chondrite-normalized patterns (no Eu anomalies), and mantle-normalized negative anomalies for Nb, Ti, and P. Pb isotopic compositions (²⁰⁶Pb/²⁰⁴Pb = 18.850–18.960; ²⁰⁷Pb/²⁰⁴Pb = 15.575–15.694; ²⁰⁸Pb/²⁰⁴Pb = 38.350–39.039) are intermediate between depleted mantle and average continental crust. The Southern zone and a portion of the Central zone are characterized by negative ε_Nd values of − 3.7 to − 0.3 and T_DM ages ranging from ~ 838 Ma to 1011 Ma. Other granitic rocks from the Central and Northern zones have higher ε_Nd values of − 0.4 to + 4.7 and younger T_DM ages of ~ 450 to 797 Ma. Granitic and mafic plutons from the Eastern satellites show a wide range of ε_Ndvalues of − 2.7 to + 6.4, and T_DM ages from 204 Ma to 2124 Ma. ⁸⁷Sr/⁸⁶Sr values of the Southern and Central zones overlap and tend to be slightly more radiogenic (⁸⁷Sr/⁸⁶Sr > 0.70426) than the Northern zone (⁸⁷Sr/⁸⁶Sr