OXYGEN AND HAFNIUM ISOTOPE GEOCHEMISTRY OF ZIRCON, QUARTZ, AND GARNET FROM THE NEAR-TRENCH CRAWFISH INLET AND KRESTOF PLUTONS, BARANOF ISLAND, ALASKA

The Paleocene to early Eocene Sanak–Baranof plutonic belt (SBPB) is a 2100 km series of near-trench plutons that intruded the entire Upper Cretaceous to Paleocene Chugach–Prince William terrane accretionary complex along the continental margin of southeast Alaska between 61-47 Ma. This study presents new oxygen and hafnium isotope analyses, and U/Pb geochronology on the Crawfish Inlet Pluton and the Krestof Pluton, from the eastern part of the Sanak-Baranof Belt, to further evaluate the hypothesis of a crust vs. mantle origins for the SBPB. The goals of this project include: correlating δ¹⁸O with available ε-Hf (Zrc) and U/Pb age (Zrc), and using thermometry by calculating inter-mineral O fractionations. Values of δ¹⁸O (Zrc) range from 6.18 ± 0.12‰ (2σ) to 7.48 ± 0.12‰, and exhibit evidence for the incorporation of recycled crust, as they uniformly plot above the range for mantle equilibrated zircon. U/Pb ages from seven samples within the CIP and one sample from a small satellite sill (CP13-08) range from 47.3±1.2 to 53.1±0.8 Ma. Hf isotopes collected from four samples within the CIP yield ε-Hf values ranging from +18.2 ± 1.5 (1σ) at 47.4 Ma to +0.8 ± 1.5 (1σ) at 54.0 Ma, indicating a distinct trend within the CIP. The results show a strong correlation between ε-Hf and O indicating that the more primitive (juvenile) granitoid zircons with higher ε-Hf values record the least evolved oxygen isotope ratios, with decreasing importance of crustal recycling during the genesis of these granitoids with time.