MAGMA MIXING OVER A SLAB WINDOW: GEOCHEMISTRY AND PETROLOGY OF THE SHEEP BAY, MCKINLEY PEAK, AND HIVE ISLAND PLUTONS, CHUGACH-PRINCE WILLIAM TERRANE, ALASKA

The Sanak-Baranof plutonic belt (SBPB) is a diachronous series of near-trench plutons that intrude the Chugach-Prince William terrane (CPW), and range in age from 61 Ma in the west to 48 Ma in the east. In the central part of the belt, they intrude Campanian to Paleocene flysch of the Valdez and Orca groups. SBPB granitoids are inferred to be the product of slab window magmatism, related to subduction of either the Kula-Farallon or the Kula-Resurrection ridge. This study examined 24 samples from the Sheep Bay, McKinley Peak plutons (both near Cordova) and Hive Island pluton (Resurrection Bay). The goal is to understand magma generation in the context of models that require a slab window and partial melting of flysch in the accretionary complex.

The nearly synchronous Hive Island (~ 56.4 ± 1.5 Ma), Sheep Bay (54.8 ± 0.7 Ma) and McKinley Peak plutons (~54.5 ± 1.7 Ma) contain an array of mafic enclaves ~2-15 cm in diameter. The enclaves range in grain size and color index but are always both finer-grained and more mafic than the host granitoid. The finest-grained and most mafic enclaves are bounded by sharp contacts whereas coarser-grained more felsic enclaves have more gradational contacts. Sheep Bay pluton contains gently dipping, tabular, graded sequences with sharply defined finer-grained and more mafic bases.

Both the host granitoids and the mafic enclaves are composed of varying proportions of plagioclase, orthoclase, biotite, and low-Al hornblende. Mafic enclaves contain significantly more biotite and hornblende and less quartz and orthoclase than the granitoid host. Enclaves are basaltic to dioritic whereas the host is dominantly granodiorite. A prehnite-bearing selvage commonly occurs along the enclave-host boundary. Preliminary trace-element data of host and and enclaves from all three plutons show similar patterns, including a general enrichment in the most incompatible elements, a Pb spike and a negative Nb anomaly, characteristics of arc-magmatism.

Ongoing analyses include new zircon age determinations, major element chemistry of both host and enclaves, and mineral chemistry of both host and enclaves. We hope to fully characterize both the host and enclave and determine a petrogenetic model that suggests two distinct magma sources for the plutons in a slab window setting.