GEOCHEMICAL AND PETROLOGIC EVIDENCE FOR MAGMA MIXING IN THE SHEEP BAY AND MCKINLEY PEAK PLUTONS, PRINCE WILLIAM SOUND, 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 63 Ma in the west to 48 Ma in the east. In east Prince William Sound, they intrude Paleocene flysch and volcanic rocks of the Orca Group. SBPB granitoids are inferred to be the product of slab window magmatism and partial melting of the overlying accretionary complex of the CPW, related to subduction of either the Kula-Farallon or the Kula-Resurrection ridge beneath the North American continent at a trench-ridge-trench triple junction.

Recently obtained U/Pb zircon dates from the Sheep Bay and McKinley Peak plutons near Cordova yield crystallization ages of 54.8 ± 0.7 Ma and 54.5 ± 1.7 Ma, respectively. Both plutons contain an array of mafic enclaves ~2-15 cm in diameter. Many features of enclave petrography, including poikilitic textures, acicular apatite, sharp but partly diffuse contact with host granitoids, and finer grain size than host granitoid, provide evidence for magma mixing and mingling. Locally, the Sheep Bay pluton displays magmatic layering with gently dipping, tabular, graded sequences with sharply defined, finer-grained, and more mafic bases. Both plutons are weakly peraluminous, with Sheep Bay samples having an average ASI of 1.032 and McKinley Peak an average of 1.072. Trace-element data of host and enclaves from both 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. Zircon from the McKinley Peak and Sheep Bay plutons yield average εHf(t) of +7.4 and +4.8, respectively. The distinctly different and more primitive isotopic signature of the McKinley Peak pluton supports previous studies that suggest the two plutons have isotopically distinct crustal components in their source region.