GEOCHEMICAL AND PETROLOGIC EVIDENCE FOR MAGMA MIXING IN THE SHEEP BAY AND MCKINLEY PEAK PLUTONS, PRINCE WILLIAM SOUND, ALASKA
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.