HF ISOTOPE SIGNATURES FROM FORE-ARC PLUTONS OF THE SANAK-BARANOF BELT, ALASKA, SUGGEST SPATIAL AND TEMPORAL CONTROL OF CRUSTAL CONTAMINATION

Upper Cretaceous to Paleocene flysch and interbedded volcanic rocks of the Chugach-Prince William terrane (CPW) are intruded by the diachronous Sanak-Baranof belt of granitic plutons for about 2000 km along the southern Alaskan margin from Sanak Island in the west to Baranof Island in the east. To the west, the Sanak Island pluton yields a crystallization age of 63.1±0.9 Ma and records a primitive zircon εHf(t) isotope signature of +9.3. In the center of the belt, the 56.5-54.5 Ma plutons in the Prince William Sound area yield more evolved zircon εHf(t) signatures of about +5. Far to the east, the Crawfish Inlet composite pluton on Baranof Island yields a range of crystallization ages from 53.1 to 47.3 Ma suggesting that this pluton was built over a period of ~6 Myr. εHf(t) values increase systematically from more evolved values (+4.7) at 53.1 Ma to more primitive values (+13.7) at 47.3 Ma.

The unusual near-trench tectonic setting for the intrusion of Sanak-Baranof plutons into the accretionary wedge complex of the CPW, and the depleted geochemical signature of some of the SB plutons, has led many workers to suggest that these plutons formed during the subduction of a mid-ocean ridge and formation of a slab window at a trench-ridge-trench triple junction. The diachronous timing of intrusion of the plutons suggests two competing models: 1) the CPW formed more or less in place and the TRT triple junction migrated from west to east during subduction of the Kula-Resurrection ridge; or 2) the CPW migrated across the Kula-Farallon ridge during coastwise translation from lower latitudes. Our new data show that the Sanak-Baranof plutons record the most evolved Hf isotope signatures at ~56-53 Ma suggesting that partial melting and assimilation of older continental crust was an important part of the petrogenesis of these plutons at this time regardless of location along the belt. In addition, the long-lived plutonism and systematic change in the Hf isotope signatures with time on Baranof Island, suggest that simple migration or translation models for the formation of the Sanak-Baranof plutons need to be modified.