GEOCHEMISTRY, U-PB AGES, AND HF ISOTOPES OF THE MT. DRAPER AND MT. STAMY PLUTONS, NUNATAK FJORD, ALASKA: IMPLICATIONS FOR THE SANAK-BARANOF PLUTONIC BELT

The Sanak-Baranof plutonic belt intrudes the Chugach-Prince William (CPW) terrane, a thick accretionary wedge complex dominated by Upper Cretaceous to Eocene turbidites that now occurs along the southern coast of Alaska from Sanak Island in the west to Baranof Island in the east. The plutons are time-transgressive with older plutons in the west and ages ranging from 63–47 Ma. A prevailing hypothesis is that the Sanak-Baranof plutons formed during ridge subduction leading to anataxis of the overlying turbidites of the CPW. The Mt. Draper and Mt. Stamy plutons intrude the Yakutat Group in Nunatak Fjord, Alaska. Sanak-Baranof plutons generally have a MORB signature, but rare earth element data from a mafic phase of the Mt. Draper pluton is enriched in LREEs compared to N-MORB.

Early attempts to date the Mt. Draper pluton with TIMS were problematic due to inheritance. We report here LA-ICPMS U-Pb zircon dates from three different Mt. Draper two-mica granite samples that yield crystallization ages of 54.00 ± 0.60 Ma, 53.92 ± 0.82 Ma, and 53.06 ± 0.72 Ma. Two of these samples have Jurassic and Cretaceous inheritance and one of those samples contains four Precambrian zircons with ages ranging from 1354–1737 Ma. A gabbro most likely associated with the Mt Draper pluton, but collected from the Nunatak (5 km east of the main body of the pluton) yields an age of 52.78 ± 0.60 Ma, and the nearby Mt. Stamy pluton has a U/Pb date of 51.42 ± 0.66 Ma. Maximum depositional ages based on detrital zircon U-Pb ages for three samples from the Schist of Nunatak Fjord, into which the Mt. Draper and Mt. Stamy plutons intrude, are 53.2 ± 2.1 Ma, 55.1 ± 2.0 Ma, and 56.9 ± 1.6 Ma and overlap the crystallization ages of the plutons, and thus require a remarkably rapid series of events. εHf (t) values from magmatic (~54 Ma) zircons from the Mt. Draper sample with Precambrian inheritance have a large range from -2.3 to +11.6 and a weighted mean of +7.6 (MSWD=19.8). εHf (t) values from another Mt. Draper sample with no evidence of inheritance range from +4.7 to +10.6 and also has a weighted mean of +7.6 (MSWD=7.1). The wide range of εHf (t) values suggests a heterogeneous source region for the Mt. Draper granites and might reflect mixing between a juvenile mantle melt and partial melts of crustal material with a more radiogenic Hf isotope signature such as the turbidites from the Yakutat Group.