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
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.