ZIRCON U-PB AND AR-AR GEOCHRONOLOGY, HF ISOTOPE, AND TRACE ELEMENT CONSTRAINTS ON TEMPORAL TRENDS IN MAGMA COMPOSITION: ALASKA PENINSULA

The Alaska Peninsula consists of the continental portion of the Aleutian arc and contains Jurassic through Quaternary igneous rocks. Reanalysis of archived samples collected during U.S. Geological Survey field campaigns are used to evaluate temporal trends in magma composition leading up to and including modern Aleutian arc magmatism on the Alaska Peninsula. New analytical data includes zircon U-Pb geochronology (ages range from 15.7 to 92.1 Ma), Hf isotope, and trace element geochemistry on 13 samples of dioritic to granodioritic rocks from the northeastern portion of the Alaska Peninsula. These zircon data complement whole rock major and trace element data on a total of 180 Alaska Peninsula samples analyzed as part of this study. Our age data coincide with previous work concluding that the inception of the proto-Aleutian arc occurred in the Late Eocene. Additionally, the zircons of all ages in this study are of continental affinity (i.e., they are enriched in U and Th relative to Yb, Hf, and Y), even though the plutons were emplaced through Jurassic rocks of the Peninsular terrane which are largely of intraoceanic arc affinity (Talkeetna arc). These results reveal that evolved crust likely contributed to Late Cretaceous through Miocene magma formation. Furthermore, using our 13 new U-Pb ages and 9 new Ar-Ar ages along with estimated ages of the remaining 180 samples based on mapped units, the Late Cretaceous through Miocene igneous rocks on the Alaska Peninsula show a decrease in whole rock La/Yb and an increase in zircon eHf through time. This trend is consistent with results for similar aged rocks within the central Alaska Range and Talkeetna Mountains of south-central Alaska (e.g., Eocene rocks have lower K₂O, La/Yb, and zircon U/Yb, and higher eNd and eHf than Late Cretaceous-Paleocene rocks). These data reveal a widespread younging-trend towards more depleted compositions that may be evidence that mantle replacement processes occurred beneath southern Alaska (e.g., asthenosphere upwelling following a delamination or slab-window event).