40AR/39AR DATES FROM IGNEOUS ROCKS IN THE LAKE CLARK REGION INDICATE ALBIAN-CENOMANIAN AND MIDDLE EOCENE MAGMATIC EVENTS

The Lake Clark region in southern Alaska experienced voluminous magmatism associated with Late Cretaceous through mid-Tertiary convergence and subduction of the Kula plate. Plutons, dikes, and volcanic flows are present, and most have intermediate to felsic calc-alkaline compositions. Geochemical data from forty-five samples suggest that the volcanic and plutonic rocks are genetically related. MORB-normalized trace-element spidergrams for both the volcanic and plutonic rocks have relatively high levels of elements such as Rb, Ba, Rb, Th, K, Pb, and Sr, and low levels of Nb and Ti. The chondrite-normalized REE pattern is relatively flat, with slight enrichment of LREE values and a slightly negative to slightly positive Eu anomaly. The similarity of the trace element abundances in the volcanic and plutonic rocks support a similar petrogenetic history.

⁴⁰Ar/³⁹Ar dates collected from six samples reveal two main periods of magmatism in the region. One sample, a gabbro, appears to be part of an older sequence of metamorphic rocks in the area. A complex age spectrum indicates the sample is at least 350 Ma that has undergone alteration/argon loss associated with ca. 100 Ma reheating. A hornblende from an intermediate shallow intrusion yields a plateau age of 101.1 ± 0.7 Ma. A biotite from a granitic pluton has a plateau age of 97.9 ± 0.4 Ma. A hornblende from a gabbro has a plateau age 97.4 ± 1.3 Ma, and its age spectrum suggests partial resetting at ~ 44 Ma. A hornblende from a granodiorite pluton records ages as old as 64.2 ± 0.8 Ma, and a coexisting biotite yields a plateau age of 48.4 ± 0.9 Ma. A biotite from a rhyolite yielded a plateau age of 43.0 ± 0.2 Ma. Together, these data indicate two magmatic events that occurred between ~101-97 Ma and from 48-43 Ma. The 64 Ma hornblende may either reflect partial argon loss from a ~100 Ma sample or may be part of a ~65 Ma magmatic event well documented to the west of Lake Clark.