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