Sample locality map and analytical data for potassium-argon ages in the Port Moller, Stepovak Bay, and Simeonof Island quadrangles, Alaska Peninsula

Potassium-argon age determinations for 84 volcanic, intrusive, and hydrothermally altered rocks from the Port Moller, Stepovak Bay, and Simeonof Island quadrangles are reported here. Of these age determinations, 78 samples were analyzed as part of Alaska Mineral Resource Assessment Program (AMRAP) studies in the Port Moller, Stepovak Bay, and Simeonof Island quadrangles. Age determinations for 6 of the samples have been previously published (Burk, 1965; Kienle and Turner, 1976; Wilson and others, 1981). This report consists of a sample location map, analytical data (table 1), and rock descriptions (table 2). In earlier studies in the map area, Burk (1965), Moore (1974), and Kienle and Turner (1976) reported age determinations on samples (samples 405M and 406, sample 62AGz 7, and samples 75088 and 75092 respectively, table 1) from the Shumagin Islands batholith (Tg). Armstrong and others (1976) reported an age determination (sample "Pyramid", table 1) on a hydrothermally altered sample from the Pyramid porphyry prospect. Finally, Wilson and others (1981) reported an age on a volcanic rock sample (77AWs 102, table 1) from the Meshik Volcanics (Tm). Previous geochronologic studies on the Alaska Peninsula (Wilson and others, 1981; Wilson and Shew, 1992; Shew and Lanphere, 1992) have shown that most ages cluster in late Eocene to earliest Miocene (about 30 to 40 Ma) or late Miocene to Quaternary (less than 10 Ma) times. In particular, samples yielding middle Miocene ages (12-21 Ma) are rare and samples yielding ages of greater than approximately 47 Ma absent, except for ages determined on plutonic rocks of the Alaska-Aleutian Range batholith and plutons (Tg) of the Kodiak, Shumagin, and Sanak Islands. Geochronologic studies in the Aleutian Islands (see Wilson and others, 1991) display a similar pattern, except for an event in the range of 9.0±0.6 to 15.6±0.8 Ma in the western Aleutians on the Near Islands and on Amchitka Island (Scholl and others, 1976; Marlow and others, 1973).

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