Tectonic significance of early Oligocene plutonism on Adak Island, central Aleutian Islands, Alaska G. P. Citron* R. W. Kay, S. Mahlburg Kay Department of Geological Sciences, Cornell University, Ithaca, New York 14853 L. W. Snee, J. F. Sutter Department of Geology and Mineralogy, Ohio State University, Columbus, Ohio 43210 ABSTRACT Six biotite and two hornblende separates from the Hidden Bay pluton, Adak Island, yielded K-Ar dates that cluster about an average of 33.13 ± 0.41 m.y. B.P. This average is interpreted as the time of intrusion of the pluton and provides the first docu- mentation of an early Oligocene magmatic event in the central Aleutian island arc. Biotite and hornblende from the nearby Kagalaska pluton on Adak Island yielded nearly concordant K-Ar dates that have an average of 13.15 ± 0.55 m.y. B.P., in good agreement with published dates of about 13 to 14 m.y. B.P. •Present address: Amoco Production Company, P.O. Box 3092, Houston, Texas 77001. INTRODUCTION Because certain plutonic rocks record past episodes of subduction along the Aleutian island arc (Fig. 1), the timing of plutonism within the arc places constraints on tectonic models of plate evolution which have been deduced from marine magnetic anomalies in the North Pacific Ocean (Hayes and Pitman, 1970; Grow and Atwater, 1970; Cooper and others, 1976; Byrne, 1979). Here we intend to (1) briefly review the two main groups of Aleutian pre-Pliocene igneous rocks, (2) demon- strate that a third (Oligocene) group exists chronologically be- tween the two main groups, and (3) relate the tectonic significance of the Oligocene episode to the development of the Aleutian arc. Of particular interest, the new data relate directly to the postu- lated early Tertiary subduction of the Kula Ridge (spreading center) at the Aleutian Trench (Scholl and others, 1975; DeLong and others, 1978; Hein and McLean, 1980a). Figure 1. Location map for the Aleutian island arc and continental margin in the North Pacific. Islands referred to in the text are blackened. Amatignak, Ulak, and llak (from west to east) are represented by the two tiny islands between and south of Amchitka and Adak. Submarine contours are at 2,000 and 6,000 m. Squares and adjacent numbers indicate DSDP drill sites. Arrow represents approximate motion of the Pacific plate relative to the North American plate. GEOLOGY, v. 8, p. 375-379, AUGUST 1980 375