16-MY OF HOTSPOT AND NONHOTSPOT VOLCANISM ON THE PATTON-MURRAY-SEAMOUNT-PLATFORM, GULF OF ALASKA

Age-progressive, linear seamount chains in the northeast Pacific appea r to have formed as the Pacific plate passed over a set of stationary hotspots; however, some anomalously young ages and the lack of an ''en riched'' isotopic signature in basalts from the seamounts do not fit t he standard hotspot model. For example, published ages (28-30 Ma) for basalts dredged from the Patton-Murray seamount platform in the Gulf o f Alaska are 2-4 m.y. younger than the time when the platform was abov e the Cobb hotspot. However, the lowermost basalt recovered by ocean d rilling on Patton-Murray yielded a Ar-40-Ar-39 age of 33 Ma. This age exactly coincides with the time when the seamount platform was above t he Cobb hotspot, consistent with a stationary, long-lived mantle plume . A 27 Ma alkalic basalt flow recovered 8 m above the 33 Ma basalt is similar in age and composition to the previously dredged basalts, and may be the alkalic capping phase typical of many hotspot volcanoes. A 17 Ma tholeiitic basalt sill recovered 5 m above the 27 Ma basalt was emplaced long after the seamount platform moved away from the hotspot, and may be associated with a period of intraplate extension. Anomalou sly young phases of volcanism on this and other hotspot seamounts sugg est that they can be volcanically rejuvenated by nonhotspot causes, bu t this rejuvenation does not rule out the hotspot model as an explanat ion for the initial creation of the seamount platform. The lack of an ''enriched'' isotopic signature in any of these basalts shows that enr iched compositions are not necessary characteristics of plume-related basalts. The isotopic compositions of the lower basalts are slightly m ore depleted than the 0-9 Ma products of the Cobb hotspot, despite the fact that the hotspot was closer to a spreading ridge at 0-9 Ma. It a ppears that this hotspot, like several others, has become more enriche d with time.