EARLIEST TERTIARY AND LATEST CRETACEOUS GRANITIC ROCKS OF SOUTHWEST ALASKA

In southern Alaska, granitic rocks of Mesozoic and Cenozoic age are widespread. Of these, plutons of roughly 60 to 70 Ma age are the most common. Volcanic rocks of this age are known only locally through the region. The plutons range in composition from quartz diorite and rare gabbroic rocks to alkali granite and syenite. Many of the hot springs in Alaska are associated with these plutons, as are numerous gold, mercury, and tin mineral occurrences. In southwest Alaska, the conventional model is that these plutons are the result of rapid shallow-angle subduction along the paleo-Aleutian trench and that potassium content increases westward, supporting the inference of subduction direction.

A database of major- and trace-element chemical analyses and radiometric ages on more than 500 samples from southwest Alaska was used to investigate the setting and character of these plutons. The data show that for plutons west of the Tertiary to Jurassic age Alaska-Aleutian Range batholith (AARb), potassium content increases eastward, opposite the expected trend. A surface generated using the K₂O/SiO₂ ratio shows the ratio rising eastward to a broad, generally northeast trending high in the east central part of the area and local high zones (typically individual plutons) in the northwest part of the area. If 60-70 Ma plutons within the AARb are included, the K₂O/SiO₂ ratio then decreases rapidly eastward from the high. The plutons of this study typically do not produce anomalies in aeromagnetic surveys, although their contact aureoles may produce moderate highs. Comparison of the K₂O/SiO₂ surface with aeromagnetic patterns shows the broad K₂O/SiO₂ high is coincident with a moderate, deep source, broad, aeromagnetic high.

Our preliminary analysis suggests that these plutons are not associated with paleo-Aleutian trench subduction for a number of reasons. If shallow subduction was the driver for pluton emplacement and the depth of magma generation was on the order of 100-200 km, then the root of the AARb would constitute a barrier for the down-going plate. Additionally, the potassium content over the region increases in the opposite direction from expected.