Compositional variation and provenance of Late Cenozoic distal tephra beds, Fairbanks area, Alaska

Abstract

Thin, fine-grained, discontinuous tephra beds from interior Alaska can be divided into two compositional groups by their mineralogy and major- and trace-element glass geochemistry. The source vents of these beds are unknown but are believed to be in the Aleutian arc, Alaskan Peninsula and Wrangell Volcanic Field. Type I tephra has evolved trace-element profiles, while Type II is primitive. Type II contains abundant hydrous minerals that are absent or rare in Type I. Trace elements were used to compare the Fairbanks tephra beds to volcanic rocks in the potential source areas. There are very few published glass trace-element analyses from these regions. This necessitates comparing whole rock with glass analyses. For this reason, incompatible elements, particularly rare-earth elements, are most useful. Whole rock analyses and petrography of lavas throughout the Aleutian arc, Alaskan Peninsula and the Wrangell Volcanic Field are similar to Type I tephra. Lavas and intrusive rocks similar to Type II tephra are rare and occur at one volcano in the Alaskan Peninsula and at three volcanoes in the Wrangell Volcanic Field.