Some accreted volcanic rocks of Alaska and their elemental abundances

Abstract

This chapter describes and gives elemental abundances of many of the accreted volcanic rocks and of a few hypabyssal rocks of Alaska. These rocks range from early Paleozoic (or perhaps late Precambrian) to Eocene age. All formed prior to accretion of the terrane containing them and thus were generated either as primary features in the ancestral Pacific Ocean or on terranes or superterranes carried by plates underlying that ocean. These accreted volcanic rocks are important in terms of continental growth by accretion of oceanic rocks. Various workers have asserted that such growth is by accretion of intraoceanic island arcs. This assertion, however, must be appreciably modified for the ca. 400,000-km^2 region of southern and central Alaska that is underlain by accreted rocks. Though these rocks are not known in sufficient detail to yield a precise figure, I estimate that no more than 70 to 75 percent of this newly formed crust consists of former island arcs and arc-derived epiclastic sedimentary rocks. Most of the tectonostratigraphic (lithotectonic) terranes of Alaska have minor exposures of volcanic rocks. Accounts of local and regional geology of the state contain cursory to extensive descriptions of such rocks. However, a catalog of such occurrences is not considered appropriate for this volume, and we discuss here only rocks studied by modern methods. The particular terranes containing these rocks are shown on Plate 13 (Barker and others, this volume), whereas all tectonostratigraphic terranes of Alaska are shown on Plate 3 (Silberling and others, this volume).

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