Evidence for Eocene mafic underplating during fore-arc intrusive activity, eastern Chugach Mountains, Alaska

Abstract

Felsic intrusive rocks in the Chugach metamorphic complex (Alaska) have clear geochemical signatures indicating mixing of magmas from different sources. In this region, the relative sequence of intrusion for four generations of these rocks is well constrained by crosscutting relations. They were intruded during a four-phase deformational history that may be related to changes in the Kula–North America–Farallon plate configuration and subduction of a ridge axis. The felsic intrusions are predominantly tonalitic; some have adakite-like geochemical signatures. Both rare earth element (REE) abundance and initial Sr and Nd isotopic ratios vary, from values similar to metasedimentary rock to more isotopically depleted values with lower REE abundance. This evidence suggests two sources for the Chugach metamorphic complex magmas: metasedimentary rocks of the accretionary prism and a mafic (mantle-derived) source, possibly underplated oceanic material. Furthermore, the adakite-like magmas probably did not form in response to melting of the slab.