EVIDENCE FOR EOCENE MAFIC UNDERPLATING DURING FORE-ARC INTRUSIVE ACTIVITY, EASTERN CHUGACH-MOUNTAINS, ALASKA

Felsic intrusive rocks in the Chugach metamorphic complex (Alaska) hav e clear geochemical signatures indicating mixing of magmas from differ ent sources, In this region, the relative sequence of intrusion for fo ur generations of these rocks is well constrained by crosscutting rela tions. They were intruded during a four-phase deformational history th at may be related to changes in the Kula-North America-Farallon plate configuration and subduction of a ridge axis. The felsic intrusions ar e predominantly tonalitic; some have adakite-like geochemical signatur es, Both rare earth element (REE) abundance and initial Sr and Nd isot opic ratios vary, from values similar to metasedimentary rock to more isotopically depleted values with lower REE abundance. This evidence s uggests two sources for the Chugach metamorphic complex magmas: metase dimentary rocks of the accretionary prism and a mafic (mantle-derived) source, possibly underplated oceanic material. Furthermore, the adaki te-like magmas probably did not form in response to melting of the sla b.