RELATIONSHIPS BETWEEN SPREADING-RIDGE SUBDUCTION AND THE FLARE-UP IN ARC MAGMATISM IN THE ALASKA-ALEUTIAN BATHOLITH AND THE COAST PLUTONIC COMPLEX

Increased arc magmatism and a 61-50 Ma spreading-ridge subduction event in southern Alaska are temporally and spatially correlated. The arc flare-up migrates from the western Alaska-Aleutian batholith (ca. 61-60 Ma), to the eastern Alaska Range (ca. 56-58 Ma McKinley intrusive suite) and concludes with the ca. 54 Ma magmatic pulse in the Coast Plutonic Complex of British Columbia. Based on geochronology from near-trench and arc plutons, at a given point along strike, arc flare-up preceded the passage of the Sanak-Baranof belt TRT triple junction by 1-2 Ma. After the passage of the triple junction, arc magmatism shut off.

Modern examples in which young oceanic crust is being subducted (e.g. southern Japan and the Cascades) exhibit different types of arc magmatism than is observed in the above-mentioned arcs. These examples produce hot, deep mantle melts and adakites. Overall, the volume of arc magmatism is low. In contrast, the Alaska-Aleutian range batholith and Coast Plutonic complex flare-ups produced high volume plutonism dominantly derived from crustal sources.

A characteristic difference between the modern and the Paleocene-Eocene Alaska-British Columbia subduction zones is the age of the subducted lithosphere. In the modern examples, the oceanic crust being subducted is still older than 5 Ma, whereas in Alaska and B.C., the subduction of a spreading ridge necessitates that even younger lithosphere was subducted. The temperature of oceanic lithosphere younger than 1-2 Ma greatly increases by more than 500°C. Subduction of crust of this age would lead to a hotter, dryer mantle wedge that could potentially induce large-scale partial melting in the overlying lower crust. Such a mechanism would explain the observed crustal signature in the 60-54 Ma Alaska-Aleutian batholith / Coast Plutonic Complex, and would fit into the larger tectonic history of the region.