MID-CRETACEOUS PLUTONIC EVOLUTION OF ALASKA AND YUKON

Twenty-five Early and mid-Cretaceous plutonic suites, defined on the basis of lithological, geochemical, isotopic, and geochronometric similarities, comprise four belts that record the evolution of the most significant magmatic episode in Alaska and Yukon. Plutonism records more than 55 million years of evolving continental margin arc formation and extension-related crustal melting. Plutonic belts were built across Insular, accretionary wedge, Intermontane, pericratonic and miogeoclinal tectonic elements across Alaska/Yukon during the final stages terrane accretion, related deformation and subsequent orogenic collapse.

Notably, the source melts for each of the plutonic suites can additionally be characterized by their primary oxidation state which can be determined by their aeromagnetic characteristics, magnetic susceptibility and whole-rock ferric:ferrous ratios which distinguish them as magnetite- or ilmenite-series.

Overall temporal trends define older, more mafic magnetite-series belts that young inland, with younger, more felsic ilmenite-series belts further cratonward. The oldest and most seaward plutonic episodes (145–125 Ma) are arc-type magnetite-series and may include pre-accretionary arc elements in Insular terrane rocks. Successive, more inboard continental arc magmatism (118–99 Ma) is also magnetite-series, with metaluminous calc-alkaline plutonism. Ilmenite-series felsic plutonism, initiated at about 112 Ma in response to crustal thickening, was dominated by the formation of large, slightly peraluminous batholiths. Younger plutonic suites (109–96 Ma) are similarly dominantly felsic and slightly peraluminous, but are more oxidized forming hybrid weakly magnetite-series belts. A final magmatic episode led to the emplacement of inboard, widely scattered, ilmenite-series granitoids during a late, minor extensional event at 98 to 92 Ma. During latest extension, magnetite-series alkaline plutonic suites and lamprophyres were emplaced at 92–90 Ma in the most inboard locations.