MAGMATISM AND OROGENIC CYCLES IN SOUTH-CENTRAL ALASKA: INSIGHTS FROM ZIRCON U-PB-HF, WHOLE ROCK ND-ISOTOPE AND GEOCHEMICAL DATA, AND FIELD OBSERVATIONS (Invited Presentation)

The age and composition of Late Cretaceous through Eocene igneous rocks of south-central Alaska are consistent with the cycle of lithosphere thickening and delamination documented in other Cordilleran orogenic systems. The trends described here represent a compilation of geochemical data on hundreds of volcanic and plutonic samples from the central Alaska Range and Talkeetna Mountains. These rocks exhibit a trend to more depleted compositions between Late Cretaceous-Paleocene and mid-Eocene ages (e.g., mid-Eocene rocks have lower K₂O, La/Yb, Nb/Yb, and higher εNd and εHf than Late Cretaceous-Paleocene rocks). The Late Cretaceous-Paleocene rocks represent the culmination of arc magmatism during shortening across the suture zone of the Wrangellia composite terrane (WCT) marked by contractional deformation and peak metamorphism between ca. 70-62 m.y. Crustal thickening with underthrusting of melt-fertile suture zone flysch (Kahiltna assemblage) contributed to the more evolved magma compositions as supported by geochemical modeling and confirmed by migmatites surrounding some plutons that yield zircon ages consistent with the Kahiltna flysch. Unroofing of early Paleocene plutons indicates that regional uplift closely followed this phase of magmatism. The more primitive Eocene rocks are indicative of less crustal influence and the introduction of depleted asthenosphere. We propose that this sequence of events may reveal the following orogenic cycle: ca. 15 m.y. of Late Cretaceous-Paleocene (~70-55 Ma) magmatism coincident with contraction along the WCT suture zone, ca. 10 m.y. of delamination of an arc root and/or thickened suture zone lithosphere, followed by more primitive and less widespread Eocene (~45-35 Ma) magmatism sourced by upwelling asthenosphere. This provides an alternative to the hypothesis that depleted asthenosphere was emplaced beneath southern Alaska through a slab window. Eocene magmatism may have been localized by extension or strike-slip associated with a change in direction of the subducting oceanic plate beneath southern Alaska from N-NE to NW at ca. 46-40 Ma. Magmatism then essentially ceased in this region with flat-slab subduction of the Yakutat plate while ongoing subduction to the west, beneath the Alaska Peninsula, initiated the Aleutian arc system.