MID-EOCENE MAGMATISM ACROSS SOUTHERN ALASKA: EVALUATING A SECULAR TREND TO MORE DEPLETED GEOCHEMICAL AFFINITY LEADING UP TO THE MODERN ALEUTIAN ARC

A salient observation of igneous rocks across southern Alaska is a trend to more juvenile and depleted compositions between Late Cretaceous-Paleocene and mid-Eocene time. Mid-Eocene plutonic and volcanic rocks have lower K₂O, La/Yb, Nb/Yb, and zircon U/Yb, and higher εNd and εHf than Late Cretaceous-Paleocene rocks. Of note is that this ‘younging-trend’ towards more depleted composition exists among granitic rocks, even without mafic end-members. The trend is widespread, evident in samples from the Talkeetna Mountains, Matanuska Valley, the central and western Alaska Range, and the northern Alaska Peninsula. This change to more juvenile magmatism occurred after both the Late Cretaceous culmination of Wrangellia composite terrane accretion and Paleocene spreading ridge subduction beneath southern Alaska and was coeval with a NW shift in Pacific plate motion between ca. 50 and 40 Ma. The shift in Pacific plate motion may have been a driver for both strike-slip along margin parallel faults and the initiation of Aleutian arc magmatism. Mid-Eocene rocks on the Alaska Peninsula and in the Alaska Range have arc-like affinity and resemble the modern Aleutian arc in their depleted chemistry. It follows then that part of the mid-Eocene belt represents the continental portion of the early Aleutian arc, which ceased in the Alaska Range with initial flat-slab subduction of the Yakutat plate (~25 Ma). Also, the mid-Eocene plutons in the Alaska Range are localized along the Denali fault, perhaps signifying a link between magmatism and deformation. Other mid-Eocene rocks, such as those in the Matanuska Valley (remnant forearc), are not of typical arc affinity and may represent transtensional magmatism along the Castle Mountain fault system. Models for mid-Eocene magmatism thus need to account for inboard arc-like rocks and near-trench magmatism, all with relatively depleted chemistry. We pose the hypothesis that relatively juvenile depleted mantle was introduced beneath southern Alaska between Paleocene and mid-Eocene time by: asthenospheric upwelling through a slab window, upwelling following lithosphere delamination, and/or corner flow of previously melted mantle. This mantle reservoir was tapped during mid-Eocene and younger tectonic events across southern Alaska including the modern Aleutian arc.