NEWLY RECOGNIZED MONOGENETIC VOLCANISM IN SOUTH-CENTRAL ALASKA (U.S.A.): THE MACLAREN RIVER VOLCANIC FIELD AND IMPLICATIONS FOR THE ARCHITECTURE OF THE SUBDUCTING YAKUTAT SLAB

Since ~30 Ma, the Yakutat microplate has subducted under North America leading to the archetypal example of flat slab subduction-slab edge arc magmatism (e.g., Wrangell Arc). The timing of Yakutat subduction initiation is well-constrained, but, the overall extent and geometry of the Yakutat slab under Alaska is still unclear, as well as whether the Yakutat slab is torn or segmented. Small volume, ca. 1 Ma to Holocene monogenetic volcanoes exist in the Alaska Range suture zone (e.g., Buzzard Maar, Jumbo Dome) and are used to delineate the current northern extent of the subducting slab. We report geological and geochemical constraints on lavas and pyroclastic deposits from three different volcanoes, near the Maclaren River and the Denali Highway, ~100 km east of these previously recognized volcanoes. One of the lavas is a calc-alkaline basaltic andesite and has been dated to 940 ± 27 ka using ⁴⁰Ar/³⁹Ar geochronology on groundmass. Other exposures include olivine+plagioclase+phlogopite+amphibole-bearing bombs and scoria; the preservation of this delicate facies indicates a recent eruption given the region’s extensive Pleistocene glacial history. These tephra are all alkaline basalt or trachybasalt, with Sr (~2300-3100 ppm) and Ba (~2700-3200 ppm) enrichments, high Ni (308-331 ppm) and Cr (442-501 ppm), coupled with Mg# = 70-73, suggestive of primary mantle melts. These rocks are also adakite-like (e.g., Sr/Y = 107-141; La/Yb =60-68). A third lithology are fine-grained, platy to massive lavas, exposed at what appears to be a small (~1x1 km) shield. These rocks have olivine+plagioclase+pyroxene, are more evolved (e.g., andesite-trachyandesite) and are subalkaline and calc-alkaline on an AFM diagram. These rocks are also adakite-like, but with lower incompatible element abundances and ratios (e.g., Sr and Ba <1000 ppm; Sr/Y = 64-73; La/Yb =21-22), but high Mg#s (67-69). All rocks show LILE enrichments and HFSE depletions on multi-element diagrams, but each group differs. Geochronology and further study is in progress to constrain the petrogenesis of these rocks. Yet, their location aligns with and chemical compositions are consistent with upwelling and melting adjacent to a tear between ~146° W and 145° W separating moderately dipping (to the east) and lower-angle-dipping portions of the Yakutat slab to the west.