STUCK IN THE MIDDLE: STRATIGRAPHY AND GEOCHEMISTRY OF VOLCANIC ROCKS FROM THE GREEN HILLS IN THE CENTRAL WRANGELL ARC, ALASKA

The Wrangell Arc (WA) straddles the Alaska-Canada border and is a transition zone between the “normal” slab edge subduction of the Aleutian subduction zone and the right lateral strike-slip tectonics to the east along the Queen Charlotte/Fairweather and Denali/Duke River fault systems. Our research is part of a larger project focused on the temporal and spatial magmatic migration of the WA. The Green Hills area is a well-exposed ice-free location adjacent to the Nabesna Glacier, in between regions of the WA studied by Trop et al. (2012) and Preece and Hart (2004). The volcanic stratigraphy, geochemistry, and geochronology of local eruptive products in this part of the WA is not well understood because of limited exposures. At Green Hills, ~720m of WA volcanic products were sampled. Volcanic deposits overlie poorly sorted, matrix-rich breccia and volcaniclastic sandstone deposited by hyperconcentrated flow and debris flow, reflecting progradation of vent-proximal volcanics across alluvial strata. The lower volcanic section is dominated by thick basaltic andesite and andesite lavas that are overlain by a felsic pyroclastic unit, near vent deposits from an eroded cinder cone, and then thin-bedded intermediate composition lavas. Petrography and geochemistry differentiate seven different eruptive units. The whole rock geochemical composition for the new samples range from basaltic andesite to dacite (wt.% SiO₂ = 54-69), plot as transitional to calc-alkaline on an AFM diagram, and the higher Si rocks are metaluminous. Three samples (SB15-3, -7, -8) have SiO₂ <60 wt. % and TiO₂ > 1.15 wt.% (1.24-1.96), which place them into WA Trend 1 (Preece and Hart, 2004), a transitional to tholeiitic suite formed via intra-arc extension. SB15-6, a trachydacite, also has Trend 1 chemistry. All other samples have bulk chemistry that overlap with WA-wide trend 2a of Preece and Hart (2004), which are calc-alkaline rocks formed via “typical’ subduction processes (e.g., melting of mantle wedge enriched in subduction components). No adakitic (Trend 2b; Preece and Hart, 2004) compositions were sampled indicating that slab-edge melting did not produce the sampled magmas. Furthermore, no evidence of alkaline magmatism that has been attributed to leaky-strike slip faulting in the WA is present at Green Hills.