TERTIARY VOLCANISM ACROSS THE WRANGELLIA COMPOSITE TERRANE OF SOUTH-CENTRAL ALASKA

Volcanic rocks of the Talkeetna Mountains (TM ) form a 200-km-long, northwest-trending belt across the Wrangellia composite terrane (WCT) and it’s suture zone in the oroclinal hinge area of south-central Alaska. Each of three main volcanic fields includes a basalt-andesite-dacite-rhyolite suite of lavas, pyroclastic deposits, and shallow intrusions. Eruptions began in each area with basaltic lavas and was followed by felsic lavas and pyroclastics. These volcanic rocks began to erupt just after the WCT was accreted to southern Alaska and show a southward-younging trend (~56-49 Ma, ~50-39 Ma, and ~49-34 Ma from the northern, central, and southern TM, respectively). The northern TM volcanics were erupted through Cretaceous flysch in the WCT suture zone and geochemically are the most enriched with La/Yb ~3 to 9 for basalts. The central and southern TM volcanics were erupted through Paleozoic and Mesozoic granitic, sedimentary, and metavolcanic rocks of the WCT and are strongly depleted with basalts similar to mid-ocean ridge basalt (MORB) in rare earth element composition (La/Yb ~0.5 to 2.5). The southern and central TM basalts also have high Zr/Nb ratios (~10-100) similar to the range of MORB. The southern and central TM volcanics had a depleted magma source that was derived from an upper mantle slab window. The northern TM volcanics had a more enriched mantle source probably derived from the remnant mantle wedge of a preceding episode of 72-56 Ma arc magmatism (Alaska Range magmatic belt). Overall, the TM volcanics were not part of a regional arc system but were erupted during a tectonic transition in southern Alaska from terrane accretion to strike-slip faulting and oroclinal bending.