POST-COLLISIONAL SLAB-WINDOW VOLCANISM ACROSS AN ACCRETED TERRANE ASSEMBLAGE IN SOUTH-CENTRAL ALASKA

Early Tertiary volcanic rocks of the Talkeetna Mountains (TM) in south central Alaska were erupted across the Wrangellia composite terrane (WCT) just after it was accreted to southern Alaska. Even though WCT accretion was integral in Alaska tectonics, there are few models that demonstrate how terrane accretion, coupled with other important tectonic events (such as ridge subduction), influenced plate subduction and associated magmatism. New strontium isotope data presented here reveal a systematic trend among the TM volcanic rocks in which basalts and andesites are most depleted adjacent to the forearc region at the southern edge of the WCT (87Sr/86Sr(i) of 0.7028 to 0.703) and are more enriched at the northern, inboard edge of the WCT at the terrane suture zone (87Sr/86Sr(i) of 0.7037 to 0.7049). A south-to-north increase in La/Yb, Nb/Zr, and Ba/Zr is consistent with the Sr isotope data. The TM volcanic rocks (~56 to 34 Ma) span a hiatus in arc magmatism in southern Alaska and were not likely erupted as part of a regional arc system (there is an absence of margin-parallel magmatism from about 55 to 45 Ma). Instead, upon accretion of the WCT, the TM volcanics were erupted during the period when 72-56 Ma arc magmatism in south central Alaska ceased and the locus of arc magmatism shifted southward to form the Aleutian system. This may have been a response to break-off or roll-back of the subducting slab after terrane accretion. Also during the time of TM volcanism, the migration of a spreading ridge along southern Alaska could have caused the hiatus in arc magmatism. Geochemical data show that the northernmost TM volcanic rocks had a more enriched mantle source, consistent with the remnant mantle wedge from earlier arc magmatism. The southern and central TM volcanics had a more depleted source of magma consistent with derivation from an upper mantle slab window. The slab window could have formed in response to ridge subduction and/or slab break. TM rocks with adakite characteristics are consistent with a slab window model; partial melting of the slab edges and/or melting of garnet-bearing crustal rocks of the WCT above a slab window could produce adakitic magmas. Overall, the TM volcanic rocks provide a unique record of crust-mantle interactions during the plate kinematics that shaped the southern margin of Alaska.