Post-Collisional Magmatism Across an Accreted Terrane Assemblage in South-Central Alaska: Connections Between Mantle Sources, Deformation, and Plate Margin Dynamics

Paleocene and Eocene volcanic and plutonic rocks in the Talkeetna Mountains (TM) of south central Alaska were emplaced across the Wrangellia composite terrane (WCT) and its suture zone during and after collision with southern Alaska. Past studies infer that these volcanic rocks were formed as part of a regional arc system. Our new Nd-Sr isotope and geochemical data along with previously described field and radiometric age data reveal that they instead represent a unique episode of continental margin magmatism. TM magmatism began at ca. 62 Ma with adakitic granitic plutons and then continued, through Eocene time, with granitic and shallow adakitic intrusions and tholeiitic to high-K basalt-andesite-rhyolite volcanism when there was a hiatus in arc magmatism across southern Alaska. Trace element and Nd- and Sr-isotopic data reveal a systematic trend among the TM volcanic rocks in which basalts are most depleted in the forearc region at the southern edge of the WCT (ɛNd(t) = 8.41 to 10.87; 87Sr/86Sr(t) = 0.702778 to 0.703193) and are more enriched at the northern, inboard, edge of the WCT (in the terrane suture zone) (ɛNd(t) = 7.24 to 1.98; 87Sr/86Sr(t) = 0.703725 to 0.704755). Intermediate to acidic rock compositions have more enriched isotopic ratios than the basalts in each region. A systematic south-to-north (continentward) increase in La/Yb, Nb/Zr, and Ba/Zr is consistent with the isotope data. The composition of crustal rocks in the accreted terrane assemblage changes from oceanic arc affinity in the south to granitic and sedimentary rocks of continental affinity in the north. These changes in basement composition are, in part, responsible for the range of Nd and Sr isotope compositions among the TM volcanic rocks. The southern TM rocks had a depleted source of basaltic magma consistent with derivation by adiabatic melting of a sub-oceanic MORB-like mantle reservoir. This mantle was emplaced beneath southern Alaska through a slab window following Late Paleocene ridge subduction. Farther inboard, the northern TM basaltic magmas had a more enriched mantle source and/or assimilated more enriched crustal rocks. In this region, high heat flow through a slab window could have induced partial melting of pre-existing sub-continental mantle and also produced crustal melts to form rhyolites. 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 have produced adakitic magmas. TM magmatism occurred in a zone of crustal extension that was oblique to the continental margin. Extension can be attributed to oroclinal rotation of western Alaska and/or transtension associated with regional strike-slip faults. Subsequent deformation of parts of the TM volcanic fields (broad folding and small-scale brittle faults) is consistent with dextral simple shear along adjacent strike-slip faults. The TM rocks, therefore, record the magmatic response to terrane accretion and ridge subduction and the kinematics of margin-parallel transport of an accreted terrane assemblage after it was sutured to the continental margin. In addition, these rocks reveal how the composition of collisional crustal elements can influence subsequent continental margin magmatism. Overall, the TM volcanic rocks provide a unique record of crust-mantle interactions during the plate kinematics that shaped the southern margin of Alaska.