GRANITIC ADAKITE PLUTONS AND BIMODAL VOLCANISM IN A COLLISIONAL TERRANE SUTURE ZONE, NORTHERN TALKEETNA MOUNTAINS, ALASKA

Large plutons and 100’s of m of volcanic rocks intrude and overlie the suture zone of the Wrangellia composite terrane in the northern Talkeetna Mountains. The plutons (tens km² in size) range from granodiorite to granite, are peraluminous, are transitional between volcanic arc and collisional granites, and exhibit adakite characteristics (heavy rare earth element depletion; high Sr/Y = 20-73). The volcanic rocks are bimodal; one end-member is basalt-andesite (48 to 62% SiO2) and the other is rhyolite (>72% SiO2). There is a distinct absence of dacite. The basalt-andesite set is comagmatic and exhibits high TiO2 (>1.5%), within-plate geochemical affinity, and Nd-Sr isotope ratios in the range between E-MORB and OIB (87Sr/86Sr (T) = 0.7033 to 0.7049; epsilon Nd (T) = 7.2 to 1.9). The rhyolites did not evolve directly from the mafic-intermediate magmas. Isotope data on one rhyolite sample (87Sr/86Sr (T) = 0.7036 and epsilon Nd (T) = 2.5) falls within the range of the mafic samples but trace element data are not consistent with simple fractional crystallization models for the origin of the rhyolites. Our 40Ar/39Ar ages for these rocks are 62.7 to 54.6 Ma for the granitic adakites, 56.0 to 53.5 Ma for basalts-andesites, and 54.9 to 49.5 Ma for rhyolites.

The adakite-like plutons likely formed by partial melting of mid-crustal garnet-bearing meta-flysch in the suture zone. This required increased heat flow at ~62-63 Ma at the end of terrane accretion. High heat flow could have occurred by delamination of lithospheric mantle, slab break-off, and/or spreading ridge subduction. In each case, upwelling asthenospheric mantle would raise the geothermal gradient beneath the suture zone. The basalt magmas formed at ~56 Ma as partial melts from this asthenospheric mantle and/or from a residual subduction-related mantle wedge and evolved by AFC processes to form andesite. Isotope data indicate that the mantle was in the range of E-MORB and OIB-type mantle end-members. The rhyolites formed last, largely as anatectic melts of upper-crustal suture zone flysch. Our data indicate that, collectively, these suture zone magmas were not formed above an actively dehydrating subducted slab but are consistent with a transition from calc-alkaline subduction magmatism to magmatism that is more typical of intraplate tectonic settings.