33 Petrology, Igneous (Posters)

Sunday, 18 October 2009
9:00 AM - 6:00 PM
Hall A

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

Paper number: 33-31
Program: Technical Sessions
Time: 9:00 AM – 6:00 PM

Authors

Abstract

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.