Magmatic Differentiation in the Accreted Talkeetna Arc, South-Central Alaska

Geochemical studies of island-arc magmatism have principally focused on the volcanic carapace of arcs because of the limited exposures of intrusive rocks in active arc settings. As a result, our understanding of how mantle-derived magmas evolve within island arcs is largely based on geochemical trends of arc volcanic suites. The accreted Talkeetna arc, however, exposes a nearly complete crustal section and provides an opportunity to compare both the plutonic and volcanic trends from a Jurassic intraoceanic arc. Our ongoing research on the Talkeetna arc includes U/Pb zircon dating and geochemical analyses that provide the basis for modeling magmatic differentiation within the arc. U/Pb zircon ages from the Chugach Mountains record active Talkeetna arc magmatism from 201-180 Ma, which is consonant with biochronology that constrains arc-related volcanism to Late Triassic to early Bajocian (206-169 Ma). Plutonic ages from the Alaska Peninsula (183-164 Ma) and the Talkeetna Mountains (177-156 Ma) suggest a northward shift in arc magmatism at ∼180 Ma, with continued plutonism into the Late Jurassic. Radiogenic isotope data from the Chugach Mountains record limited isotopic variation of ⁸⁷Sr/⁸⁶Sr_initial = 0.7032-0.7037 and ¹⁴³Nd/¹⁴⁴Nd_initial = 0.512707-0.512743. The restricted range in the isotopic ratios is similar to primitive modern arcs such as the Izu-Bonin-Mariana arc and demonstrates the juvenile nature of the main section of Talkeetna plutonic rocks. Isotopic ratios from the Talkeetna Mountains define a wider range of ⁸⁷Sr/⁸⁶Sr_initial = 0.7035-0.7053 and ¹⁴³Nd/¹⁴⁴Nd_initial = 0.512627-0.512735. The geographic distribution of the data suggests that the more evolved ratios may reflect assimilation of an older crustal component. The primitive isotopic ratios within the Talkeetna arc are consistent with an oceanic origin, while the slightly enriched isotopic ratios in the Talkeetna Mountains may reflect either a collisional event or a pre-existing crustal boundary that was intruded by arc magmas. The rare exposures in the Talkeetna arc provide direct constraints on models of magmatic differentiation. Greene et al. (2002) used least-squares calculations to reproduce Talkeetna arc lava compositions with 48-57 wt% SiO₂ by fractional crystallization of mineral compositions found in arc gabbronorites. We will use similar least-squares models to determine whether more evolved lava and plutonic compositions (57-77 wt% SiO₂) can also be explained by simple crystal fractionation of observed cumulate phases, or whether these compositions require multistage differentiation.