UNDERSTANDING ISLAND ARC THERMAL STRUCTURE THROUGH U-PB AND 40AR-39AR GEOCHRONOLOGY OF THE TALKEETNA ARC SECTION, SOUTH-CENTRAL ALASKA

Geologic evidence suggests that island arc accretion is one of the principle mechanisms of Phanerozoic continental growth. However, estimates of bulk arc compositions raise important questions regarding this model of crustal genesis. Island arcs are basaltic in composition whereas bulk continental crust is andesitic. If arcs form from basaltic mantle melts, why is the bulk continental crust andesitic?

Mineralogy, geochemistry, and field relations indicate that the Talkeetna arc section, South Central Alaska, represents a cross section of an accreted Jurassic island arc from the crust-mantle boundary (Moho) to surficial volcanic rocks. The arc sequence provides a rare opportunity to study island arc growth and evolution. This study focuses on the geochronology of the Talkeetna section.

Previous research on the Talkeetna arc has relied predominantly on hornblende and biotite K-Ar and ⁴⁰Ar/³⁹Ar geochronology. Over 60 hornblende and biotite analyses have yielded ages of 135–194 Ma, with >95% of the ages <190 Ma. Our new sensitive high-resolution ion microprobe (SHRIMP) zircon analyses of an arc-related granodiorite, hornblende gabbro and hornblende quartz diorite yield ages of 186 ± 3 Ma, 190.4 ± 2.3 Ma and 194.1 ± 1.9 Ma, respectively. These results are similar to the oldest K-Ar and ⁴⁰Ar/³⁹Ar ages, but 50 Myr older than the youngest K-Ar ages, suggesting that either i) the zircon samples are from rare, older arc rocks, ii) the arc underwent protracted cooling, or iii) the K-Ar and ⁴⁰Ar/³⁹Ar ages were affected by late reheating. The first explanation is unlikely as the granodiorite and hornblende diorite samples are either syn-plutonic with or intrude the older mafic section. Our ongoing U-Pb and ⁴⁰Ar/³⁹Ar analyses will differentiate between the second and third scenarios and provide constraints on the timing of arc magmatism and accretion and arc thermal structure. The thermal history of the lower to middle Talkeetna arc crust will lend insight into whether P-T conditions might have favored the phase transformations necessary to trigger lower crustal delamination, perhaps explaining the compositional paradox.