GROWTH AND EVOLUTION OF THE ACCRETED TALKEETNA ARC, SOUTH-CENTRAL ALASKA: SOLUTIONS TO THE "ARC PARADOX"

The volcanic Talkeetna Formation, the Border Ranges ultramafic-mafic complex, and associated intermediate plutonic rocks exposed in the Chugach Mountains of south-central Alaska, represent a cross section through the accreted Jurassic Talkeetna arc and provide a rare opportunity to study how island arcs grow, evolve, and morph into continental crust.

Our new TIMS and SIMS U/Pb zircon ages of 184.3 ± 0.4, 185.0 ± 0.4 Ma, 186.6 ± 0.4 Ma, 192.4 ± 2.9 Ma, 193.2 ± 0.4 Ma, and 198.6 ± 0.4 Ma, indicate that the Talkeetna arc was active from ~184-199 Ma. This is consonant with biochronology that constrains arc growth between the Late Triassic and early Bajocian (206-169 Ma), and yields arc growth rates of 40-60 km³ km^-1 Ma^-1, similar to recent estimates for the active Aleutian arc (Holbrook et al., 1999). The lifespan of the Talkeetna arc, combined with thermobarometry of the arc lower crust by Mehl et al. (2001) and Raleigh-Taylor instability modeling by Jull and Kelemen (2001), suggests that the lower crust of the Talkeetna arc could have become convectively unstable prior to accretion and sunk into the asthenosphere. The removal of mafic lower crust would drive the bulk arc composition toward more continental values, and may explain the paradox between the compositions of intermediate continental crust and more mafic modern arcs.

Intermediate to felsic plutonic rocks in the Talkeetna Mountains intrude the volcanic Talkeetna Formation, but were not previously considered to be part of the Talkeetna arc. However, our new TIMS and SIMS U/Pb zircon ages of 175.6 ± 0.4 Ma, 180.8 ± 2.4 Ma, and 183.8 ± 2.1 Ma, overlap those reported above for the Chugach Mountains, indicating that the Talkeetna arc continues north into the Talkeetna Mountains. Inclusion of these intermediate to felsic plutons will lead to a more intermediate calculated bulk arc composition and also help solve the arc paradox.