Growth and Evolution of the Accreted Talkeetna Arc, South-Central Alaska: Solutions to the "Arc Paradox"

The accreted Talkeetna arc, exposed in the Chugach Mountains of south-central Alaska, represents a cross section from the Moho to surficial volcanic deposits of a Jurassic intra-oceanic arc. We are using SIMS and partial dissolution-TIMS U/Pb zircon analyses of plutonic rocks in the Talkeetna arc to study how island arcs grow, evolve, and morph into continental crust. Our partial dissolution analyses (PDA), following the chemical abrasion technique developed by Mattinson (2000, 2001a, 2001b), used high-T (800°C) annealing of radiation damage, followed by successive acid digestions of grain populations, to generate reproducible high precision ages. Low-T (160-170°C) "clean-up" digestions typically showed signs of Pb loss, consistent with removal of discordant zircon, whereas high-T (>170°C) and residue steps gave concordant age plateaus. Repeat analyses of samples showed similar dissolution patterns and ages agreed within two sigma errors. The SIMS and TIMS U/Pb zircon analyses from the Talkeetna arc yielded ages of 184.3 +/- 0.4 Ma, 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, indicating that the arc was active from ~184-199 Ma. Previous ⁴⁰Ar/³⁹Ar hornblende cooling ages for the arc range from ~175-182 Ma, suggesting a total arc lifespan of ~20 My. This is consonant with fossil ages within arc volcanic rocks that range from early Sinemurian to upper Toarcian (~180-202 Ma) and biochronology that bounds arc growth between Late Triassic and early Bajocian (~169-206 Ma). Intermediate to felsic plutonic rocks in the Talkeetna Mountains intrude the volcanic carapace of the arc, but were not previously considered to be part of the Talkeetna arc. Our new SIMS and TIMS ages of 163.9 +/- 3.6 Ma, 170.0 +/- 4.2 Ma, 171.3 +/- 5.1 Ma, 175.6 +/- 0.4 Ma, 180.8 +/- 2.7 Ma, and 183.8 +/- 2.1 Ma, overlap the ages reported above for the Chugach Mountains, suggesting that portions of the intermediate plutonism could be related to the Talkeetna arc. Our new age data from the Talkeetna and Chugach Mountains may explain the apparent paradox of intermediate continental crust being produced by the accretion of mafic intra-oceanic arcs. The lifespan of the Talkeetna arc, combined with Raleigh-Taylor instability modeling by Jull and Kelemen (2001) and thermobarometry on lower crustal garnet gabbronorites (Mehl et al., 2001), suggests that the lower crust of the Talkeetna arc could have become convectively unstable, and sunk into the asthenosphere prior to accretion. The removal of mafic lower crust would drive the bulk arc composition toward more continental values. Additionally, previously unrecognized intermediate arc plutonic rocks in the Talkeetna Mountains would lead to a more intermediate composition for the Talkeetna arc and also help solve the arc paradox.