NEW ESTIMATES FOR THE BULK COMPOSITION OF THE JURASSIC TALKEETNA ARC

The Jurassic Talkeetna arc section extends from mantle peridotite to volcanics, including a few 100 meters of pyroxenite, rare garnet granulites, cumulate gabbronorites, and felsic plutonic rocks. Nd isotopes and zircon systematics are similar to modern oceanic arcs, with little or no contamination from older crust.

New thermobarometry [1] indicates that there is a substantial gap in outcrop between tonalities and quartz diorites emplaced at 5 to 9 km and gabbronorites crystallized at 17 to 24 km. The missing mid-crustal section could be composed of gabbronorite, felsic plutonic rocks, or a mixture of the two.

Using average values for the the Talkeetna-Tonsina area [2], the most felsic bulk crust estimates (61 wt% SiO2) are derived from a 5 km thick volcanic section, an 11 km thick mafic plutonic section and a 19 km thick felsic mid-crustal layer. For most oxides, felsic estimates are similar to continental crust, though Na2O in Talkeetna is higher, and La/Yb and K2O are much lower. Intermediate compositions for the Talkeetna crust are derived if the volcanic section is thicker, and the mafic plutonic section is 18 km thick, leaving a felsic mid-crust 8 km thick. This yields an andesitic bulk arc composition at the mafic end of continental estimates. The most mafic compositions are derived if felsic plutonic rocks are only a few km thick, yielding a high aluminum, basaltic andesite bulk crust with 52.5 wt%, as in previous estimates [3,4].

None of these bulk compositions are possible liquids in equilibrium with mantle peridotite. Adding more gabbronorite brings them no closer. Adding 10 km of primitive Talkeetna pyroxenite yields possible boninite to basalt with 58-52 wt% SiO2 and an Mg# of 0.7. The most primitive lavas and dikes in the Talkeetna section include both andesites (Mg# 0.63) and basalts (Mg# 0.61).

[1] Hacker et al in prep [2] Kelemen et al, Treatise on Geochem 03 [3] Greene et al J Pet 06 [4] DeBari & Coleman JGR 89