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Rioux, Matthew, 2006

The growth and differentiation of arc crust: Temporal and geochemical evolution of the accreted Talkeetna arc, south-central Alaska

Bibliographic Reference

Rioux, Matthew, 2006, The growth and differentiation of arc crust: Temporal and geochemical evolution of the accreted Talkeetna arc, south-central Alaska: University of California, Santa Barbara, Ph.D. dissertation, 121 p.


The accreted Talkeetna arc, south-central Alaska, is an archetypal example of an intra-oceanic arc crustal section. Arc-related units include all levels of a lithospheric column and provide a rare opportunity to directly study arc magmatic processes at depth. New U-Pb zircon ages and whole-rock geochemical analyses elucidate the tectonic development of the Talkeetna arc and provide insight into the role of evolved magmas in intra-oceanic settings. Plutons from the Chugach Mountains record U-Pb zircon ages of 201.5 - 181.4 Ma and have a restricted range of initial isotopic ratios that are similar to modern intra-oceanic arcs. This initial arc plutonism was followed by a northward shift in the magmatic axis at ~180 Ma. Plutons from the E. Talkeetna Mountains yield U-Pb zircon ages of 177.4 - 168.9 Ma and are isotopically similar to the Chugach Mountains intrusions. However, plutons from the W. Talkeetna Mountains batholith have more evolved isotopic ratios and contain inherited Paleozoic zircons. These data are interpreted to represent assimilation of adjacent Wrangellia crust into the arc magmas. As a whole, the zircon and radiogenic isotope data indicate that the main volume of Talkeetna arc magmas formed with little or no involvement of pre-existing crustal material. These observations justify the use of the Talkeetna arc as a type section for intrusive intra-oceanic arc crust. Seismic profiles of several modern intra-oceanic arcs suggest that intermediate to felsic plutonic rocks may constitute a significant volume of primitive arcs. Evolved plutonic rocks in the Talkeetna arc range from 54 - 75 wt.% SiO2. Geochemical modeling using an iterative residual minimization technique and MELTS calculations demonstrate that the felsic compositions in the Talkeetna arc can be generated by closed-system fractional crystallization. The predicted phase proportions of the cumulate residua are consistent with the observed modal mineralogy of arc gabbronorites. The identification of evolved plutonic rocks in intra-oceanic arcs indicates that arcs have the potential to generate crust with major-element abundances (excluding K) that are similar to average continental crust. However, distinct differences in the K2O content and La(N)/Yb(N) ratios between evolved Talkeetna plutons and average continental material indicates that arc crust must undergo additional differentiation to form average continental crust.

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