40Ar/39Ar ages and bulk-rock chemistry of the lower submarine units of the central and western Aleutian Arc

Rachel Bezard*, Kaj Hoernle, Jörg A. Pfänder, Brian Jicha, Reinhard Werner, Folkmar Hauff, Maxim Portnyagin, Blanka Sperner, Gene M. Yogodzinski, Simon Turner

*Corresponding author for this work

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    Abstract

    In order to further constrain the timing of the Aleutian Arc initiation as well as its early evolution, an extensive 40Ar/39Ar dating and geochemical (major and selected trace elements) campaign (40 samples) of the lower units of the Aleutian ridge has been carried out on samples dredged from deep fore-arc canyons and rear-arc tectonic structures. The new dataset slightly increases the minimum inception age for the Aleutian system, with the two oldest samples dated at 46.1 ± 3.3 Ma and 47.80 ± 0.57 Ma. Both mid Eocene ages were obtained on tholeiitic mafic volcanic rocks from the western section of the arc. The new data also support the occurrence of three distinct periods of enhanced magmatic activity (magmatic pulses) during the pre-Quaternary evolution of the arc (at 38–27, 16–11 and 6–0 Ma), as previously suggested based on a more limited and dominantly subaerial dataset. Moreover, the data refine the duration of the first pulse of activity, which ended 2 Ma later than previous estimates. The first and last pulses may be associated with rotations of the subducting plates while the second pulse might result from regional tectonic changes. The significant overlap between the age distribution of the submarine and subaerial samples suggests that much of the earlier parts of the arc may have been uplifted and subaerially exposed. The expected crustal growth associated with the pulses is unlikely to have significantly impacted magmatic residence times, since no variation in the degree of differentiation of the rocks can be observed during or after the pulses. On the other hand, the type of magmas erupted may have changed during the arc evolution. Prior to the first pulse, activity appears to have been dominantly tholeiitic. On the other hand, the first pulse was characterized by coeval tholeiitic, transitional and calc-alkaline magmas, with calc-alkaline activity increasing after the first ~3 Ma. Subsequently, a dominantly calc-alkaline period occurred from 29 to 8 Ma, followed by a progressive return of coeval tholeiitic, transitional and calc-alkaline activity. These temporal changes in magma types correspond to likely variations in arc crustal thickness beneath the active front, and could therefore be a response to physical changes of the overriding plate.

    Original languageEnglish
    Article number106147
    Pages (from-to)1-18
    Number of pages18
    JournalLithos
    Volume392-393
    DOIs
    Publication statusPublished - Jul 2021

    Keywords

    • Ar-Ar geochronology
    • Geochemistry
    • Subduction zones
    • Arc crust
    • Aleutian Arc
    • Pacific

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