167-1 Late Triassic Initiation of the Talkeetna Arc along the Wrangellia Composite Terrane Margin, Kodiak Archipelago and Alaska Peninsula, Alaska

Session: Integrating Geochronology and Geochemistry to Decipher the Tectonic Evolution of Orogenic Belts

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Abstract:

The Late Triassic-Jurassic Talkeetna Arc is interpreted as an oceanic arc and defines the Peninsular terrane of the Wrangellia composite terrane. Despite comprising a significant component of the northern North American Cordillera, the basement of the Peninsular terrane and the initiation of subduction for this system remain understudied. Our study investigates Permo-Triassic unconformity development involving the Peninsular terrane basement that preceded initiation of the Talkeetna Arc. We also examine Late Triassic mafic volcanism that we relate to initiation of the Talkeetna Arc system. On the Alaska Peninsula, a major unconformity represents a depositional hiatus of up to 40 million years. This unconformity separates a sequence of likely Pennsylvanian-Permian volcaniclastic strata capped by middle Permian limestone from the Norian-Rhaetian Kamishak Formation. The Kamishak Formation is a mixed carbonate and volcaniclastic system that was deposited during the early stages of Talkeetna Arc activity. Development of this unconformity on the Peninsular terrane was synchronous with a depositional hiatus across the Wrangellia and Alexander terranes, suggesting that all three terranes occupied the same plate margin by the Permian.

Unconformity development was followed by subsidence and eruption of 3-4 kilometers of pillow basalts, the Lower Shuyak Formation of the Kodiak Archipelago. Our chemostratigraphic analysis shows significant depletion of an initially enriched mantle source followed by a transition to island arc tholeiites (IATs) at ~232 Ma (middle Carnian) based on new U-Pb zircon geochronology (CA-ID-TIMS). Lavas classified as IATs are enriched in Th relative to depleted Nb, Ta, and LREEs. We interpret the transition to depleted IATs to represent fluid-fluxed melting of a depleted mantle source following subduction initiation. Calc-alkaline plutons of the Afognak batholith intrude the Lower Shuyak Formation no later than ~212 Ma, marking the establishment of the arc axis. In our interpretation, the Afognak batholith was built on oceanic crust formed in a supra-subduction zone position during and shortly after subduction initiation. The sequence of Permo-Triassic regional shortening followed by subsidence and mafic volcanism is consistent with a forced subduction initiation mechanism for the Late Triassic initiation of the Talkeetna Arc.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

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