INVESTIGATING THE INITIATION OF THE TALKEETNA ARC THROUGH VOLCANIC AND SEDIMENTARY RECORDS OF THE SHUYAK FORMATION, KODIAK ARCHIPELAGO, ALASKA

Despite being an archetypal example of an oceanic arc crustal section, the pre- and syn- subduction initiation record of the latest Triassic to Jurassic Talkeetna Arc remains sparsely studied. The oldest known record of Talkeetna Arc magmatism is the Afognak batholith (ca. 218-203 Ma), a series of plutons exposed on the Kodiak Archipelago. Afognak batholith magmatism was coeval with, or closely followed, signs of tectonic reorganization including a regional Triassic unconformity and the development and subsequent drowning of a Norian carbonate platform on the Alaska Peninsula. The objective of this study is to document the Late Triassic volcanic and sedimentary records of the Kodiak Archipelago with a focus on the Upper Triassic Shuyak Formation to determine the timing of subduction initiation and the nature of the early arc’s basement. Here we present geochemical analyses of pillow basalt of the lower Shuyak Formation and U-Pb zircon ages for felsic tuffs in the context of measured stratigraphic sections in the upper Shuyak Formation.

The lower Shuyak Formation comprises at least 4 km of pillow basalt and pillow breccia intruded by tonalite of the Afognak batholith. One goal of this study is to determine if the lower Shuyak Formation represents pre-existing oceanic crust intruded by the early Talkeetna Arc, flood basalts possibly related to widespread Carnian-Norian basaltic volcanism associated with the Wrangellia composite terrane, or the upper part of a supra-subduction zone ophiolite that marks the initiation of the subduction zone that produced the Talkeetna Arc. The upper Shuyak Formation comprises at least 2 km of volcaniclastic strata interbedded with abundant felsic tuffs. These volcaniclastic strata represent reworked volcanic detritus deposited in basins adjacent to the nascent Talkeetna Arc. U-Pb zircon geochronology via chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS) is used to constrain the onset of magmatism and volcanism by dating tonalite from the Afognak batholith and felsic tuffs from the Shuyak Formation, respectively. The integration of stratigraphic and geochronological datasets provides new insights into the upper plate processes associated with the early history of an oceanic volcanic arc.