Mesozoic thermal evolution of the Yukon‐Tanana Composite Terrane: New evidence from ⁴⁰Ar/³⁹Ar data

The Yukon‐Tanana composite terrane (YT) in the northern North American Cordillera records a complex polyphase history. Twenty‐three new ⁴⁰Ar/³⁹Ar dates from YT tectonites of Yukon and eastern Alaska form the basis for subdivision of the YT into different cooling packages. These data, together with structural and kinematic interpretations presented elsewhere, enable us to define four spatially, kinematically, and temporally distinct, yet overlapping, deformation events. The Teslin‐Taylor Mountain and Nisutlin terranes are distinguished herein on the basis of cooling history, from the orthogneiss assemblage. Divisions of the YT into these terranes is consistent with division based on structural, kinematic, and metamorphic criteria. Metamorphic minerals of the Teslin‐Taylor Mountain and Nisutlin terranes give Early Jurassic cooling dates. Hornblende and white mica from high‐P/T Teslin‐Taylor Mountain and Nisutlin tectonites in Yukon yield isochron dates of ∼195 Ma; similarly, two Teslin‐Taylor Mountain hornblende‐biotite (hb‐bi) mineral pairs from eastern Alaska yield isochron dates of 188 (hb) and 186 (bi), and 187 (hb) and 185 Ma (bi), respectively. Near‐coincident mineral pair dates suggest cooling rates of ∼100°C/Ma through 500°–300°C from both regions. Rocks of the orthogneiss assemblage (here Cassiar terrane) record Late Jurassic to dominantly mid‐Cretaceous cooling ages. Orthogneiss in southern Yukon yields discordant ⁴⁰Ar/³⁹Ar mineral pair dates indicative of slow cooling (∼5°C/Ma), from Late Jurassic to mid‐Cretaceous time (hb ∼147 Ma; muscovite ∼117 Ma; bi ∼110 Ma). Low‐angle, east‐vergent ductile shear deformation predated cooling of orthogneiss assemblage rocks in this region. In easternmost Alaska, two white mica‐biotite mineral pairs recovered from the orthogneiss assemblage yield concordant ⁴⁰Ar/³⁹Ar plateau dates of ∼109 Ma. These dates, taken together with an Early Cretaceous metamorphic hornblende cooling date reported from the same region by others, indicate that YT orthogneiss assemblage tectonites in eastern Alaska cooled during the Early Cretaceous at a rate of ∼20°C/Ma through 500°–300°C. We interpret that these differences in cooling dates and cooling rates determined for the Teslin‐Taylor Mountain and orthogneiss terranes of the YT reflect important differences in the structural and tectonic evolution of these two terranes. We propose a model in which the Teslin‐Taylor Mountain, Nisutlin, and Slide Mountain terranes comprise the leading edge of the upper plate of a convergent margin which overrode North American parautochthonous strata in early Jurassic time. While this upper plate package was uplifted, the rocks cooled through at least 300°C; as a result of overthrusting, lower plate parautochthonous strata were tectonically buried and metamorphosed. Lower plate rocks cooled through 300°C, only after overthrusting, as a result of erosion in southern Yukon, and as a result of top‐to‐the‐northwest and top‐to‐the‐southeast directed crustal extension in eastern Alaska. Through this model we suggest that the orthogneiss assemblage of the YT represents parautochthonous North American strata overridden by the easternmost accreted terranes. High‐angle, northwest striking dextral shear zones cut all YT packages and Cassiar parautochthonous shelf strata in southern Yukon in Late Cretaceous time. Muscovite and biotite from tectonites of one such ductile shear zone, the d'Abbadie fault, yield concordant cooling dates of ∼97 Ma. Other subparallel shear zones cut, or are cut by, northwest striking Late Cretaceous plutons.