The Mystic subterrane (partly) demystified: New data from the Farewell terrane and adjacent rocks, interior Alaska

Abstract

The youngest part of the Farewell terrane in interior Alaska (USA) is the enigmatic Devonian-Cretaceous Mystic subterrane. New U-Pb detrital zircon, fossil, geochemical, neodymium isotopic, and petrographic data illuminate the origin of the rocks of this subterrane. The Devonian-Permian Sheep Creek Formation yielded youngest detrital zircons of Devonian age, major detrital zircon age probability peaks between ca. 460 and 405 Ma, and overall age spectra like those from the underlying Dillinger subterrane. Samples are sandstones rich in sedimentary lithic clasts, and differ from approximately coeval strata to the east that have abundant volcanic lithic clasts and late Paleozoic detrital zircons. The Permian Mount Dall conglomerate has mainly carbonate and chert clasts and yielded youngest detrital zircons of latest Pennsylvanian age. Permian quartz-carbonate sandstone in the northern Farewell terrane yielded abundant middle to late Permian detrital zircons. Late Triassic-Early Jurassic mafic igneous rocks occur in the central and eastern Mystic subterrane. New whole-rock geochemical and isotopic data indicate that magmas were rift related and derived from subcontinental mantle. Triassic and Jurassic strata have detrital zircon age spectra much like those of the Sheep Creek Formation, with major age populations between ca. 430 and 410 Ma. These rocks include conglomerate with clasts of carbonate ± chert and youngest detrital zircons of Late Triassic age and quartz-carbonate sandstone with youngest detrital zircons of Early Jurassic age. Lithofacies indicating highly productive oceanographic conditions (upwelling?) bracket the main part of the Mystic succession: Upper Devonian bedded barite and phosphatic Upper Devonian and Lower Jurassic rocks. The youngest part of the Mystic subterrane consists of Lower Cretaceous (Valanginian-Aptian) limestone, calcareous sandstone, and related strata. These rocks are partly coeval with the oldest parts of the Kahiltna assemblage, an overlap succession exposed along the southern margin of the Farewell terrane. Our findings support previous models suggesting that the Farewell terrane was proximal to the Alexander-Wrangellia-Peninsular composite terrane during the late Paleozoic, and further suggest that such proximity continued into (or recurred during) the Late Triassic-Early Jurassic. But middle to late Permian detrital zircons in northern Farewell require another source; the Yukon-Tanana terrane is one possibility.

Changing exhumation patterns during Cenozoic growth and glaciation of the Alaska Range: Insights from detrital thermochronology and geochronology

“…The detrital U/Pb ages from modern rivers match bedrock igneous ages and suggest that the vast majority of zircon and apatite in modern rivers are derived from this plutonic belt. We directly compare our modern river ages with bedrock ages for a portion of the western Alaska Range (Figure a) where a large number of ZUPb ages have been determined from igneous bedrock [ Jones et al , ] and detrital bedrock [ Box et al , ; Karl et al , ; Dumoulin et al , ]. Igneous bedrock ages are distinct from detrital bedrock ages in all cases except for 90–110 Ma ages, which are present in both.…”

Detrital zircon geochronology along a structural transect across the Kahiltna assemblage in the western Alaska Range: Implications for emplacement of the Alexander-Wrangellia-Peninsular terrane against North America

“…The northwestern area of the cross section (Figs. 2 and 3) exposes deformed strata of the Dillinger and Mystic subterranes of the Farewell terrane (Dumoulin et al, 2018a(Dumoulin et al, , 2018b. These thin-bedded carbonates and siliciclastic turbidite strata are cut by top-to-the-northwest thrust faults and have been deformed into northwest-vergent overturned folds (Bundtzen et al, 1997; see subsequent section on "Deformation of the Kahiltna assemblage").…”

“…macrofossil occurrences (Csejtey et al, 1992;Bundtzen et al, 1997;Reed and Nelson, 1980). However, Dumoulin et al (2018b) suggested that some, and perhaps all, of the Valanginian and Hauterivian-Barremian fossil localities along or near the northwestern margin of the northwestern succession are part of the underlying Mystic subterrane of the Farewell terrane, and not the Kahiltna assemblage.…”

“…The assemblage is bounded to the southeast by Triassic and older rocks of the Wrangellia terrane and Triassic and Jurassic rocks of the Peninsular terrane (Jones et al, 1977;Nokleberg et al, 1994;Detterman et al, 1996;Rioux et al, 2007;Greene et al, 2010;Beranek et al, 2014). Exposures on the northwest side of the belt for much of its extent are Paleozoic and Mesozoic carbonate and clastic strata of the Farewell terrane (Dumoulin et al, 2018a(Dumoulin et al, , 2018b. At its northeastern end, Kahiltna assemblage occurs on both sides of the Denali fault and is bounded to the north and northwest by poorly understood rocks of the McKinley-Windy terrane.…”

Neoproterozoic–early Paleozoic provenance evolution of sedimentary rocks in and adjacent to the Farewell terrane (interior Alaska)

“…Instead, shallow-water carbonate rocks and limestone turbidites make up most of the Devonian section in the Farewell terrane, along with subordinate heterolithic sandstones. Upper Devonian strata in the Mystic subterrane have features indicating a high-productivity setting, such as phosphatic nodules and sedimentary barite deposits (Reed and Nelson, 1980;Bundtzen and Gilbert, 1991;Dumoulin et al, 2014b); these distinctive deposits have no counterparts in the Alexander terrane or Arctic Alaska.…”