PROTEROZOIC ROCK UNITS IN THE SOUTHERN BROOKS RANGE, ARCTIC ALASKA, TIES TO BALTICA?

The Brooks Range of Arctic Alaska is a fold-thrust belt related to the closure of the Angayucham ocean and the emplacement of a Jurassic arc onto the passive Arctic continental margin during Brookian orogenesis. Several deformational events related to contraction and subsequent extension produced blueschist to greenschist facies mineral assemblages in the Central Belt and Schist Belt to the south in the metamorphic core of the range. Synthesis of existing data in combination with new mapping and U-Pb detrital zircon (DZ) geochronology of units in the John River drainage (Wiseman quad) have the ages and provenance of pre-Mississippian basement within the metamorphic core. Five DZ age-signatures are currently recognized that represent unique depositional environments and/or protoliths of metasedimentary units. Preliminary results, from north to south, indicate that Hunt Fork Shale of the upper John River has a mid-late Devonian “Ambler-age” magmatism signature with maximum depositional ages (MDAs) between ~380-360 Ma. Middle Devonian-Ordovician signatures comprise an inferred rift basin sequence of metasedimentary strata of the Beaucoup Formation, which structurally underlies the Hunt Fork Shale. The Beaucoup Formation overlies a north-dipping shear zone, and the underlying package of higher-grade marbles, metasandstone, and schists also yield Ambler-age signatures. Neoproterozoic-Mesoproterozoic age rocks underlie a broad region along the Central Belt and Schist Belt boundary (MDAs between 1000-600 Ma) and are intruded by leucogranite along 60 Mile Creek dated as 642±2 Ma. The southern part of the Schist Belt and fault bound Phyllite Belt to the south contains Hunt Fork Shale-aged and younger(?) protolith equivalents with MDAs ranging from 364-334 Ma. Definition of these DZ spectra that are linked to distinct stratigraphic packages allow initial mapping of units in the metamorphic core of the Brooks Range. In addition, Mesoproterozoic and Neoproterozoic strata yield DZ signatures that link them to similar rock packages elsewhere in the Brooks Range and Seward Peninsula, and collectively they display a strong Baltica provenance signature.