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Cover. DGGS Geologist Mandy Willingham on an outcrop of Fortymile River metasediments near the Lake George-Fortymile detachment (photograph taken 07/04/2018 by K. R. Sicard).

This report presents 40 Ar/ 39 Ar step-heating geochronology results for igneous and metamorphic rocks from the Alaska Division of Geological & Geophysical Surveys' (DGGS) geologic mapping project in the Northeast Tanacross map area in the Tanacross D-1 and parts of the D-2, C-1, and C-2 quadrangles, Alaska. Field samples were collected by the DGGS Mineral Resources section during detailed geologic mapping campaigns in 2017 and 2018. The Northeast Tanacross map area lies within the Yukon-Tanana Upland and covers the boundary between the Fortymile River and Lake George assemblages (Dusel-Bacon and others, 2006). The map area encompasses documented porphyry Cu-Mo-Au deposits including Taurus (Harrington, 2010), Fishhook (also known as SW Pika), and Pika Canyon (U.S. Geological Survey, 2008), and is adjacent to the Fortymile Mining District (Yeend, 1996). Previously published geologic mapping includes a reconnaissance 1:250,000scale map (Foster, 1970) and a slightly modified map published by the USGS (Wilson and others, 2015). DGGS published 1:63,360-scale geologic maps and a report for the area (Wypych and others, 2019; Wypych and others, in press; Naibert and others, in press), which contribute to the understanding of the metamorphic and igneous map units in the area, including delineating the boundary between parautochthonous North America (Lake George assemblage) and the allochthonous Yukon-Tanana terrane (Fortymile River assemblage) in multiple places. The age data in this report constrain the crystallization ages of igneous rocks and the cooling histories of metamorphic rocks in the map area and will be used to improve upon recent mapping and further refine the uplift history of eastern Interior Alaska. DOCUMENTATION OF METHODS Sample Collection Fresh, unweathered samples from surface outcrops were collected by DGGS field geologists and were selected for dating based on the presence of sufficiently large crystals appropriate for dating. Sample location coordinates (in WGS84 datum) were obtained using GPS-enabled field tablets, with a typical reported accuracy of about 10 meters. Samples were examined under a binocular microscope and in thin section to select unaltered mineral phases before sample preparation.

The Late Cretaceous (ca. 75 Ma) Philipsburg Batholith in SW Montana provides an excellent setting to examine the emplacement mechanisms of large-volume silicic plutons in the Sevier fold-and-thrust belt of the western U.S. Cordillera. Magnetic fabrics, determined using the anisotropy of magnetic susceptibility from 119 sites and anisotropy of anhysteretic remanent magnetization data from a subset of these sites, suggest that the Philipsburg Batholith is a tabular body and that its emplacement involved subhorizontal magma fl ow controlled by local thrust faults, including the Georgetown-Princeton thrust, which served as conduits for magma ascent. Field relations and magnetic fabric data, which are typically well characterized at the site level and dominantly subhorizontal except near major thrust faults, suggest that the Philipsburg Batholith was emplaced within a fault-bend fold at the top of a ramp in the Georgetown-Princeton thrust system. New 40 Ar/ 39 Ar age spectrum data indicate that the entire Philipsburg Batholith was emplaced rapidly at ca. 74.8 ± 0.1 Ma. Paleomagnetic directional data indicate that the Philipsburg Batholith has experienced ~9°-16° ± 8° of west-side-down tilt (tilt estimate and associated error are based on a new approach described herein) since emplacement in the Late Cretaceous. The data do not support any internal deformation of the batholith since emplacement and suggest that Sevier-age thin-skinned deformation in the Flint Creek Range had ceased by ca. 75 Ma. The modest magnitude of observed tilt could have occurred during subsequent Cenozoic extension. The inferred mode of emplacement of the Philipsburg Batholith may typify how large volumes of silicic magma are emplaced in shallow, fold-and-thrust belt settings.