Preliminary bedrock geologic map of a portion of the Fortymile mining district, Alaska

Mesozoic thermal history and timing of structural events for the Yukon-Tanana Upland, east-central Alaska: 40Ar/39Ar data from metamorphic and plutonic rocks

“…3), indicating a potentially rapid progression of igneous crystallization and cooling from -700 to -450°C. The local development of weakly to moderately well-developed fabrics within parts of the 186-Ma plutons (Szumigala et al 2000b) is also consistent with a late-to posttectonic origin. Regional uplift related to thrusting also would explain the coincidence of -188-186 Ma 40 Ar/ 39 Ar cooling ages in plutonic and metamorphic rocks throughout much of the southeastern Eagle quadrangle (Fig.…”

“…These older ages were determined using the laser heating method as opposed to the furnace heating method used on our sample 1. Sample 2, a Hbl monzonite from the Napoleon Creek pluton (Szumigala et al 2000b), yields a well-defined Hbl plateau age of 186.6 ± 1.0 ( Fig. 5C).…”

“…In the eastern Yukon-Tanana Upland (Fig. 3), the Nisutlin assemblage is structurally overlain by Paleozoic amphibolite-facies rocks, including Grt amphibolite, Bt ± Hbl ± Grt gneiss and schist, marble, quartzite, metachert, pelitic schist (Foster 1976;Hansen et al 1991), and small bodies of tonalitic orthogneiss (Szumigala et al 2000b;Day et al 2000) and augen gneiss with pink K-feldspar augen. These amphibolite-facies rocks have been referred to previously as the Y4 subterrane of the Yukon-Tanana terrane (Foster et al 1994), the Teslin-Taylor Mountain terrane (a term proposed by Hansen (1990) to genetically link the rocks in the Taylor Mountain area with similar rocks in the Teslin suture zone in southern Yukon Territory; Fig.…”

“…This observation suggests that the plutons postdate both the first (northeast-vergent) deformation and the second (northwest-vergent), and more widespread, deformation recorded in Fortymile River assemblage tectonites. Contacts between these plutons and the adjacent metamorphic wall rocks are generally not exposed and, in many places, steep faults have been mapped between exposures of weakly foliated plutonic rocks and penetratively deformed metamorphic rocks (Szumigala et al 2000b; J.M. Proffett, unpublished report for Kennecott Exploration 1999), precluding structural comparisons between wall rock and pluton fabrics.…”