40AR/39AR DATA FROM THE TALKEETNA MOUNTAINS C-4 QUADRANGLE AND ADJOINING AREAS, CENTRAL ALASKA.

This report presents 40AR/39AR step-heating geochronology results for igneous and metamorphic rocks from the Alaska Division of Geological & Geophysical Surveys' (DGGS) Talkeetna Mountains C-4 Quadrangle and Adjoining Areas geologic mapping project. The U.S. Geological Survey (USGS) National Cooperative Geologic Mapping Program partially funded work by DGGS to map approximately 450 mi2 of the Talkeetna Mountains region of central Alaska at 1:50,000 scale. This area contains significant exposures of Late Triassic mafic volcanics and gabbro sills that have been the focus of region-wide exploration for the Strategic and Critical platinum-group elements (PGEs). The area also exposes numerous inactive and possibly active faults that project through the area of the proposed Susitna-Watana hydroelectric dam and reservoir. Because this area was poorly mapped at a reconnaissance scale of 1:250,000, a team of eight DGGS geologists spent 42 days in the field to produce a new geologic map and a greatly improved understanding of the geology, structural history, and mineral potential of the area. Hornblende-bearing gabbro returned a Late Triassic crystallization age consistent with regional results for Nikolai Greenstone-related magmatism. We determined Early Jurassic metamorphic ages for hornblende and sericite from two samples of upper greenschist to amphibolite grade metavolcanic rocks. Our results indicate that a granodiorite pluton intruding Wrangellia in the study area has a Middle Jurassic age consistent with the Talkeetna Arc, indicating that the Wrangellia and Peninsular terranes were joined at that time. Cretaceous plutons and porphyry intrusions in the map area can be divided into two types: reduced granitic to felsic porphyry intrusions with latest Cretaceous ages, and an earlier, oxidized type with early Late Cretaceous ages. This observation corresponds to similar patterns in magmatism in the western Alaska Range, and may indicate geological potential for similar style intrusive-related gold, copper, and molybdenum mineralization in the study area. Finally, we obtained Eocene ages for andesite and rhyolite; these ages constrain the timing of displacement on some of the faults in the map area. [ABSTRACT FROM AUTHOR]