40Ar/39Ar data from the eastern Moran area, Tanana B-6 and C-6 quadrangles, and the Ruby mining district, Ruby B-5 and B-6 quadrangles, Alaska
Description
This report presents 40Ar/39Ar step-heating geochronology results for igneous and metamorphic rocks from the eastern Moran area. Field samples were collected by the DGGS Mineral Resources section during detailed geologic mapping campaigns in 2011. The data provided in this report add significant detail to the thermal history of the Moran area. These new data indicate that the minimum age of prograde metamorphism of Ruby terrane rocks ranges from 148.5 +/- 1.7 to 140.4 +/- 1.7 Ma, and retrograde greenschist metamorphism is 122.6 +/- 2.3 Ma. The retrograde metamorphism is roughly coeval with the age of fabric development parallel to the Kaltag fault (128.3 +/- 1.7) and Tozitna thrust/detachment fault (123.2 +/- 1.5 Ma). The new data also indicate that the Melozitna pluton is composite, with a biotite cooling age of 116.5 +/- 1.3 from coarse-grained granite, while cooling ages for dikes cutting the granite range from 110.1 +/- 1.3 to 102.8 +/- 1.2 Ma. The age of mineralized veins in the area are variable and include 119.0 +/- 1.3 Ma galena veins in the Tozimoran drainage and an interpreted age of 66.5 +/- 2.6 for an auriferous vein from the Monday Creek area, which is synchronous with ages of biotite samples from granite and schist from the Ruby Mining district. The complete report and digital data are available through the DGGS website: http://doi.org/10.14509/30117.