Petrology and tectonic significance of augen gneiss from a belt of Mississippian granitoids in the Yukon-Tanana terrane, east-central Alaska

Metamorphic history of Alaska

“…However, limited evidence from U-Pb zircon data from quartzite others, 1984b, 1986) and sillimanite gneiss from unit 46 (Aleinikoff and others, 1984a), and from structural relations between augen gneiss and wallrocks in east-central Alaska and Yukon Territory, suggest that an earlier metamorphic episode may have accompanied the Mississippian intrusion of batholithic sheets of what is now augen gneiss (Dusel- Bacon and Aleinikoff, 1985). These structural relations are: (1) that some areas of augen gneiss coincide with large structural and metamorphic culminations (Mortensen, 1983); and (2) that the concordant contacts of the augen gneiss bodies, and parallelism between lithologic contacts and the gently dipping regional penetrative fabric, suggest the augen gneiss bodies were intruded synkinematically into ductile crust (Dusel- Bacon and Aleinikoff, 1985).…”

Geologic map of the Kechumstuk fault zone in the Mount Veta area, Fortymile mining district, east-central Alaska

“…Anastomosing mica-rich shear bands separate zones of both equigranular matrix and augen into sigmoid-shaped zones (mesolithons). Dusel- Bacon and Aleinikoff (1985) Biotite orthogneiss (Late Devonian)-Predominantly light to medium gray, medium-grained, layered trondhjemitic to granodioritic orthogneiss with lesser amounts of biotite schist, quartzite, and paragneiss. Undated in the map area, but similar rocks in the Big Delta quadrangle yielded Late Devonian (367 ± 7 Ma) U-Pb SHRIMP dates on zircon cores (Day and others, 2007).…”

Geologic studies in Alaska by the U.S. Geological Survey, 1991

“…2) consist of two similar amphibolite-facies subterranes of the Yukon-Tanana terrane. The Lake George subterrane is characterized by augen gneiss derived from dynamothermal metamorphism of a belt of Mississippian granitoids (Dusel- Bacon and Aleinikoff, 1985), as well as quartz-biotite schist and gneiss, pelitic schist, garnet amphibolite, and quartzite. A similar lithologic assemblage, albeit differing in the relative abundances of rock types (specifically, having very little augen gneiss and more quartzite) makes up the Chena River subterrane of the Yukon-Tanana terrane (terminology proposed in Nokleberg and others, 1989).…”