Augen gneiss and orthogneiss

Unit symbol: MDag
Age range Early Mississippian and Late Devonian (372.2 to 346.7 Ma)
Lithology: Metamorphic
Group name: Granitic rocks and orthogneiss
Peraluminous granitic gneiss that contains augen of potassium feldspar generally interpreted as a blastoporphyritic texture. These metaigneous bodies are exposed in the Tanacross, Eagle, Big Delta, Circle, Mount Hayes, Tanana, Melozitna, and Chandalar quadrangles and range from weakly to strongly foliated quartzofeldspathic orthogneiss. Augen gneiss in the Big Delta quadrangle has yielded a 341±3 Ma U/Pb (TIMS) and 371±3 Ma (SHRIMP) age; the SHRIMP age is interpreted as a crystallization age (Dusel-Bacon and others, 2004). Similar augen gneiss in the other quadrangles yield U/Pb SHRIMP ages between about 370 and 332.6±5.7 Ma (Aleinikoff and others, 1986; Dusel-Bacon and Aleinikoff, 1996; Newberry and others, 1998a; Dusel-Bacon and others, 2004; Day and others, 2014). Although the body in the Tanana quadrangle yielded an age that was interpreted as 390±25 Ma (Patton and others, 1987)— significantly older than other dated bodies in this unit and similar to the age of orthogneiss in the Brooks Range (ranges from 395 to 365 Ma; unit Dogn)—this age was interpreted on the basis of the upper intercept of a concordia plot (Patton and others, 1987) where the discordia cord was nearly parallel with the concordia; the large uncertainty is indicative of this poor fit. Unit also includes augen gneiss associated with the West Point complex of Smith and others (1994) and foliated, muscovite-biotite granitic orthogneiss bodies within the West Point complex. The West Point complex of Smith and others (1994) is exposed in the northeastern part of the Big Delta quadrangle and consists of upper amphibolite facies metamorphic rocks that were intruded by abundant pre- and post-metamorphic, felsic to intermediate igneous rocks. Smith and others (1994) reported a TIMS U/Pb age of 671±34 Ma for the orthogneiss, but a U/Pb SHRIMP analysis reported by Dusel-Bacon and others (2003a) reported an average age of 113±2 Ma, from 7 of the most concordant rim analyses, a subset of 16 rim samples analyzed from a total of 33 zircon grains. Both authors interpreted their ages as emplacement ages, but the majority of the zircons reported by Dusel-Bacon and others (2003a) had Devonian-age cores, which leads us to suggest that this was likely a Devonian pluton metamorphosed in the Cretaceous; nearby augen gneiss has yielded a U/Pb age of 355±4 Ma (Dusel-Bacon and others, 2006, interpreted as an intrusion age. Boundaries of the West Point complex are gradational (Smith and others, 1994). The gneiss bodies in the Melozitna and Chandalar quadrangles are as yet undated. Although the Chandalar body included here is near the Brooks Range, its position south of the Kobuk Fault System leads us to assign it to this unit, whereas all the metaigneous bodies north of the Kobuk Fault System are assigned to map unit Dogn. Many of these augen gneiss and orthogneiss bodies yield Cretaceous cooling ages, typically between 115 and 105 Ma, but some are as young as 90 Ma. Rb-Sr biotite, K-feldspar, plagioclase, and whole-rock isochron ages of about 110 Ma on some of these rocks were interpreted by Wilson and others (1985) and Nokleberg and others (1992a, b) as the age of metamorphism. Emplacement age determinations on the orthogneiss bodies of this map unit overlap ages determined on orthogneiss in the Brooks Range (unit Dogn)

Source map information

Source map Werdon, M.B., Newberry, R.J., Szumigala, D.J., and Pinney, D.S., 2001, Geologic map of the Eagle A-2 quadrangle, Fortymile mining district, Alaska: Alaska Division of Geological and Geophysical Surveys Preliminary Interpretive Report 2001-3A, 1 sheet, scale 1:63,360.
Symbol MDog
Description Medium- to coarse-grained, variably foliated, biotite–quartz–feldspar gneiss. Lacks K-feldspar porphyroclasts but possesses relict equigranular to slightly porphyritic igneous textures and granitic to granodioritic to trondhjemitic bulk composition and mineralogy. Typical modal mineralogy is 20–30 percent quartz, 0–20 percent K-feldspar, 30–80 percent plagioclase, 0–15 percent biotite (typically chloritized), and 0–6 percent white mica. Spatial association with augen gneiss (MDag) and igneous composition implies a plutonic origin for this rock type. Present as irregular bodies, generally <8 km in surface area; interlayering with surrounding metamorphic rocks suggests a sill morphology for most bodies. Locally cut by quartz iron carbonate veins with pyrite. Magnetic susceptibility generally low to moderate, 0.02–3 x 10x-3 SI. There are no published dates for this unit, but spatial association with augen gneiss (MDag) suggests a similar age.
Lithology Metamorphic

Correlated geologic units

Label MDyao
Description Augen gneiss and schist, Mississipian or Devonian intrusion age, Yukon-Tanana terrane
Geologic age Famennian to Tournaisian
Geologic setting Intrusive
Lithology Form Importance
Biotite-gneiss < Gneiss < Metamorphic Amphibolite Major
Orthogneiss < Metaigneous < Metamorphic Amphibolite Major