Granitic gneiss

Unit symbol: Dogn
Age range Late and Middle Devonian (393.2 to 358.9 Ma)
Lithology: Metamorphic
Group name: Granitic rocks and orthogneiss
Generally consists of “metamorphosed intrusive rocks that are predominantly muscovite-biotite granite ranging in composition from alkali-feldspar granite to tonalite” (Nelson and Grybeck, 1980). Primarily exposed in the Survey Pass and Chandalar quadrangles, but other small exposures of this unit occur on the Seward Peninsula and in the Wiseman, Ambler River, Shungnak, and Baird Mountains quadrangles. In the Survey Pass quadrangle, unit is coarse-grained augen gneiss and granite gneiss that occur locally within Arrigetch Peaks and Mount Igikpak plutons (Nelson and Grybeck, 1980). Brosgé and Reiser (1964) and Dillon and others (1996) described the unit in the Chandalar quadrangle as generally gneissic chloritized biotite granite, quartz monzonite, and granodiorite that locally includes chloritized hornblende granite and granodiorite, all of which are white to tan or cream-colored. On the Seward Peninsula, unit consists of texturally homogeneous, light-brownish-gray, light-orange to gray, fine-grained, biotite-plagioclase-quartz gneiss and granitic orthogneiss. The foliation in the gneiss is defined by aligned and segregated muscovite and minor biotite (Till and others, 2011). Variously mapped as Cretaceous, Mesozoic, or Paleozoic, U/Pb dating has shown these bodies to be Late and Middle Devonian intrusions that have undergone a Cretaceous resetting and therefore yield Cretaceous K/Ar and 40Ar/39Ar ages. Early Rb/Sr analysis yielded an age of 373±25 Ma (Silberman and others, 1979; Nelson and Grybeck, 1980). Dillon and others (1979, 1980) reported the first modern U/Pb ages (there were earlier, discredited, Pb-alpha ages in the Ambler River and Shungnak quadrangles; see Mayfield and others, 1983) that indicate a Late Devonian age (365±15 Ma) for the Arrigetch Peaks and Mount Igikpak plutons. Subsequently, Aleinikoff and others (1993) determined additional U/Pb ages ranging from 395 to 388 Ma in the Chandalar quadrangle, and Till and others (2008a) reported a U/Pb age of 386±1 Ma in the Shungnak quadrangle. These ages overlap the age range of augen gneiss and orthogneiss in the Yukon-Tanana Upland and Ruby terrane (unit MDag). Till and others (2011) repot a U/Pb age of 390±3 Ma age for orthogneiss on the Seward Peninsula

Source map information

Source map Till, A.B., Dumoulin, J.A., Harris, A.G., Moore, T.E., Bleick, Heather, and Siwiec, Benjamin, 2008, Bedrock geologic map of the southern Brooks Range, Alaska, and accompanying conodont data: U.S. Geological Survey Open-File Report 2008-1149, 88 p., 2 sheets, scale 1:500,000.
Symbol Dg
Unit name Granitic orthogneiss
Description Tan to gray weathering, equigranular to porphyroblastic, fine to coarse grained metagranitic bodies ranging in size from less than a kilometer to over 20 kms across, found in both the Schist and Central belts. Generally granitic in composition and made up of quartz, K-feldspar, albite, muscovite, and biotite. In the Chandalar quadrangle, the Horace Mountain pluton is dioritic to granodioritic in composition, and contains hornblende. Chlorite, sericite, Fe-Ti oxides, epidote minerals, and calcite are also present in many of the plutons. Remnants of intrusive contact relations are locally preserved. Exposures of skarn have been mapped at the contacts and within the orthogneiss bodies. Skarns at the contacts of some of the plutons are mineralized (Sn or Cu).
Lithology Metamorphic

Correlated geologic units

Label MDyao
Description Medium-grained granitic gneiss, Devonian intrusion age
Geologic age Givetian to Late-Devonian
Geologic setting Intrusive
Lithology Form Importance
Orthogneiss < Metaigneous < Metamorphic Amphibolite, epidote-amphibolite Major
Orthogneiss < Metaigneous < Metamorphic Amphibolite Indeterminate, major