Geologic Map of Alaska: Orthogneiss of the Seward Peninsula

Orthogneiss of the Seward Peninsula

Unit symbol:	Zgns
Age range	Neoproterozoic (850 to 541 Ma)
Lithology:	Metamorphic
Group name:	Gneiss of northern Alaska

Pale-gray-, tan- and orangish-tan-weathering outcrops and rubble fields of foliated and nonfoliated metagranitic syenogranite to monzogranite that form a large, tabular, concordant body within high-grade rocks (unit PzPxkg) of the Kigluaik Mountains on the Seward Peninsula (Till and others, 2011). Foliation is apparent because of local alignment of potassium feldspar, plagioclase, and biotite; quartz and biotite lenses or layers up to a centimeter thick also parallel foliation. Allanite is locally 1 to 2 percent of the rock and is zoned with brown cores and orange rims; accessory zircon is also present. Layers of quartz amphibolite 1–2 m thick parallel the metamorphic foliation in the orthogneiss. Amato and Miller (2004) show the orthogneiss on both the north and south flanks of the antiform in the core of the Kigluaik Mountains (their unit pCtog). Orthogneiss has yielded a TIMS U/Pb zircon protolith age of 555±15 Ma (Amato and Wright, 1998) and a SHRIMP age of 565±6 Ma (Amato, 2004)

Source map information

Source map	Till, A.B., Dumoulin, J.A., Werdon, M.B., and Bleick, H.A., 2010, Preliminary bedrock geologic map of the Seward Peninsula, Alaska, and accompanying conodont data: U.S. Geological Survey Open-File Report 2009-1254, 1 pamphlet, 57 p., 2 plates, scale 1:500,000, and database.
Symbol	Zo
Unit name	Orthogneiss, High-grade metamorphic and associated igneous rocks
Description	Pale gray-, tan- and orangish-tan-weathering outcrops and rubble fields of foliated and unfoliated metagranitic rocks that form a large, tabular, concordant body within the high-grade rocks of the Kigluaik Mountains. Called the "Thompson Creek orthogneiss" after exposures near the Grand Central River in the Nome D-1 quadrangle (Till, 1980), the rock is syenogranite to monzogranite in composition (Streikeisen, 1976). K-feldspar, plagioclase, and biotite are locally aligned in foliation; quartz and biotite lenses or layers up to a centimeter thick also parallel foliation. Allanite is locally 1-2% of the rock and is zoned with brown cores and orange rims; accessory zircon is also present. Quartz amphibolite layers that are parallel to metamorphic foliation in the orthogneiss occur in the exposures on Thompson Creek and are 1-2 meters thick. Amato and Miller (2004) show the Thompson Creek orthogneiss on both the north and south flank of the antiform in the core of the Kigluaiks (their unit "pCtog"). The orthogneiss has yielded U-Pb zircon ages of 555 ¦ 15 Ma (TIMS; Amato and Wright, 1998) and 565 ¦ 6 Ma (SHRIMP; Amato, 2004)
Lithology	Metamorphic

Correlated geologic units

Label	Zgns
Description	Orthogneiss of the Seward Peninsula
Geologic age	Neoproterozoic
Geologic setting	Metaigneous, high-grade

Lithology	Form	Importance
Biotite-gneiss < Gneiss < Metamorphic		Major
Orthogneiss < Metaigneous < Metamorphic		Major