Gneiss, schist, and quartzite

Unit symbol: PzPxygs
Age range Mississippian, Devonian, and older (635 to 358.9 Ma)
Lithology: Metamorphic
Group name: Gneiss, schist, and amphibolite (Yukon-Tanana crystalline complex)
Coarse- to fine-grained gneiss and quartzite; well-foliated and banded to massive; locally cataclastic; ranges from pelitic schist that contains abundant sillimanite to gneisses of probable igneous origin. All rocks are well foliated, and dominant foliation is folded. Protolith may include both Paleozoic and Precambrian sedimentary and igneous rocks (Weber and others, 1978). Unit is widespread throughout out the Yukon-Tanana Upland and the included meta-plutonic rocks may be equivalent to the Pelly Gneiss in Yukon, Canada. Also includes gray, medium-grained, mylonitic, quartzofeldspathic biotite-sillimanite gneiss that forms core of a gneiss dome in the central Big Delta quadrangle (Dusel-Bacon and Foster, 1983). Several U/Pb zircon ages have been determined on this unit, and Mississippian to Devonian crystallization ages are common for the metaigneous rocks within this unit and within unit |ymi (Aleinikoff and Nokleberg, 1985; Aleinikoff and others, 1986). Day and others (2003), Dusel-Bacon and Williams (2009), Aleinikoff and Nokleberg (1985) all indicate that inheritance of zircon grains is common. Detrital zircon analyses by Aleinikoff and others (1984, 1986) and Dusel-Bacon and Williams (2009) from quartzite in the unit yielded Proterozoic and Archean zircons

Source map information

Source map Foster, H.L., 1992, Geologic map of the eastern Yukon-Tanana region, Alaska: U.S. Geological Survey Open-File Report 92-313, 26 p., 1 plate, scale 1:500,000.
Symbol Pzgn
Unit name Gneiss and schist
Description Includes quartz-biotite +/- garnet gneiss, augen gneiss and other orthogneiss, pelitic gneiss, quartzite, amphibolite, schistose gneiss, and quartz-mica schist. Augen gneiss locally mapped separately as unit Pzgna or as overprint pattern. Sillimanite gneiss is also locally mapped separately as unit Pzgns. More schistose or quartzitic parts of the unit are mapped separately as unit Pzgnsq. Rocks of the unit as a whole are mostly of middle amphibolite facies
Lithology Metamorphic

Correlated geologic units

Label PzZypg
Description Gneiss, schist, and quartzite of the Yukon-Tanana crystalline complex; Pelly Gneiss in Yukon in part
Geologic age Ediacarian to Devonian
Geologic setting Metamorphic, undivided
Lithology Form Importance
Biotite-gneiss < Gneiss < Metamorphic Major
Quartzite < Metaclastic < Metasedimentary < Metamorphic Minor
Quartz-feldspar-schist < Schist < Metamorphic Minor
Marble < Metacarbonate < Metasedimentary < Metamorphic Incidental