Biotite gneiss, marble, schist, quartzite, and amphibolite

Unit symbol: PzPxybg
Age range Devonian or older (635 to 358.9 Ma)
Lithology: Metamorphic
Group name: Gneiss, amphibolite, schist, quartzite, and marble (Yukon-Tanana crystalline complex)
Primarily exposed in the southeastern Eagle and northeastern Tanacross quadrangles, this unit is considered part of the Fortymile River assemblage (Werdon and others, 2001; Szumigala and others, 2002; Dusel-Bacon and others, 2006). Dominantly a metasedimentary unit, according to Foster (1992), the most common rock type is quartz-biotite gneiss. The unit also includes a component of amphibolite they interpreted as metamorphosed mafic volcanic rocks. Important metasedimentary components include quartzite, paragneiss, biotite and muscovite schist, and marble. As shown here, map unit includes the PzpCb of Foster (1976) and the Pzgn and Pzbg units of Foster (1992). Foster had included abundant orthogneiss in these units; more detailed mapping by Werdon and others (2001) and Szumigala and others (2002) separated the orthogneiss, which is here included in unit MDag. Werdon and others (2001) and Szumigala and others (2002) described these rocks as the amphibolite-facies units of the Fortymile River assemblage and subdivided them on lithologic grounds. Among their units is a mixed unit, (pMa of Werdon and others (2001) and Szumigala and others (2002)), that is dominantly amphibolite. Dusel-Bacon and Cooper (1999) reported that this amphibolite has arc chemistry. Rocks of this map unit that have metasedimentary origin contain small amounts of staurolite or kyanite (Werdon and others, 2001; Szumigala and others, 2002). Werdon and others (2001) describe a coarse-grained, mostly light-gray marble that Foster (1992) reports is that is abundant and found in layers, lenses, and pods, including some large masses 1 km2 or more in area. The rocks were regionally metamorphosed to amphibolite facies under moderate- to high-pressure conditions (Dusel-Bacon and others, 1995). Locally, retrograde contact metamorphic effects are superimposed on the regional metamorphism (Foster, 1992). The ages of the protoliths are unknown, but a few poorly preserved echinoderm fragments (Foster, 1976) suggests that they are Paleozoic in age. U/Pb zircon dating of the associated metaigneous rocks yields Mississippian and Devonian ages, providing a minimum age for the metasedimentary rocks (Dusel-Bacon and others, 2006). The major regional metamorphism is believed to have occurred in Late Triassic to Early Jurassic time on the basis of incremental heating experiments (Cushing and others, 1984a, b; Foster and others, 1987; Hansen and others, 1991; Dusel-Bacon and others, 2002)

Source map information

Source map Day, W.C., O’Neill, J.M., Dusel-Bacon, Cynthia, Aleinikoff, J.N., Siron, C.R., 2014, Geologic map of the Kechumstuk fault zone in the Mount Veta area, Fortymile mining district, east-central Alaska (ver. 1.1, March 12, 2014): U.S. Geological Survey Scientific Investigations Map 3291, scale 1:63,360,
Symbol Pzq
Unit name Quartzite-rich metasediments, undivided
Description Predominantly light gray, fine-grained, laminated micaceous quartzite with lesser amounts of dark green micaceous layers of metamorphosed shale, amphibolite, micaceous schist, metagraywacke, and minor metapelite. Interlayered with white to light-gray marble (Pzm). Section exposed on western part of map area progresses up structural section (west to east) and seemingly forms an upward coarsening sequence from micaceous schist and metagraywacke up through dark green phyllite, argillite, light gray to white, medium-grained platy, and laminated quartzite; to light gray cherty marble, laminated, white to light gray fine-grained quartzite; to light gray thinly bedded marble; to tan to light brown fine-grained, platy micaceous quartzite interlayered with massive marble. Metamorphic grade is uppermost greenschist facies to lower amphibolite facies. Protolith was sequence of sandstone with variable clay content, mudstone, graywacke, locally mafic (tuff?) interlayers, and limestone horizons in a relatively shallow, near-shore environment
Lithology Metamorphic

Correlated geologic units

Label PzZybg
Description Biotite paragneiss, marble, schist, quartzite, and amphibolite, eastern Yukon-Tanana
Geologic age Paleozoic to Devonian
Geologic setting Metamorphic, undivided
Lithology Form Importance
Biotite-gneiss < Gneiss < Metamorphic Major
Marble < Metacarbonate < Metasedimentary < Metamorphic Major
Quartzite < Metaclastic < Metasedimentary < Metamorphic Minor
Orthogneiss < Gneiss < Metamorphic Incidental