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 Szumigala, D.J., Newberry, R.J., Werdon, M.B., Athey, J.E., Stevens, D.S.P., Flynn, R.L., Clautice, K.H., and Craw, P.A., 2002, Geologic map of the Eagle A-1 quadrangle, Fortymile Mining District, Alaska: Alaska Division of Geological and Geophysical Surveys Preliminary Interpretive Report 2002-1a, 1 sheet, scale 1:63,360.
Symbol pMsg
Unit name ROCKS METAMORPHOSED TO AMPHIBOLITE FACIES: FORTYMILE RIVER ASSEMBLAGE - SCHIST AND PARAGNEISS
Description Mixed unit characterized by abundant aluminous metasedimentary rocks. Small-scale isoclinal folds are locally visible, suggesting that the entire unit is isoclinally folded. Primarily interlayered, medium- to coarse-grained, schist (commonly crenulated and folded) and paragneiss, with minor local biotite quartzite, marble, and fine-grained plagioclase-rich (metavolcanic?) gneiss. The schist contains a minimum of 30 percent mica and typically contains 1–10 percent rotated, poikiloblastic garnet, 10–15 percent biotite, 15–75 percent muscovite, 2–20 percent plagioclase, 20–50 percent quartz, and 0–5 percent K-feldspar. Garnets are commonly 1–5 mm, other minerals typically 1–2 mm in size. Kyanite (0.2–0.5 mm, to 5 percent), staurolite (0.3–1 mm, to 5 percent), and tourmaline (0.1 mm, to 1 percent) are variably present, especially in the northwestern part of the Eagle A-1 Quadrangle. Staurolite, kyanite, and tourmaline are more abundant than shown on the geologic map, as they are virtually never identifiable in hand specimen. The common occurrence of staurolite with kyanite and tourmaline suggests that their occurrence is caused by aluminum–boron-rich bulk composition and not variations in metamorphic grade within the Fortymile River Assemblage. The paragneiss contains less than 30 percent micas and typically 40–60 percent quartz, 5–20 percent biotite, 1–20 percent muscovite, 10–25 percent plagioclase and 0–2 percent garnet, all with average grain sizes of 0.5–1 mm. The plagioclase-rich gneiss contains mineral abundances similar to that of the tonalite gneiss (Motn), but grain sizes of 0.20.5 mm suggest it may represent local (andesitic?) metavolcanic layers within a largely metasedimentary sequence. Magnetic susceptibilities for rocks of this unit are highly variable, with values between 0 and 20 x 10-3 SI, although low values (< 0.3 x 10-3 SI) are more common
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