Roof pendants of the Coast plutonic complex of Brew and Morrell (1979b)

Unit symbol: Pzgn
Age range Paleozoic (541 to 252.2 Ma)
Lithology: Metamorphic
Group name: Metamorphic rocks associated with the Coast plutonic complex of Brew and Morrell (1979b)
Dominantly grayish-brown-weathering, well foliated, well layered, locally lineated, fine- to coarse-grained quartz-biotite-feldspar gneiss and lesser amounts of garnet-quartz-biotite-plagioclase schist, as well as greenish-gray or grayish-green-weathering, moderately to poorly foliated and layered, medium- to coarse-grained hornblende gneiss and subordinate hornblende and biotite schist; largely consists of roof pendants of or large xenoliths within the Coast plutonic complex of Brew and Morrell (1979b) in eastern southeast Alaska. Unit crops out as irregular and elongate masses within the Coast plutonic complex of Brew and Morrell (1979b) (Brew and Ford, 1985). In the Ketchikan quadrangle, unit is characterized by rusty-brown-weathering pelitic paragneiss and schist that has conspicuous root-beer-brown biotite and by subordinate quartzofeldspathic gneiss distinguished mainly by its light color and low content of mafic minerals. Unit also includes migmatite, gneissic plutonic rocks, marble and calcsilicate gneiss, pegmatite, quartzite, amphibolite, and aplite (Berg and others, 1988). Widely distributed throughout unit is light-gray-weathering marble and calcsilicate rock in lenses associated with minor muscovite-quartz gneiss and biotite gneiss (Brew and others, 2009). Marble is fine- to coarse-grained and locally occurs in marble several hundreds of meters thick, which may have been reefs and (or) may be large detached fold hinges (S.M. Karl, unpub. data). Marble also occurs as 1-cm- to 10-cm-scale layers intercalated with equal amounts of biotite schist (Brew and Grybeck, 1984; S.M. Karl, and D.A. Brew, unpub. data). In the Atlin quadrangle, well-foliated, homogeneous biotite-hornblende-plagioclase orthogneiss and heterogeneous migmatite are present in minor amounts (Brew and others, 2009). Metamorphic grade ranges from amphibolite to granulite facies (Gehrels and others, 1992). In the Sumdum quadrangle, quartzite layers adjacent to marble in western Tracy Arm contain detrital zircons that have a variety of Precambrian ages: 1.24 to 1.0 Ga and 1.68 Ga for multigrain fractions, and 1.40 to 1.37 Ga for a single grain (Gehrels and others, 1991). Quartzite in eastern Tracy Arm contains detrital zircons that yielded U/Pb zircon ages of 2.0–1.79 Ga, ~2.31 Ga, 2.75–2.53 Ga, and ~3.0 Ga (Gehrels and others, 1991). Rocks of this unit have been correlated with rocks of the Yukon-Tanana terrane, but the multigrain detrital zircon data are of limited value in this analysis and the lithologic correlation is equivocal

Source map information

Source map Brew, D.A., and Ford, A.B., 1985, Preliminary reconnaissance geologic map of the Juneau, Taku River, Atlin, and part of the Skagway 1:250,000 quadrangles, southeastern Alaska: U.S. Geological Survey Open-File Report 85-395, pamphlet 23 p., 2 sheets, scale 1:250,000.
Symbol Tkbg
Unit name Biotite gneiss
Description MAINLAND BELT: METAMORPHIC ROCKS OF COAST PLUTONIC-METAMORPHIC COMPLEX: Dominantly well foliated, well layered, locally lineated fine to coarse grained quartz biotite feldspar gneiss with lesser amounts of garnet quartz biotite plagioclase schist and still less hornblende plagioclase schist and gneiss; weathers grayish brown, gray fresh; probably derived from the same protoliths as the Phyllite (TKp) and Biotite Schist (TKbs) units; generally, but not exclusively, lies to the northeast of those units; exposed in isolated screens within the granitic rocks northeast of the Sill Belt.
Lithology Metamorphic

Correlated geologic units

Label Zta
Description Tracy Arm assemblage, undivided metamorphic rocks of southeast Alaska associated with the Coast Plutonic Complex. May include minor Mesozoic age rocks
Geologic age Proterozoic to Paleozoic
Geologic setting Metamorphic, undivided
Lithology Form Importance
Muscovite-schist < Mica-schist < Schist < Metamorphic Major
Biotite-schist < Biotite-gneiss < Gneiss < Metamorphic Indeterminate, major
Amphibole-schist < Schist < Metamorphic Indeterminate, major
Biotite-schist < Mica-schist < Schist < Metamorphic Indeterminate, major