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 Koch, R.D., and Berg, H.C., 1996, Reconnaissance geologic map of the Bradfield Canal quadrangle, southeastern Alaska: U.S. Geological Survey Open-File Report 81-728A, scale 1:250,000.
Symbol MzPzpo
Unit name Paragneiss and orthogneiss
Description Paragneiss and orthogneiss - includes psammitic schist and gneiss, pelitic schist and gneiss, marble, amphibolite, and mafic schist.
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