Gambier Bay Formation, undivided

Unit symbol: Dgb
Age range Devonian (419.2 to 358.9 Ma)
Lithology: Metamorphic
Group name: Gambier Bay Formation, undivided
Medium-green actinolite schist, semischist, and dark-green garnet amphibolite that was first named by Loney (1964) on Admiralty Island. Unit also contains subordinate gray pelitic schist, calc-schist, chloritic quartzite, and felsic schist and marble. On Kupreanof Island, unit includes chlorite phyllite, schist, and semischist, graphitic schist, siliceous sericite schist, chloritic calc-schist, greenstone, marble, and meta-limestone. Subordinate light-tan to gray quartz-sericite semischist contains quartz porphyroblasts in a groundmass of feldspar, sericite, and pyrite and may have a quartz-porphyritic volcanic protolith. Greenstone semischist locally retains relict primary structures, including vuggy volcaniclastic textures and marble lenses and clasts. Metamorphic mineral assemblages include chlorite-epidote-calcite, quartz-muscovite-chlorite-albite, quartz-talc-calcite, and quartz-albite-muscovite-chlorite, which indicate greenschist-grade metamorphism (S.M. Karl, unpub. data). Unit thickness is unknown. On Kupreanof Island, Karl and others, (1999) report brachiopods from the late Emsian and Eifelian (latest Early to Middle Devonian), conodonts from the late Emsian to Late Devonian, and corals from the Silurian to Middle Devonian; together, the fossils have been interpreted to indicate a unit age of Middle Devonian. Unit yields Permian metamorphic ages (S.M. Karl, unpub. data). Locally, marble separated as map unit Dgbm

Source map information

Source map Karl, S.M., and Brew, D.A., 2002, Unpublished data.
Symbol Dvs
Unit name Greenstone of Gambier Bay Formation
Description Greenstone semischist, with subordinate greenschist, talc schist, marble, sericite quartz semischist, and gray sericitic phyllite. Medium-green actinolite schist. Greenstone semischist retains relict primary structures, including vuggy volcaniclastic textures with marble lenses and clasts. Greenstones with igneous protoliths have relict hornblende and augite, and clusters of euhedral plagioclase. Immobile trace element chemistry plotted on the diagrams of Wood (1981) and Pearce (1982) indicate the metabasites have calc-alkaline and MORB compositions, and rare earth element/chondrite ratios range from depleted to slightly enriched at 100 times chondrite suggesting magmatic sources are arc-related (Karl, unpublished data). Metamorphic mineral assemblages include chlorite-epidote-calcite, quartz-muscovite-chlorite-albite, quartz-talc-calcite, quartz-albite-muscovite-chlorite. Tremolite, actinolite, and sodic amphibole have also been identified in some places. Unit also includes subordinate light tan to gray quartz-sericite semischist with quartz porphyroblasts and feldspar and sericite microlites and accessory pyrite. Discontinuous quartz segregations to 15 cm in thickness. Locally intercalated with up to 30% argillaceous marble and minor metachert. Locally contains gray pelitic schist and intervals of felsic schist to 3 m thick. Isoclinally folded. In Chilkat Mountains and Chilkat Lake area, includes amphibolite, greenschist, and greenstone. On Admiralty Island, includes subordinate black phyllite, graphitic metasiltstone, siliceous marble, metafelsite, and metavolcanic breccia. On Kupreanof Island, includes chlorite phyllite, schist, and semischist, graphitic schist, siliceous sericite schist, chloritic calc-schist, greenstone, marble, and meta-limestone. Pervasive isoclinally folded fabric; commonly crenulated, locally rodded.
Lithology Metamorphic

Correlated geologic units

Label Dgg
Description Gambier Bay Formation, pyrite-bearing garnet-chlorite-amphibole-quartz-feldspar schist and gneiss
Geologic age Devonian
Geologic setting Metamorphic, undivided
Lithology Form Importance
Gneiss < Metamorphic Major
Quartz-feldspar-schist < Schist < Metamorphic Major