Foliated tonalite sill of Coast plutonic complex of Brew and Morrell (1979b)

Unit symbol: TKts
Age range Tertiary, Paleocene, and latest Cretaceous (72.1 to 56 Ma)
Lithology: Igneous - Plutonic
Group name: Foliated tonalite sill and pegmatite, Coast plutonic complex of Brew and Morrell (1979b)
Homogeneous, well foliated, non-layered; locally lineated; medium- to coarse-grained, hornblende-dominant, biotite-bearing tonalite and subordinate quartz diorite found as steeply dipping, foliated, and locally lineated sills along the west side of the Coast plutonic complex of Brew and Morrell (1979b). Commonly referred to as “The Great Tonalite Sill” (see, for example, Brew and Grybeck, 1984 or Brew and Friedman, 2002). Gray on fresh surfaces and weathering darker gray, it has an average color index of 25, has equigranular to seriate texture, and locally contains hornblende phenocrysts up to 2 cm in length; some bodies have distinctive skeletal garnet. Inclusions and schlieren of dioritic composition are common; gneiss inclusions occur locally (Brew and Ford, 1985). Associated migmatite consists of intimately intermixed paragneiss and orthogneiss and has widespread lit-par-lit injection gneiss (Redman and others, 1984). Paleosomes include amphibolite, metamorphic grade hornblende and biotite schist and gneiss, calc-silicate gneiss, and granodioritic to dioritic meta-intrusive rocks (Karl and others, 1999). Field and U/Pb zircon data indicate emplacement in Late Cretaceous and Paleocene time, during waning stages of deformation and metamorphism in Coast Mountains (Gehrels and others, 1991; Brew and Ford, 1985). Radiometric ages vary; K/Ar ages on biotite and hornblende tend to be early Eocene and Paleocene and significantly discordant, whereas U/Pb analyses (Barker and others, 1986; Berg and others, 1988; Saleeby, 2000; Rubin and Saleeby, 2000), largely multigrain analyses, yielded Paleocene and latest Cretaceous discordant ages

Source map information

Source map Karl, S.M., Haeussler, P.J., and McCafferty, Anne, 1999, Reconnaissance geologic map of the Duncan Canal-Zarembo Island area, southeast Alaska: U.S. Geological Survey Open File Report 99-168, 1 sheet, scale 1:125,000.
Symbol Tto
Unit name Tonalite
Description Medium gray, medium-grained, biotite-hornblende tonalite of the Great Tonalite Sill. Seriate, commonly lineated hornblende up to 2 cm long. CI 16-40. Syndeformational magmatic foliation.
Lithology Igneous

Correlated geologic units

Label TKtf
Description Foliated tonalite sill of Coast Plutonic Complex
Geologic age Maastrichtian to Paleocene
Geologic setting Intrusive
Lithology Form Importance
Quartz-diorite < Dioritic < Plutonic < Igneous Dike or sill Major
Tonalite < Granitic < Plutonic < Igneous Dike or sill Major