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Source map Brew, D.A., and Karl, S.M., 2002, Unpublished data.
Symbol TKtm
Unit name Migmatite associated with the tonalite sill
Description Migmatite associated with the tonalite sill. In Skagway area, complexly intermixed paragneiss, orthogneiss, and migmatite. Paragneiss consists of quartzofeldspathic gneiss, with marble, quartzite, amphibolite, and biotite schist. The orthogneiss resembles the Ferebee pluton except it has a stronger foliation. Migmatite consists of intimately intermixed paragneiss and orthogneiss, with widespread lit-par-lit injection gneiss. In Juneau area, Amphibolite facies (hornblende) biotite quartz feldspar schist and gneiss invaded and deformed by tonalite; schist and gneiss are fine to medium grained, locally include calc-silicate layers, and typically weather rusty; invader is generally the Biotite-Hornblende and Hornblende-Biotite Tonalite, etc. unit (TKto), characterized by its foliation and local aligned hornblende phenocrysts. Successively invaded by younger neosomes to the east and can be recognized as paleosomes in almost all migmatites of the Coast plutonic metamorphic complex. In Petersburg area, migmatite consisting of deformed, wavy-banded gneiss with raft structures and mafic inclusions, invaded by tonalitic neosomes, associated with Tto. Paleosomes include amphibolite, metamorphic grade hornblende and biotite schists and gneisses, calc-silicate gneiss, and granodioritic to dioritic meta-intrusive rocks.
Lithology Igneous

Correlated geologic units

Label TKtm
Description Tonalite migmatite of the tonalite sill of the Coast Plutonic Complex
Geologic age Maastrichtian to Paleocene
Geologic setting Intrusive
Lithology Form Importance
Migmatite < Metamorphic Amphibolite Major