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Source map Brew, D.A., and Karl, S.M., 2002, Unpublished data.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 TKtf
Unit name Tonalite sill
Description Foliated hornblende-biotite. In Skagway area, foliated hornblende-biotite granodiorite and tonalite, with related hornblende diorite to biotite quartz monzonite. Medium-grained, subhedral seriate, color index 15-20, biotite usually more abundant than hornblende. Quartz 15-20%, K feldspar 5-15%. Magnetite is common, locally common sphene and zircon. Correlated with the tonalitic plutons that form thick sills extending for at least 800 km along the west margin of the Coast Mountains plutonic metamorphic complex. In Juneau area, homogeneous, well foliated, non-layered; locally lineated; medium to coarse grained; color index averages 25, range 12 to 40; gray fresh, weathers darker gray; locally hornblende porphyritic with phenocrysts up to 2 cm; some bodies have distinctive skeletal garnet; inclusions and schlieren of dioritic composition common; gneiss inclusions occur locally; petrographic features include equigranular to seriate texture. In Tracy Arm, biotite-hornblende and hornblende-biotite tonalite, quartz diorite, and minor granodiorite. In the Petersburg area, the tonalite is medium gray, medium-grained, biotite-hornblende tonalite. CI 16-40. Homogeneous, foliated, non-layered; medium- to coarse-grained; gray fresh, weathers darker gray; locally hornblende porphyritic with Seriate, commonly lineated, phenocrysts up to 2 cm; inclusions and schlieren of dioritic composition common; gneiss inclusions occur locally. Syndeformational magmatic foliation. In the Ketchikan area, about 60 percent of the unit is undifferentiated bodies of dark gray weathering, uniformly foliated to gneissic quartz diorite; the rest consists of paragneiss (21 percent), granodiorite (11 percent), and subordinate migmatite, amphibolite, pegmatite, and quartz monzonite. The quartz diorite commonly is at least weakly foliated. Thin sections of typical samples of quartz diorite show medium grained granular to planar aggregates of andesine (54 percent), quartz (18 percent), hornblende (13 percent), biotite (12 percent), and accessory apatite, augite, garnet, magnetite, muscovite, potassium feldspar, and sphene. Mapped with Quattoon pluton in British Columbia, and foliated tonalitic bodies forming the eastern margin of the Coast batholith in southeast Alaska . Color Index 20-30
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