Geologic Map of Alaska: Younger phase, Coast plutonic complex of Brew and Morrell (1979b)

Younger phase, Coast plutonic complex of Brew and Morrell (1979b)

Unit symbol:	Tcp
Age range	Tertiary, Eocene (56 to 33.9 Ma)
Lithology:	Igneous - Plutonic
Group name:	Younger phase, Coast plutonic complex of Brew and Morrell (1979b)

Consists of biotite-dominant, hornblende- and sphene-bearing granodiorite and subordinate quartz monzonite, quartz diorite, and leucogranite found in Coast Mountains of southeast Alaska (Berg and others, 1978, 1988; Elliott and Koch, 1981; Brew and others, 1984; Webster, 1984; Brew and Grybeck, 1984; Brew and Ford, 1977, 1985; Souther and others, 1979; Redman and others, 1984; Barker and others, 1986, Gehrels and Berg, 1992, Karl and others, 1999, Brew and Friedman, 2002). These plutons are found in a belt adjacent to the international border along the east side of southeast Alaska. Associated is migmatite that consists of schist, gneiss, tonalite, and granodiorite that is invaded by the plutonic rocks of this unit. The migmatite includes stockwork agmatite and banded hornblende gneiss and biotite-hornblende gneiss, amphibolite, biotite quartz schist, and some calc-silicate rocks (Brew and Grybeck, 1984, Koch and Berg, 1996). Radiometric ages vary; K/Ar ages on biotite and hornblende yield both concordant and discordant Eocene ages, whereas U/Pb analyses (Barker and others, 1986; Alldrick and others, 1987; Berg and others, 1988) yield Eocene ages. Age determinations primarily reflect timing of intrusions, the original country rock many have been significantly older

Source map information

Source map	Karl, S.M., 2012, Unpublished data.
Symbol	Tegd
Unit name	Granite and granodiorite
Description	Sphene-hornblende-biotite granodiorite and granite. Color index 5-20 Coast Mountains In Atlin area, includes fine- to medium-grained brownish gray weathering, garnet-muscovite-biotite granite, CI about 5 of the White Pass plutonic suite. Locally K-feldspar porphyritic, massive to weakly foliated. In Juneau area, homogeneous, nonfoliated to poorly foliated; medium to coarse grained; color index 6 to 25; light gray to buff fresh, weathers darker gray; euhedral sphene crystals (to 4 mm) common; biotite locally dominant over hornblende; varies to quartz monzonite; minor biotite alaskite and magnetite; petrographic features include slightly inequigranular, hypidiomorphic granular texture; zoned (An40-An25) subhedral plagioclase. In Ketchikan area, Consists mainly of massive to weakly foliated granodiorite and quartz monzonite; the map unit also encompasses outcrops of paragneiss, dikes, and other intrusive rocks too small to show on the map. Typical specimens are medium grained and hypidiomorphic granular, and their average mineral composition is plagioclase (45 percent), potassium feldspar (23 percent), quartz (22 percent), biotite (6 percent), and hornblende (4 percent); the most prominent accessory mineral is sphene, which occurs as golden yellow euhedra as large as 5 mm
Lithology	Igneous

Correlated geologic units

Label	Tegd
Description	Coast Plutonic Complex
Geologic age	Selandian to Eocene
Geologic setting	Intrusive

Lithology	Form	Importance
Granite < Granitic < Plutonic < Igneous	Pluton	Major
Granodiorite < Granitic < Plutonic < Igneous	Pluton	Major
Tonalite < Granitic < Plutonic < Igneous	Pluton	Minor