Porphyritic granodiorite phase of Coast plutonic complex of Brew and Morrell (1979b)

Unit symbol: Tcpp
Age range Tertiary, Eocene (56 to 33.9 Ma)
Lithology: Igneous - Plutonic
Group name: Porphyritic granodiorite phase of Coast plutonic complex of Brew and Morrell (1979b)
Consists mainly of weakly foliated to massive, medium-grained, hornblende-biotite granodiorite, and locally of foliated and gneissic granodiorite as well as distinctive porphyritic, coarse-grained, homogeneous, biotite-hornblende granodiorite that locally grades into quartz diorite and quartz monzonite; typically contains about 5 percent euhedral phenocrysts of potassium feldspar that are as large as 5 cm. Mafic minerals generally make up as much as 25 percent of this pluton. Intrudes foliated tonalite (unit TKts) and is intruded by leucocratic granodiorite (unit Tcp) (Berg and others, 1988; S.M. Karl, unpublished data). Includes associated pervasively intruded migmatite that consists of tonalitic to granodioritic gneiss and orthogneiss of diverse migmatitic fabrics; agmatitic metabasite and ultramafic rocks; quartzite; quartz-biotite schist and gneiss; and marble and irregularly banded calc-silicate rocks. The age of the migmatite primarily reflects timing of intrusions, and original country rock many have been significantly older. In the western part of the unit, in the Ketchikan and Prince Rupert quadrangles, part of this unit may more properly be associated with the tonalite sill (unit TKts). Radiometric ages vary; K/Ar ages on biotite and hornblende are early Eocene and commonly discordant (Smith and Diggles, 1981; Berg and others, 1988; Douglass and others, 1989), whereas U/Pb analyses (Berg and others, 1988, Gehrels and others, 1991), largely multigrain analyses, yielded Eocene and latest Cretaceous discordant ages

Source map information

Source map Karl, S.M, 2013, Unpublished data
Symbol Tgdp
Unit name K-feldspar phorphyritic granodiorite
Description Locally porphyritic; unfoliated to foliated; medium-grained granodiorite in the Coast Mountains. Color Index 4 to 25; gray to buff fresh, weathers to medium gray; with rare mafic inclusions; petrographic features include slightly inequigranular, hypidiomorphic granular texture; biotite more abundant than hornblende; as much as 20 percent euhedral to subhedral K-spar phenocrysts locally as long as 6 cm. Intrudes Coast plutonic-metamorphic complex rocks as sill-like bodies. In Atlin area, includes homogeneous, nonfoliated to poorly foliated; medium to coarse grained granodiorite of the Skagway-Sloko plutonic suite; 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 Skagway quadrangle, unit includes medium-grained, subhedral, seriate, granodiorite with hexagonal books of biotite, euhedral hornblende, and a color index of about 15. Quartz ranges from 15-20%, K-feldspar up to 10%. Intrudes Ferebee pluton and associated migmatite unit. In Petersburg quadrangle, gray to buff, foliated and banded, gneissic, fine- to medium-grained, biotite granodiorite and quartz monzonite; porphyritic, and characterized by K-feldspar phenocrysts to 3.5 cm. Locally contains garnet. Intrudes Tto; intruded by Tlgm-Tlg-Tgd-Tgdp. Also includes granodiorite that is foliated, generally leucocratic, locally porphyritic and banded; medium-grained; light gray fresh, weathers darker gray; with K-feldspar phenocrysts or porphyroblasts up to 3x5 cm, locally augen-like; locally containing inclusions of quartz and hornblende. Differs from other granodiorites (Tgdb and Tgdp) by gneissic structure. In Ketchikan quadrangle, consists mainly of weakly foliated to massive, medium-grained, hornblende biotite granodiorite, and locally of foliated and gneissic granodiorite. Mafic minerals generally make up 15 to 25 percent of this pluton. Hornblende is predominant in its southern part; biotite generally equals or locally exceeds hornblende in most of its northern part. Sparse euhedral phenocrysts of potassium feldspar to 2 cm. Intrudes foliated tonalite (TKft) and is intruded by leucocratic granodiorite (Tegd). Also includes Consists of porphyritic, coarse-grained, homogeneous, biotite hornblende granodiorite that locally grades into quartz diorite and quartz monzonite. It typically contains about 5 percent of euhedral phenocrysts of potassium feldspar as large as 5 cm. Approximate average composition of the pluton is plagioclase (50 percent), quartz (25 percent), potassium feldspar (15 percent), and mafic minerals (10 percent). Intrudes TKft; intruded by Tegd correlative bodies
Lithology Igneous

Correlated geologic units

Label Tgdp
Description Porphyritic granodiorite distinctive phase of Coast Plutonic Complex
Geologic age Late-Paleocene to Eocene
Geologic setting Intrusive
Lithology Form Importance
Granodiorite < Granitic < Plutonic < Igneous Pluton Major
Quartz-diorite < Dioritic < Plutonic < Igneous Pluton Minor
Granite < Granitic < Plutonic < Igneous Pluton Minor