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foliated tonalite (unit TKts) and is intruded by leucocratic granodiorite (unit Tcp) (Berg and others, 1988; S.M. Karl, unpublished data).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 Brew, D.A., Himmelberg, G.R., and Ford, A.B., 2009, Geologic map of the Atlin quadrangle, southeastern Alaska: U.S. Geological Survey Scientific Investigations Map 2929, 1 sheet, scale 1:250,000.
Symbol Test
Unit name Intrusive rocks of Skagway, sphene-biotite-hornblende tonalite and granodiorite
Description Locally foliated, medium- to coarse-grained, gray weathering sphene-biotite-hornblende tonalite and granodiorite, C.I. 10-30. Rarely K-feldspar porphyritic.
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