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foliated tonalite (unit TKts) and is intruded by leucocratic granodiorite (unit Tcp) (Berg and others, 1988; S.M. Karl, unpublished data).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Source map Karl, S.M, 2013, Unpublished data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 MzPzog
Unit name Orthogneiss
Description Intermediate to mafic composition orthogneiss in Coast Mountains. Endicott Arm gneiss dome core rocks are homogeneous medium- to coarse-grained, poorly foliated biotite-quartz-feldspar gneiss. The core rocks are in general in sharp contact with the surrounding schist and darker gneiss, but, locally, screens or inclusions of schist occur near the margins parallel to the contact and to the foliation in the core. The gneiss domes are aligned north-northwest at a small angle to the dominant northwest strike of rock units and structures. Unit also includes rocks with magmatic epidote and/or garnet correlated with 90 Ma plutons in the Petersburg and Bradfield Canal quadrangles. In Ketchikan area, includes undifferentiated metamorphosed plutonic rocks that are intruded by foliated K-feldspar megacrystic granodiorite. Consists mostly of foliated and gneissic granodiorite, but also contains a wide variety of other rock types. Based on samples consists mainly of granodiorite (51 percent), quartz monzonite (14 percent), and quartz diorite (13 percent), but locally includes such nonintrusive components as hornfels, marble, paragneiss, and gneiss of undetermined origin (18 percent). Orthogneiss locally grades to paragneiss. Also includes aplite, diorite, granite, hornblende, lamprophyre, and one occurrence of pyroxenite. Unit may include tonalite of the western margin of the Coast Mountains batholith, as well as older plutons
Lithology Metamorphic

Correlated geologic units

Label Tcpm2
Description Another Coast Plutonic Complex migmatite
Geologic age Campanian to Eocene
Geologic setting Intrusive
Lithology Form Importance
Migmatite < Metamorphic Amphibolite Indeterminate, major