| 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 |
