Gabbro and diorite of southeast Alaska

Unit symbol: Kgb
Age range Cretaceous (145 to 66 Ma)
Lithology: Igneous - Plutonic
Group name: Mafic and ultramafic rocks in southern Alaska
Primarily gabbro, hornblende gabbro, hornblendite, leucogabbro, and subordinate norite, syenite, and pyroxenite on Prince of Wales Island (Eberlein and others, 1983), Kuiu Island (Brew and others, 1984), and Chichagof Island (also described as the layered gabbro near the head of Tenakee Inlet by Loney and others[1975]), where it is associated with Early Cretaceous granodiorite and diorite (units Keg and KJse) (Loney and others, 1975; Johnson and Karl, 1985). Also includes hornblende diorite and gabbro in the Haines area that grades laterally into granodiorite and quartz diorite. Potassium-argon ages are Late Cretaceous (Redman and others, 1984). Hornblende and biotite diorite of the Jualin Diorite (Knopf, 1911) along east shore of Lynn Canal in the northern Juneau quadrangle has produced a hornfels aureole in adjacent metasedimentary and metavolcanic rocks. Gehrels (2000) reported a U/Pb zircon age of 105 Ma on this pluton. At an unusual occurrence in the Craig quadrangle, epidote replaces the gabbro in irregular masses up to several meters in diameter or as veinlets cutting the rock (Herreid and others, 1978). In thin section, the replacement origin of the epidote is clearly shown by gradational embayed contacts with gabbro. The gabbro, which is a small border phase of Cretaceous granodiorite, contains medium-sized subhedral crystals of sericitized labradorite and subordinate hornblende, pyroxene, ilmenite, sphene, and apatite (Herreid and others, 1978)

Source map information

Source map Redman, E.C., Retherford, R.M., and Hickok, B.D., 1984, Geology and geochemistry of the Skagway B-2 quadrangle, southeastern Alaska: Alaska Division of Geological and Geophysical Surveys Report of Investigations 84-31, 34 p., 4 sheets, scale 1:40,000.
Symbol Kkhd
Unit name Hornblende diorite
Description Hornblende diorite, subhedral, seriate, medium-grained, color index 15-20. Inhomogeneous texture results form variations in color index. Orbicular texture at peak 4412, zoned orbicules 10-40 cm long. Hornblende is dominant mafic mineral, with rare augite, some hornblende is cored by augite. Hornblende is lineated, trending 140 degrees, plunging moderately NW or SE. Quartz is 5-10%, 5% K feldspar, but locally as much as 30% K feldspar. Accessory sphene, magnetite, epidote.
Lithology Igneous

Correlated geologic units

Label Kdqd
Description Mafic plutonic rocks, mostly diorite and quartz diorite with subordinate tonalite and gabbro 85-110 Ma. Southeast Alaska
Geologic age Aptian to Santonian
Geologic setting Intrusive, tonalite-granodiorite
Lithology Form Importance
Diorite < Dioritic < Plutonic < Igneous Major
Quartz-diorite < Dioritic < Plutonic < Igneous Major
Tonalite < Granitic < Plutonic < Igneous Minor
Gabbro < Gabbroic < Plutonic < Igneous Incidental