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Source map Karl, S.M., and Brew, D.A., 2002, Unpublished data.
Symbol Kgg
Unit name Gabbro
Description Body on Lincoln Island described by Barker (1957) as: "Gabbro-- Sericitized hornblende uralite gabbro." On Admiralty Island, in Mole Harbor includes augite-hornblende gabbro. In Petersburg quadrangle, on Kupreanof, Zarembo, and Blashke Islands, includes massive, medium-grained, medium to dark gray gabbro. Grades from clinopyroxene gabbro at wherlite contact to hornblende gabbro at contact with country rock
Lithology Igneous

Correlated geologic units

Label Kmgb
Description Gabbro (not zoned ultramafic)
Geologic age Cretaceous
Geologic setting Intrusive
Lithology Form Importance
Gabbro < Gabbroic < Plutonic < Igneous Pluton Major