Younger gabbro of southeast Alaska

Unit symbol: Tgbe
Age range Tertiary, Miocene to Eocene (56 to 5.333 Ma)
Lithology: Igneous - Plutonic
Group name: Gabbroic rocks in southern Alaska
Map unit includes two belts of mafic intrusive rocks. The younger belt, earliest Miocene to late Oligocene, consists of “layered and locally zoned bodies of two-pyroxene ± olivine ± biotite ± hornblende ± quartz gabbro and subordinate troctolite, peridotite, leucogabbro, diorite, and tonalite” (Gehrels and Berg, 1992). This younger belt consists of stocks on northern Kupreanof and Kuiu Islands (Brew and others, 1984), Chichagof Island (Johnson and Karl, 1985; Loney and others, 1975), and north of Cross Sound in the Fairweather Range (Brew and others, 1978). Stocks on Revillagigedo Island and adjacent area of mainland yield K/Ar ages between 24.9±0.75 and 23.2±0.7 Ma, early Miocene and late Oligocene (Smith and Diggles, 1981). A large body (La Perouse gabbro) northwest of Cross Sound yielded an 40Ar/39Ar apparent age of 28.0±8.0 Ma, Oligocene (Loney and Himmelberg, 1983). The second (older belt) plutonic suite yields bimodal Eocene (early and latest) radiometric ages and is found on Yakobi and Chichagof Islands. Found west of the Border Ranges Fault in the Sitka quadrangle, these rocks are dominantly composed of medium- to dark-gray, locally brown-gray, medium- to coarse-grained gabbronorite and norite (Johnson and Karl, 1985). They contain abundant sulfides and form the host rock for nickel-sulfide ore bodies (Johnson and Karl, 1985) and yield discordant K/Ar ages of 34.0±1.0 and 39.6±1.2 Ma from biotite and hornblende (Karl and others, 1988) and 40Ar/39Ar ages of 50.5±.1 and 50.3±.1 Ma on biotite (Bradley and others, 2003)

Source map information

Source map Johnson, B.R., and Karl, S.M., 1985, Geologic map of western Chichagof and Yakobi Islands, southeastern Alaska: U.S. Geological Survey Miscellaneous Investigations Series Map I-1506, pamphlet, 15 p., 1 sheet, scale 1:125,000.
Symbol Tgn
Unit name Gabbronorite and norite
Description Composed dominantly of medium to dark gray (C.I. 40-75), locally brown gray, medium to coarse grained, orthopyroxene + plagioclase + clinopyroxene gabbronorite and norite containing minor amounts of plagioclase bearing orthopyroxenite. Olivine is present in some norite, and postcumulous, brown hornblende is ubiquitous. Contains abundant sulfides and forms the host rock for the Bohemia Basin and Mirror Harbor nickel-sulfide ore bodies.
Lithology Igneous

Correlated geologic units

Label Tgn
Description Gabbronorite and norite
Geologic age Ypresian to Priabonian
Geologic setting Intrusive
Lithology Form Importance
Gabbronorite < Gabbro < Gabbroic < Plutonic < Igneous Pluton Major
Norite < Gabbro < Gabbroic < Plutonic < Igneous Pluton Major