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 Karl, S.M., Haeussler, P.J., Himmelberg, G.R., Zumsteg, C.L., Layer, P.W., Friedman, R.M., Roeske, S.M.,and Snee, L.W., 2015, Geologic map of Baranof Island: U.S. Geological Survey Scientific Investigations Map SIM-3335, pamphlet 82 p., 1 sheet, scale 1:250,000.
Symbol Tegb
Unit name Gabbro
Description Dark gray, coarse-grained, massive altered gabbro and norite that is altered to secondary hornblende, albite, magnetite, and abundant apatite (Buddington, 1926; Reed and Gates, 1942). A small body at the mouth of Snipe Bay is about 15 m in thickness, and has intrusive contacts in graywacke semischist that trend parallel to the fabric of the host rocks. The gabbro does not have a foliation and postdates the metamorphic fabric in the host rocks. Mineralization in the body at Snipe Bay consists of disseminated magnetite, nickeliferous pyrrhotite, chalcopyrite, and pentlandite, in a lens of massive sulfide about 2 m in thickness, which is interpreted to be a magmatic segregation deposit (Buddington, 1925; Reed and Gates, 1942). The magnetite is enclosed in hornblende and pyrrhotite. The altered gabbro is different in composition from the dunite-wherlite and clinopyroxenite at Red Bluff Bay. The copper-nickel mineralization of the altered gabbro also differs from the chromite mineralization in the ultramafic rocks at Red Bluff Bay and chromite in the serpentinite lenses in the Patterson Bay fault zone. A maximum Eocene age for the altered gabbro in Snipe Bay is inferred from crosscutting relations with metamorphosed host rocks of the Sitka Graywacke (Kss) and proximity to the Eocene Redfish Bay pluton
Lithology Metamorphic

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