Older ultramafic rocks of southeast Alaska

Unit symbol: DSum
Age range Devonian and Silurian (443.4 to 419.2 Ma)
Lithology: Igneous - Plutonic
Group name: Older ultramafic rocks of southeast Alaska
Pyroxenite, hornblendite, and related ultramafic rocks on southern Prince of Wales Island (MacKevett, 1963; Gehrels and Saleeby, 1986, 1987), on southern Dall Island (Gehrels, 1991), and on east-central Prince of Wales Island (Loney and others, 1987). Gehrels and Berg (1992) interpreted ultramafic rocks on southern Prince of Wales Island as Silurian on the basis of gradational relations with syenitic rocks of Silurian age (unit SOgi, here). Early Silurian K/Ar apparent ages have been determined on the body at Salt Chuck on east-central Prince of Wales Island (Loney and others, 1987) and on Sukkwan Island (Himmelberg and Loney, 1995). The intrusive body on Dall Island yielded a K/Ar apparent age of Early Devonian (400.1 Ma, from hornblende) (Himmelberg and Loney, 1995) although Gehrels (1991) had mapped it as Ordovician

Source map information

Source map Brew, D.A., compiler, 1996, Geologic map of the Craig, Dixon Entrance, and parts of the Ketchikan and Prince Rupert quadrangles, southeastern Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-2319, 53 p., 1 sheet, scale 1:250,000.
Symbol SOpx
Unit name Intrusive and related rocks of younger part of Cape Chacon plutonic province - Pryoxenite, hornblendite, and gabbro
Description Pyroxenite, hornblendite, and gabbro. Compositions differs at different localities. Salt Chuck - discordant, pipe-like stock of olivine pyroxenite and gabbro, diorite, and hornblendite. Stone Rock Bay - porphyritic augite pyroxenite; intruded by quartz monzonite and granite. Sukkwan Island - medium to coarse grained hornblende gabbro, hornblende pyroxenite, and hornblendite. Long and Dall Islands - medium to coarse grained augite pyroxenite and pyroxene gabbro.
Lithology Igneous

Correlated geologic units

Label Sum
Description Ultramafic rocks, southeast Alaska
Geologic age Late-Ordovician to Telychian
Geologic setting Intrusive
Lithology Form Importance
Gabbro < Gabbroic < Plutonic < Igneous Pluton Major
Pyroxenite < Ultramafic < Plutonic < Igneous Pluton Major