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 Gehrels, G.E., 1992, Geologic map of southern Prince of Wales Island, southeastern Alaska: U.S. Geological Survey Miscellaneous Investigations Series Map I-2169, 23 p., 1 sheet, scale 1:63,360.
Symbol SOpx
Unit name Pyroxenite and hornblendite
Description Coarse-grained to pegmatitic pyroxenite, hornblende pyroxenite, and hornblendite at Stone Rock Bay and Cape Chacon. Hornblende rims augite and is associated with abundant opaque minerals. Hornblende pegmatite contains hornblende crystals to 10 cm long. Gradational to SOsy.
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