Ultramafic rocks of southeast Alaska

Unit symbol: Kum
Age range Early Cretaceous (113 to 100.5 Ma)
Lithology: Igneous - Plutonic
Group name: Mafic and ultramafic rocks in southern Alaska
“Ultramafic intrusive bodies of magnetite-bearing hornblende clinopyroxenite and subordinate dunite, peridotite, and hornblendite (Taylor, 1967). Several complexes are concentrically zoned from a core of dunite to rocks containing progressively less olivine and more hornblende and magnetite. Zoned bodies commonly intrude a two-pyroxene gabbro known to be of Late Triassic age on Duke Island (Gehrels and others, 1987). Geologic and geochemical considerations suggest that rocks in these bodies may be genetically related to some Cretaceous and Jurassic volcanic rocks (Berg and others, 1972; Irvine, 1973). Potassium-argon apparent ages of the ultramafic rocks indicate emplacement during Early Cretaceous time (Lanphere and Eberlein, 1966)” Gehrels and Berg (1992). Himmelberg and Loney (1995) also provide extensive information on the characteristics and petrogenesis of many of these ultramafic intrusions

Source map information

Source map Berg, H.C., Elliott, R.L., and Koch, R.D., 1988, Geologic map of the Ketchikan and Prince Rupert quadrangles, southeastern Alaska: U.S. Geological Survey Miscellaneous Investigations Series Map I-1807, pamphlet, 27 p., 1 sheet, scale 1:250,000.
Symbol Kum
Unit name Ultramafic rocks
Description PRINCE OF WALES-GRAVINA-ANNETTE-MARY-DUKE ISLANDS Dunite, peridotite, clinopyroxenite, and hornblendite, locally serpentinized. The dunite and peridotite in these ultramafic rocks contain small amounts of chromite, asbestos, and platinum group metals, and the clinopyroxenite locally contains accumulations of titaniferous magnetite. Ultramafic complex on Duke Island is zoned and layered. The ultramafic bodies on Duke and Percy Islands have been prospected for iron and chromite.
Lithology Igneous

Correlated geologic units

Label Kmum
Description Ultramafic rocks, especially including zoned ultramafic bodies of southeast Alaska
Geologic age Albian
Geologic setting Intrusive
Lithology Form Importance
Hornblendite < Ultramafic < Plutonic < Igneous Indeterminate, major
Dunite < Peridotite < Ultramafic < Plutonic < Igneous Indeterminate, major
Pyroxenite < Ultramafic < Plutonic < Igneous Indeterminate, major