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., scale 1:250,000.
Symbol MzPzmi
Unit name Metamorphosed intrusive rocks
Description Mainly regionally metamorphosed or otherwise recrystallized or altered diorite, quartz diorite, and gabbro. Can consist of dark greenish gray hypidiomorphic granular diorite and ferromagnesian porphyritic diorite containing magmatic plagioclase, magmatic or secondary hornblende(?), and secondary epidote, chlorite and calcite. West of Port Stewart a body of massive to foliated hornblende biotite diorite, quartz diorite, and gabbro coincides with a positive aeromagnetic anomaly. This body may be a structurally detached fragment of a Cretaceous zoned ultramafic intrusive complex near Union Bay, about 6 km beyond the west boundary of Ketchikan quadrangle.
Lithology Metamorphic

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