Mafic and ultramafic rocks

Unit symbol: KJmu
Age range Cretaceous to Jurassic or older (163.5 to 66 Ma)
Lithology: Igneous - Plutonic
Group name: Mafic and ultramafic rocks in southern Alaska
Mafic and ultramafic rocks that are widely distributed in the southern part of Alaska and have poor age control. Includes a small dark greenish- or brownish-gray, coarse- to medium-grained, plagioclase-bearing, sill-like ultramafic intrusive in the Healy quadrangle (Csejtey and others, 1992). It appears to intrude Late Triassic calcareous rocks of unit ^cs and to have been metamorphosed during a mid-Cretaceous regional event that affected rocks in the eastern Healy quadrangle. In the southeastern Healy quadrangle, a small discordant pluton of alkali gabbro (monzogabbro) has an inferred age of Late Jurassic (Csejtey and others, 1992) on the basis of K/Ar and U/Pb radiometric dates. This pluton intrudes an argillite and metagraywacke unit that yields Late Jurassic detrital zircon ages. In the northern Anchorage quadrangle, north of the Border Ranges Fault Zone, Winkler (1992) reported small, structurally bounded, pervasively sheared, discordant bodies of serpentinized ultramafic rocks wholly enclosed in pelitic schist (Kps). Winkler (1992) reported that the timing of origin was unknown, but early Late Cretaceous K/Ar dates (91–89 Ma) were presumed to provide a minimum age for their emplacement. In the northern Nabesna quadrangle, Richter (1976) mapped several small ultramafic bodies that consist of serpentinite, serpentinized peridotite, dunite, and subordinate clinopyroxenite; he suggested they were of possible Cretaceous age. Includes two units on Baranof Island. At Red Bluff Bay on the southeast coast of the island, red-weathering, fine-grained dunite-wehrlite and clinopyroxenite form a body 3 km by 6 km. In the interior of the island, structurally concordant sills as much as 1.5 km in length and 0.5 km wide of yellowish-brown-weathering, clinopyroxene-antigorite and talc-tremolite-chrysotile serpentinite are commonly associated with strands of the Patterson Bay Fault (Karl and others, 2015). In the Bethel quadrangle, Box and others (1993) also report small, pervasively slickensided bodies of serpentinite, serpentinite-matrix mélange, and silica-carbonate altered serpentinite, which they inferred to be Late Cretaceous in age

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 Mzum
Unit name Ultramafic rocks
Description Red-weathering, black-fresh, fine-grained dunite-wherlite and clinopyroxenite form a body 3 km by 6 km in Red Bluff Bay. Dunite, wherlite, and clinopyroxenite are both layered and massive, are not zoned, and retain cumulus textures. Massive dunite gradational to wherlite forms the western half of the body at Red Bluff Bay, and is faulted against dunite-wherlite that is intruded by irregular masses and veins of clinopyroxenite, and that alternates with clinopyroxenite in cumulus layers 2-50 cm thick in the eastern half of the body (Himmelberg and Loney, 1995). Chromian spinel layers and lenses occur in concentrations that have been prospected, but the chromite is rich in Fe and subeconomic (Guild and Balsley, 1942; Loney and others, 1975). Magnetite occurs as a secondary mineral resulting from serpentinization and recrystallization. All observed contacts at the margin of the Red Bluff Bay ultramafic body are faults, and no thermal aureole has been observed in adjacent metasedimentary and metamorphic rocks
Lithology Igneous

Correlated geologic units

Label Kmum
Description Serpentinized ultramafic rocks
Geologic age Early-Cretaceous to Aptian
Geologic setting Intrusive
Lithology Form Importance
Peridotite < Ultramafic < Plutonic < Igneous Indeterminate, major
Serpentinite < Metaigneous < Metamorphic Indeterminate, major