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


Correlated geologic units

Label Kmum
Description Metagabbro, metadiabase, and minor amphibolite
Geologic age Cretaceous
Geologic setting Intrusive
Lithology Form Importance
Greenstone < Metaigneous < Metamorphic Amphibolite Indeterminate, major