Ultramafic complexes of western Alaska

Unit symbol: JPztu
Age range Jurassic or older (298.9 to 152.1 Ma)
Lithology: Igneous - Plutonic
Group name: Mafic and ultramafic rocks in central, western, and northern Alaska
Consists of ultramafic rock complexes surrounding the Yukon-Koyukuk Basin and along the north side of the Yukon Flats basin. “The complexes consist of: (1) a cumulate magmatic suite composed of interlayered dunite, wehrlite, olivine clinopyroxenite, and gabbro, (2) a mantle suite composed of harzburgite, dunite, and minor clinopyroxenite, and (3) a metamorphic sole consisting of a highly tectonized layer of amphibolite, garnet amphibolite, and pyroxene granulite. The harzburgite in the mantle suite typically is partly to mostly serpentinized. Chromite is generally restricted to centimeter-scale layers in dunite and as an accessory mineral” (Patton and others, 2009). Also included in this unit are ultramafic rocks assigned to the Kanuti ultramafic belt (Patton, 1974) and the Pitka ultramafic complex (Brosgé and others, 1974). The Pitka ultramafic complex was originally described as an eclogite and amphibolite unit during a rapid reconnaissance of the Beaver quadrangle (Brosgé and others, 1973); more detailed analysis by Brosgé and others (1974) showed that it consists “largely of banded garnet-amphibolite, foliated dunite, and harzburgite with pronounced cleavage, gneissic leucogabbro, and only minor eclogite.” Ghent and others (2001) reported that the complex had undergone granulite facies metamorphism and reported 40Ar/39Ar ages between 169.5±0.3 and 164.8±1.1 Ma. The Kanuti ultramafic belt, whose range has been extended from the original 125-km-long belt (Patton and Miller, 1970) through additional mapping, occurs “as tabular masses as much as 1,000 m thick composed of partially serpentinized dunite-harzburgite in the lower part and gabbro in the upper part. They dip 10° to 60° northwestward beneath the Cretaceous and Tertiary volcanic and sedimentary deposits of the Yukon-Koyukuk basin” (Patton, 1974). In the Nulato quadrangle, two exposures of massive chromite as much as 1.5 m thick were noted in dunite (Patton and Moll-Stalcup, 2000). “The complexes are intruded by narrow dikes of fresh clinopyroxenite, hornblendite, gabbro, and gabbro pegmatite. K/Ar isotopic cooling ages from the magmatic suite average 159 Ma and two 40Ar/39Ar determinations yielded a plateau age of 162 Ma. K/Ar isotopic cooling ages from the metamorphic sole at the base of the complexes range from 172 to 155 Ma and one 40Ar /39Ar determination from the metamorphic sole yielded a plateau age of 161 Ma” (Patton and others, 2009). Unit includes the Dishna River mafic and ultramafic rocks of the Iditarod and Ophir quadrangles (Miller, 1990; Miller and Bundtzen, 1994), on which three questionable K/Ar ages have been determined on hornblende: 222±23 and 228±25 Ma (replicate) and 92.2±2.8 Ma. The older sample had extremely low K2O and was thought to have incorporated excess argon; the younger was possibly reset by nearby plutonism (Miller and Bundtzen, 1994)

Source map information

Source map Patton, W.W., Jr., Wilson, F.H., Labay, K.A., and Shew, Nora, 2009, Geologic map of the Yukon-Koyukuk basin, Alaska: U.S. Geological Survey Scientific Investigations Map SIM-2909, 26 p., 2 sheets, scale 1:500,000.
Symbol Jumc
Unit name Mafic-ultramafic complexes
Description The complexes consist of: 1) a cumulate magmatic suite composed of interlayered dunite, wehrlite, olivine clinopyroxenite, and gabbro, 2) a mantle suite composed of harzburgite, dunite, and minor clinopyoxenite, and 3) a metamorphic sole consisting of a highly tectonized layer of amphibolite, garnet amphibolite, and pyroxene granulite. The harzburgite in the mantle suite typically is partly to mostly serpentinized. Chromite is generally restricted to centimeter-scale layers in dunite and as an accessory mineral. The complexes are intruded by narrow dikes of fresh clinopyroxenite, hornblendite, gabbro, and gabbro pegmatite.
Lithology Igneous

Correlated geologic units

Label Jtu
Description Ultramafic and mafic rocks; Dishna River mafic and ultramafic rocks, Middle to Early Jurassic
Geologic age Permian to Kimmeridgian
Geologic setting Intrusive
Lithology Form Importance
Ultramafic < Plutonic < Igneous Pluton Major