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 MzPzum
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 The complexes appear to form the roots of the Koyukuk volcanic arc that was active from Jurassic to late Early Cretaceous. The protolith age of the mantle suite is uncertain and may be as old as Paleozoic. Unit exposed along southeastern and northern borders of Yukon-Koyukuk Basin, in the south-central part of Unalakleet quadrangle where it is spatially associated with the trondhjemite and tonalite unit Jgt, and in one small area along the western border of the basin in southern Candle quadrangle. Also included in this unit is a small complex exposed in a window(?) through the Innoko terrane in the south-central part of Ophir quadrangle
Lithology Igneous

Correlated geologic units

Label Jaum
Description Ultramafic rocks, Permian to Jurassic, Angayucham associated
Geologic age Permian to Early-Jurassic
Geologic setting Intrusive
Lithology Form Importance
Peridotite < Ultramafic < Plutonic < Igneous Pluton Major
Dunite < Peridotite < Ultramafic < Plutonic < Igneous Pluton Major
Anorthosite < Plutonic < Igneous Pluton Indeterminate, major
Gabbro < Gabbroic < Plutonic < Igneous Pluton Indeterminate, major