Peridotite of dismembered ophiolite of the Yukon-Tanana region

Unit symbol: MzPzyo
Age range Mesozoic, Triassic? or older (272.3 to 201.3 Ma)
Lithology: Igneous - Volcanic
Group name: Mafic and ultramafic rocks in central, western, and northern Alaska
Serpentinized peridotite, harzburgite, and dunite. Unit restricted to small outcrops in the Eagle, Charley River, Big Delta, Fairbanks, and Circle quadrangles (Foster, 1992; Foster and others, 1994). Corresponds to units assigned by many workers to the Seventymile terrane, which is thought to be a dismembered ophiolite. In the Big Delta quadrangle, unit includes units Pzd and Pzu of Weber and others (1978), and, in the Eagle quadrangle, unit Mzp of Foster (1976). Unit occurs as a klippe or is structurally infolded with mafic and pelitic schist of the Yukon-Tanana crystalline complex. Unit has not been dated, but age is inferred from association with sedimentary rocks of the Seventymile terrane, which contain early Late Triassic and late Paleozoic fossils (Dusel-Bacon and Harris, 2003). In the Eagle quadrangle, metagabbro mapped by Foster (1976) may be a component of the ophiolite; it is included in unit |yms here, mafic schist and amphibolite. The Joseph ultramafic body in the Eagle quadrangle yielded discordant hornblende and biotite dates of 174.7±5.1 Ma and 185.1±5.4 Ma, respectively (Foster, 1976)

Source map information

Source map Weber, F.R., Foster, H.L., Keith, T.E.C., and Dusel-Bacon, Cynthia, 1978, Preliminary geologic map of the Big Delta quadrangle, Alaska: U.S. Geological Survey Open-File Report 78-529A, scale 1:250,000.
Symbol Pu
Unit name Periodotite, partly serpentinized
Description Dark green to black; weathers a reddish orange brown; massive. Diabase and diorite inclusions, commonly meters in diameter. A zone of quartz and magnesite, with some dolomite, 1 to 50 m thick. The silica-carbonate rock is bright orange, orange brown, or cream with local green stains.
Lithology Igneous

Correlated geologic units

Label JPsu
Description Serpentinized ultramafic rocks
Geologic age Carboniferous to Early-Jurassic
Geologic setting Intrusive
Lithology Form Importance
Serpentinite < Metaigneous < Metamorphic Major
Hornblendite < Ultramafic < Plutonic < Igneous Indeterminate, major
Dunite < Peridotite < Ultramafic < Plutonic < Igneous Indeterminate, major