Dunite and serpentinite, undivided

Unit symbol: KPzum
Age range Cretaceous? to Paleozoic (298.9 to 66 Ma)
Lithology: Igneous - Plutonic
Group name: Mafic and ultramafic rocks in southern Alaska
Small bodies of serpentinite, peridotite, dunite, gabbro, diorite, metabasite, and minor talc schist and roddingite are widespread throughout the southern part of the State. They exhibit a broad range of intrusive and (or) metamorphic textures and fabrics. Age control is generally poor and inferred ages range from late Paleozoic to Mesozoic. In the Talkeetna quadrangle, unit includes serpentinite and minor talc schist that Reed and Nelson (1980, their units MzPzu and MzPzd) interpreted was derived from peridotite. In the Mount Hayes quadrangle, variably serpentinized pyroxenite, peridotite, dunite, schistose amphibolite and hornblende-plagioclase gneiss derived from gabbro (Nokleberg and others, 1992a) is exposed close to the Denali Fault System and along the Broxson Gulch thrust in the Mount Hayes quadrangle. These bodies are on the opposite side of the Denali Fault System relative to the ultramafic bodies of unit KJmu above. The protolith was thought to be either Paleozoic or Triassic and metamorphosed in the Early Cretaceous. K/Ar and 40Ar/39Ar analysis of biotite and hornblende from pyroxenite yielded nearly concordant ages between 125.9 and 121.2 Ma (Nokleberg and others, 1992b) for the part of the unit associated with the Denali Fault System and 97.67 to 91.89 Ma for the Broxson Gulch thrust associated part of the unit (Nokleberg and others, 1992b). Nokleberg and others (1992a) infer that the Broxson Gulch rocks may have been comagmatic with the Nikolai Greenstone of Triassic age (Trn) or, alternatively, are Late Cretaceous intrusions

Source map information

Source map Nokleberg, W.J., Aleinikoff, J.N., Lange, I.M., Silva, S.R., Miyaoka, R.T., Schwab, C.E., and Zehner, R.E., with contribution for selected areas from Bond, G.C., Richter, D.H., Smith, T.E., and Stout, J.H., 1992, Preliminary geologic map of the Mount Hayes quadrangle, eastern Alaska Range, Alaska: U.S. Geological Survey Open File Report 92-594, scale 1:250,000 [Received as CAD coverage produced in 1996].
Symbol umm
Unit name Ultramafic and mafic igneous rocks occurring along Broxson Gulch thrust
Description Chiefly dunite, olivine-clinopyroxene cumulates, serpentinite, metagabbro, and metadiabase. Unit occurs in scattered fault bounded exposures. Local, low-grade metamorphism.
Lithology Igneous

Correlated geologic units

Label Mzum
Description Ultramafic and mafic igneous rocks fault associated in south-central and southeastern Alaska
Geologic age Mesozoic
Geologic setting Intrusive
Lithology Form Importance
Dunite < Peridotite < Ultramafic < Plutonic < Igneous Pluton Major
Pyroxenite < Ultramafic < Plutonic < Igneous Pluton Major
Greenstone < Metaigneous < Metamorphic Greenschist Indeterminate, major
Serpentinite < Metaigneous < Metamorphic Greenschist Minor