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Metamorphosed mafic igneous rocks

Unit symbol: Dmi
Age range Late Devonian (382.7 to 358.9 Ma)
Lithology: Metamorphic
Group name: Granitic rocks and orthogneiss
Consists of a variety of metaigneous rocks of presumed Devonian age in central Alaska. Main part of unit lies north of the Denali Fault System and consists of a mixture of massive greenstone that locally contains thin septa of phyllite (Richter, 1976), and fine-grained, mylonitic, metavolcanic rocks interlayered with lesser metasedimentary rocks (Nokleberg and others, 1992a; Csejtey and others, 1992). Metavolcanic rocks are interpreted to consist chiefly of metamorphosed quartz keratophyre, dacite, andesite, and lesser metarhyodacite and metabasalt derived from fine-grained tuffs and flows (Nokleberg and others, 1992a) or metabasalt (Csejtey and others, 1992). Regionally metamorphosed to greenschist facies. Unit most certainly extends through the Tanacross quadrangle, but mapping is insufficient to allow us to distinguish this unit there. Unit includes the Spruce Creek sequence of Bundtzen (1981) and the Muskox sequence of Newberry and others (1996). Two samples of metarhyolite schist from the Spruce Creek sequence in the Mount McKinley quadrangle yielded TIMS U/Pb zircon ages of 370±5 and 369±4 Ma (Dusel-Bacon and others, 2006). South of the Denali Fault System, unit is sheared serpentinite tectonically intermixed with chert and pillow basalt, which form lenticular and podiform tectonic blocks (Csejtey and others, 1992). Radiolaria from the chert are Late Devonian (Famennian). South of the Denali Fault System, in the Healy and Mount McKinley quadrangles, this unit is part of the Chulitna sequence, and it was originally interpreted by Jones and others (1981) to be part of a dismembered ophiolite sequence, and it was reinterpreted by Clautice and others (2001) as feeders for the volcanic rocks included in unit TrPzvs

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 jcv
Unit name Fine-grained schistose metavolcanic rocks and metasedimentary rocks
Description Chiefly fine-grained, mylonitic, metavolcanic rocks interlayered with lesser metasedimentary rocks. Metavolcanic rocks consist chiefly of metamorphosed quartz keratophyre, dacite, and andesite and lesser metarhyodacite and metabasalt derived from fine-grained tuffs and flows. Metasedimentary rocks chiefly quartz-mica schist and some calc-schist, marble, skarn, and quartzite. Protoliths thought to be pelite, quartz sandstone, siltstone, marl and limestone. Metamorphism ranges from lower greenschist to lower amphibolite facies. Intensely deformed exhibiting strong schistosity which dips gently to moderately north or south.
Lithology Metamorphic

Correlated geologic units

Label Pzsc
Description Metabasalt and subordinate metasedimentary rocks, Late Devonian
Geologic age Late-Devonian to Devonian
Geologic setting Undivided
Lithology Form Importance
Metavolcanic < Metaigneous < Metamorphic Greenschist Major
Pelitic-schist < Metaclastic < Metasedimentary < Metamorphic Greenschist Indeterminate, major
Phyllite < Metaclastic < Metasedimentary < Metamorphic Greenschist Indeterminate, major
Metasedimentary < Metamorphic Greenschist Minor
Marble < Metacarbonate < Metasedimentary < Metamorphic Greenschist Incidental