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 Csejtey, Bela, Jr., Mullen, M.W., Cox, D.P., and Stricker, G.D., 1992, Geology and geochronology of the Healy quadrangle, south-central Alaska: U.S. Geological Survey Miscellaneous Investigations Series Map I-1961, 63 p., 2 sheets, scale 1:250,000.
Symbol Dmb
Unit name Metabasalt and subordinate metasedimentary rocks
Description 600 m thick intercalated sequence of schistose and phyllitic, generally medium to dark greenish-gray, fine-grained and massive metabasalt; dark gray to black, generally fine-grained carbonaceous pelitic schist and phyllite and associated metasiltstone; and a few thin interbeds of black metachert, and fine- to medium-grained, dark-gray marble.
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