Metamorphic rocks of the MacLaren metamorphic belt of Smith and Lanphere (1971)

Unit symbol: Mzmu
Age range Cretaceous and Jurassic or older? (201.3 to 66 Ma)
Lithology: Metamorphic
Group name: MacLaren metamorphic belt of Smith and Lanphere (1971)
Dominantly fine- to medium-grained garnet-bearing schist and hornblende amphibolite, and subordinate calc-silicate schist and quartzite, all derived from siltstone, graywacke, marl, and andesite (Smith, 1981; Smith and others, 1988; Nokleberg and others, 1992a). Also includes intercalated gneissose granitic to gabbroic metaigneous rocks. Underwent regional metamorphism to amphibolite facies and, locally, subsequent greenschist facies retrograde metamorphism. Metamorphic grade appears to increase gradationally eastward from the Healy to Mount Hayes quadrangles and from south to north across a series of thrust faults mapped within the Mount Hayes quadrangle, toward the gneissose granitic rocks of unit TKgg (Smith, 1981). K/Ar hornblende and biotite ages range from 67.9 to 29.3 Ma; biotite consistently yields the youngest ages, which suggests prolonged cooling. U/Pb dates on zircon rims from phyllite using the SHRIMP (sensitive high resolution ion microprobe) were 153.3±2.7 to 147.9±3.0 Ma or Late Jurassic, and metamorphosed volcaniclastic rocks yielded 86.4±2.2 Ma or Late Cretaceous (Mooney, 2010). Includes units of the MacLaren terrane of Nokleberg and others (1992a) in the Mount Hayes quadrangle. In the southeastern part of the adjacent Healy quadrangle, rocks that Csejtey and others (1992) originally mapped as part of the flysch sequence, we infer had the same protolith as the ones in the rest of this unit and, therefore, we place those rocks in this unit

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 mph
Unit name Phyllite
Description Chiefly fine-grained white mica-quartz phyllite, and minor quartz-plagioclase-white mica phyllite, clinozoisite-chlorite-quartz phyllite, calcite-chlorite phyllite, and quartz-rich marble and metaandesite. Protoliths chiefly siltstone, metavolcanic siltstone, marl, limestone, and andesite. Intensely deformed, with a strong schistosity. Metamorphosed to upper greenschist facies.
Lithology Metamorphic

Correlated geologic units

Label Mzpca
Description Phyllite, Late Jurassic or older
Geologic age Jurassic
Geologic setting Metamorphic, undivided
Lithology Form Importance
Phyllite < Metaclastic < Metasedimentary < Metamorphic Greenschist Major