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 Smith, T.E., Albanese, M.D., and Kline, G.L., 1988, Geologic map of the Healy A-2 quadrangle, Alaska: Alaska Division of Geological and Geophysical Surveys Professional Report 95, 1 sheet, scale 1:63,360.
Symbol Kgn
Unit name Gneiss and high-grade schist
Description Gray-brown to rust-weathering pelitic gneiss and silver-brown schist. Dominantly contain biotite, quartz, and plagioclase +/- garnet with local sillimanite-bearing variants; accessory minerals include graphite, apatite, tourmaline, and zircon. Gneissic texture on mainly layered with local folds. Form large tors. Unit is equivalent to pelitic gneiss in adjacent Healy A-1 quadrangle where K-Ar age of 66.2 m.y on hornblende.
Lithology Metamorphic

Correlated geologic units

Label Mzsa
Description Gneiss, schist, and amphibolite, Late Cretaceous metamorphic age
Geologic age Jurassic to Cretaceous
Geologic setting Metamorphic, undivided
Lithology Form Importance
Amphibolite < Metamorphic Amphibolite, epidote-amphibolite Major
Mica-schist < Schist < Metamorphic Amphibolite, epidote-amphibolite Major
Pelitic-schist < Metaclastic < Metasedimentary < Metamorphic Amphibolite, epidote-amphibolite Incidental