Quartz-mica schist of the Brooks Range

Unit symbol: DPxaqm
Age range Devonian to Proterozoic (1000 to 358.9 Ma)
Lithology: Metamorphic
Group name: Brooks Range schist belt
Exposed along the southern flank of the Brooks Range, unit is largely equivalent to unit Dsq of Till and others (2008a). Quoting Till and others (2008a) from their description of Dsq, this unit consists of “gray, dark-gray, or brownish-gray weathering, dominantly pelitic or semipelitic schist that constitutes the major lithologic unit of the Schist belt. Outcrops vary from blocky and resistant (quartz-rich varieties) to platy and less resistant (mica-rich varieties), as the abundance of quartz versus mica and albite varies at centimeter to meter scales. Scattered lenses of mafic schist, calcareous schist, albite-mica schist, graphitic metaquartzite, and marble up to 10's of meters thick are typical of the unit but volumetrically minor. Mafic lenses may be massive or schistose. In the Wiseman area, rare lenses of ultramafic rocks are up to 10's of meters thick. The dominant foliation is defined by parallel millimeter- to meter-scale variations in quartz versus mica or albite, or discontinuous layers and lens-shaped quartz segregations (Gottschalk, 1990; Little and others, 1994; Till, A.B., 2008, unpublished data). * * * At map scales, the dominant foliation is typically broadly arched or folded around axes that are subparallel to the east-west trend of the Schist belt.” No fossils have been collected from the schist, but protolith age can be partially bracketed by the age of detrital zircons from the unit. Twenty-seven detrital zircons from micaceous metaquartzite varied from pitted spherical to slightly abraded euhedral grains (Moore and others, 1997b); the rounded population (n=16) yielded single-grain 207Pb/206Pb ages that suggest the quartz-mica schist protolith included Archean and Proterozoic rocks. The euhedral grains (n=11) gave concordant single-grain 238U/206Pb ages between 371 and 361 Ma, which indicates that at least part of the protolith package is Devonian in age (Moore and others, 1997b). Middle Devonian granitic orthogneiss (unit Dogn, here) is present within the quartz-mica schist unit in the Chandalar quadrangle, but geologic mapping is insufficient to show whether there was an original intrusive relationship between Till’s units Dg and Dsq, or if the orthogneiss was folded in with unit Dsq during penetrative Mesozoic deformation (Till and others, 2008a). Therefore, it is not clear whether or not the age of the orthogneiss bears directly on the age of the protolith of the quartz-mica schist. “The quartz-mica schist shared its early deformational and metamorphic history with other units of the Schist belt * * *. Common metamorphic minerals in pelitic schist include quartz, muscovite, chlorite, plagioclase, chloritoid, and accessory sphene, tourmaline, rutile, opaque, graphite, and calcite. Some pelitic schists contain garnet, and many contain glaucophane or pseudomorphs of chlorite and albite after glaucophane. Metabasite typically contains a combination of actinolite, albite, epidote, garnet, chlorite, sphene, and quartz; many contain glaucophane or pseudomorphs after glaucophane. This unit may be a lithologic and metamorphic correlative to the Solomon schist of the Nome Group, Seward Peninsula, which has also yielded detrital zircons as young as Late Devonian (Till and others, 1986, 2006b)” (Till and others (2008a)

Source map information

Source map Brosge, W.P., and Reiser, H.N., 1964, Geologic map and section of the Chandalar quadrangle, Alaska: U.S. Geological Survey Miscellaneous Geologic Investigations Map I-375, scale 1:250,000.
Symbol Dqm
Unit name Quartz-mica schist
Description Quartz-mica schist; dark-gray to black, gray to reddish-brown-weathering, quartz-muscovite schist and schistose quartzite.
Lithology Metamorphic

Correlated geologic units

Label PzZaqs
Description Quartz-mica schist. Assigned to Arctic Alaska terrane
Geologic age Neoproterozoic to Devonian
Geologic setting Sedimentary, shallow-marine-siliciclastic
Lithology Form Importance
Mica-schist < Schist < Metamorphic Greenschist Major