Calcareous schist of Brooks Range

Unit symbol: DPxacs
Age range Devonian to Proterozoic (1000 to 358.9 Ma)
Lithology: Metamorphic
Group name: Brooks Range schist belt
“Light-gray-, brown- and locally orange-weathering, lithologically heterogeneous mix of marble and carbonate-rich, quartz-rich, and mafic schist derived from metasedimentary and metaigneous protoliths; one of two major units that extends along the length of the Schist belt [of Till and others, 2008a]. Within the unit, lithologies are interlayered at scales varying from millimeters to 10's of meters. Calcareous schist, albitic schist, marble, and metaquartzite (massive, mica-poor varieties of pelitic schist) are commonly interlayered; pelitic interlayers and pelitic components in calcareous schists are also characteristic” (Till and others, 2008a). Marble generally forms less than 25 percent of unit but locally may represent as much as 40 percent of the unit. The marble is in layers, lenses, and boudins of coarsely crystalline, pure calcite meters to tens of meters thick forming bare, steep slopes and ledges. Rare dolostone occurs as lenses up to several meters thick. Graphitic carbonate rocks (marble and dolostone), quartz-rich schist, and albite-rich schist are typical of the unit, as are chlorite-bearing marble, dolostone, and metaquartzite. Metabasite, metadiorite, and chlorite-albite schist vary greatly in abundance along the length of the unit (Till and others, 2008a). “It is likely that the calcareous schist unit is composed of several lithologic packages that have unknown depositional relationships. In the western part of the schist belt, * * * calcite-chlorite-albite schists, chlorite-albite schists, and marbles were apparently derived from sources rich in carbonate and mafic components. * * * Near Wiseman, two subunits can be distinguished, though some lithologies occur in both. One subunit is similar to the carbonate-mafic association in the west. The other * * * is dominated by metachert * * * and calcareous schist, and contains several types of metaconglomerates. The metachert commonly contains cm-scale lenses and thin, mm-thick layers of spessartine (Mn-rich) garnet and mafic metatuff. Metabasite bodies are associated with the metachert as well (A.B. Till, unpublished data)” (Till and others, 2008a). In discussing age control for this unit, Till and others (2008a), report conodont collections that range in age between Middle Ordovician and Middle Devonian. In the Baird Mountains quadrangle granitic orthogneiss that has yielded a Neoproterozoic U/Pb zircon age (705±35 Ma, Karl and Aleinikoff, 1990, in rocks assigned to unit Zgn here) apparently intrudes marble of this unit, which Till and others (2008a) interpreted to mean that “at least part of the unit must be Late Proterozoic or older.” Spatially associated with this unit are Middle and Late Devonian orthogneiss bodies (unit Dogn); Newberry and others (1997) report skarn around the Middle Devonian orthogneiss in the Chandalar quadrangle. “Metamorphic assemblages in the calcareous schist unit show that it experienced the same early high-pressure/low-temperature metamorphic and deformational history as the quartz-mica schist [unit D<aqm, here, and unit Dsq of Till and others, 2008a, who cite Gottschalk, 1990; Little and others, 1994; and Dinklage, 1998]. Chloritoid, glaucophane, pseudomorphs after glaucophane, and pseudomorphs after lawsonite are present in pelitic and mafic layers in the unit (Little and others, 1994; Dinklage, 1998; Till, A., unpublished data)” (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 Da
Unit name Albite-chlorite mica schist
Description Gray, green, and yellowish-gray chlorite-muscovite schist; generally with coarse-grained albite and quartz; little biotite.
Lithology Metamorphic

Correlated geologic units

Label DZa
Description Calcareous schist - albite-rich subunit, western part of Schist belt
Geologic age Neoproterozoic to Devonian
Geologic setting Metamorphic, undivided
Lithology Form Importance
Marble < Metacarbonate < Metasedimentary < Metamorphic Greenschist Indeterminate, major
Quartzite < Metaclastic < Metasedimentary < Metamorphic Greenschist Indeterminate, major
Mica-schist < Schist < Metamorphic Greenschist Indeterminate, major