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Younger schist

Unit symbol: Pzncs
Age range Devonian and Silurian? (443.4 to 358.9 Ma)
Lithology: Metamorphic
Group name: Layered sequence (Nome Complex)
Unit is a composite of units best described by Till and others (2011, units Dcs, and Ds) and consists of pelitic, calcareous, and graphitic schist interlayered on a scale of centimeters to meters in the Nome and Solomon quadrangles. Unit is predominantly pale-brown and gray, weakly to well foliated schist that consists primarily of plagioclase, calcite, quartz, white mica, and graphite. Pelitic schist weathers light- to dark-gray and contains chloritoid, glaucophane, or pseudomorphs of chlorite and albite after glaucophane, and rarely, garnet. Calcareous schist is light- to dark-brownish-gray and commonly contains chloritoid and rare garnet, as well as dolomite. Locally gray pelitic schist is interlayered with orange- or brown-weathering impure marble, calcareous schist, or mica- and graphite-rich schist. Dark-gray- to black-weathering, graphitic, locally micaceous schist “contains mm-scale laminae enriched in graphite, white mica, and iron oxide, and occurs in layers meters to tens of meters thick” (Till and others, 2011). Unit Dcs of Till and others (2011) consists of pelitic, calcareous, and graphitic schist and typically does not outcrop; rather, it usually occurs as loose rubble on hills, or in stream cuts; in low areas unit is covered by tundra. Unit Ds, Till’s pelitic schist unit, forms tors of resistant, well-foliated quartz-rich schist; pelitic schist is the dominant lithology in this unit and calcareous schist is minor. This schist also contains chloritoid, and locally graphite, glaucophane, and garnet. Many detrital zircon samples collected from the Dcs unit contain small populations of Middle and Late Devonian zircons and large populations of early-middle Silurian zircons, as well as older populations (Till and others, 2006b, 2008b). Only a single sample from Ds yielded sufficient detrital zircons for analysis; the youngest zircon population contains grains that are Early to Middle Devonian (Till and others, 2006b). Till and others (2011) described a distinctive white to light gray, generally impure marble that occurs in small lenses within or adjacent to this unit in the Nome quadrangle. The clast-bearing marble typically contains sand- to pebble-size clasts of dark-gray to black, organic-rich marble. Most clast-bearing intervals appear to be clast-supported, some may be graded, and the clasts are rounded to angular, commonly laminated, and generally about 2 cm in diameter, though some are as big as 12 cm long. Two collections of conodonts having CAI values of 5 are known. “One collection yielded a single element of Silurian(?) through Triassic age. The other collection, also a single conodont, is an Sb element of late Permian through Triassic (likely Triassic) age” Till and others (2011). Till and others (2011) suggest that the Triassic marble was juxtaposed with the rocks of the Nome Complex prior to metamorphism and, therefore, chose to not extend the protolith age of the Nome Complex to Triassic; the Jurassic metamorphism of the unit (Armstrong and others, 1986) provides an upper limit for the age of the protolith of the unit

Source map information

Source map Till, A.B., Dumoulin, J.A., Werdon, M.B., and Bleick, H.A., 2010, Preliminary bedrock geologic map of the Seward Peninsula, Alaska, and accompanying conodont data: U.S. Geological Survey Open-File Report 2009-1254, pamphlet, 57 p., 2 plates, scale 1:500,000, and database.
Symbol Dcs
Unit name Pelitic, calcareous, and graphitic schist, Nome Group
Description Pelitic, calcareous, and graphitic lithologies interlayered on a scale of centimeters to meters, exposed in the central Nome and Solomon quadrangles. Interlayering is gradational and intimate. The unit is composed predominantly of pale brown and gray weakly- to well-foliated schists dominated by plagioclase, calcite, quartz, white mica, and graphite. In the central Solomon quadrangle, where calcareous lithologies are subordinate to pelitic lithologies, the unit forms smooth rounded hills, or more commonly, is present in valleys and areas of subdued topography. Rarely forms outcrops; usually occurs as loose rubble on hills, stream cuts, or in low areas unit is covered by tundra. Ridge-top outcrops and ridge-flanking tors are more common in the Nome quadrangle, but not abundant. Pelitic rocks are light to dark gray weathering and contain pale-colored chloritoid, glaucophane or pseudomorphs of chlorite and albite after glaucophane, and in rare cases, garnet; at some localities, micaceous laminae alternate with quartz-rich laminae. Accessory minerals include epidote, clinozoisite, apatite, tourmaline, sphene, graphite, ilmenite, pyrite, rutile, and zircon. Calc-schist is light to dark brownish-gray colored and commonly contains plagioclase, chloritoid, calcite, some dolomite, and rare garnet. Plagioclase commonly occurs as syn- and post-penetrative deformation metamorphic porphyroblasts. Accessory minerals include apatite, tourmaline, graphite, pyrite, rutile, and zircon. Gray-weathering, plagioclase-rich lithologies are interlayered with orange- or brown-weathering impure marble or calcareous schist in some localities, mica- and graphite-rich schists in other localities. Layers rich in chlorite and albite, with and without white mica and calcite, are generally less than 0.5 m thick. The unit includes relatively thin, but widely distributed, layers of light to dark gray or pale orange weathering mm-scale color laminated marble. Marbles are slightly impure, most commonly with small concentrations or disseminations of quartz, white mica, and graphite; they also can include iron carbonate, iron oxide, and rare sulfide minerals. Less common, but also broadly distributed, are layers typically less than 20 cm thick of pale tan- to white-weathering metaquartzite with fine laminations defined by minor iron carbonate, iron oxide, and white mica. Dark-gray to black-weathering, graphitic metasiliceous rock, locally micaceous, contains mm-scale laminae enriched in graphite, white mica, and iron oxide, and occurs in layers meters to tens of meters thick; these exposures are generally thinner than the graphitic metasiliceous layers in unit DOx. In the Nome quadrangle, a dark-gray to black weathering rock with striking white mm- to cm-thick lenses, layers, and irregular blobs of polycrystalline quartz forms layers a meter to several meters thick. The matrix to the quartz concentrations is typically rich in white mica, graphite, chlorite, and albite in varying abundances. Many exposures are resistant. Some of the polycrystalline quartz lenses and blobs trace intrafolial isoclinal folds; more equant blobs occur on fold hinges. These features and other deformational characteristics of the Nome Complex are consistent with a protolith for this rock that had quartz-rich alternating with mica-rich layers, although its present appearance is suggestive of a conglomerate. This lithology is equivalent to the "lumpy schist" or "pCPzspm" of Bundtzen and others (1994). A lens of carbonate clast conglomerate, 10 to 15 m thick and >15 to 20 m long, occurs within Dcs in the Nome quadrangle near the Aurora Creek prospect. The conglomerate is poorly sorted, matrix supported, interlayered with schist, and has a matrix of calcareous schist that contains abundant quartz and white mica. Clasts are rounded to angular, elongate to disc-shaped, and as much as 75 cm in maximum diameter. Most are light gray to beige dolostone and medium gray marble. A distinctive lithologic package occurs along the western margin of the unit in the Nome quadrangle. The package coincides with a long, linear resistivity low and magnetic high (Burns and others, 2005a,b). Pale green and pale gray weathering, locally brown-weathering schists locally display mm- to cm-thick layers of sugary white quartz. Rocks typically are interlayered concentrations of quartz with lesser iron carbonate and epidote versus white mica, medium-dark colored chloritoid, and locally epidote, iron carbonate, and chlorite. Chloritoid grains may occur in polycrystalline lenses within white mica laminae. Epidote grains contain brown (allanite?) cores. Brown-weathering layers are calcareous. This lithology is part of the mineralized zone at the Aurora Creek and Christophosen Pb-Zn prospects. Many detrital zircon samples collected from this unit contain small populations of Middle and Late Devonian zircons and large populations of early-middle Silurian zircons as well as older populations (Till and others, 2006; Till and others, 2008a). Much or all of the unit is therefore Devonian or younger. Thickness of the unit is greater than 1.2 kilometers. A distinctive marble occurs in several outcrops that form a small lens within or adjacent to Dcs in the Nome quadrangle. The marble is white to light gray, generally impure (5 to 15 percent quartz, 1 to 5 percent white mica), and typically contains sand- to pebble-size clasts of dark gray to black, organic-rich marble. Most clast-bearing intervals appear to be clast-supported and some may be graded. Clasts are rounded to angular in shape, commonly laminated, and generally ?2 cm in diameter; a few clasts may be as much as 12 cm long. The clast-bearing marble produced conodonts with CAI values of 5 at two localities (Table A-1). One collection yielded a single element of Silurian(?) through Triassic age. The other collection, also a single conodont, is an Sb element of late Permian through Triassic (likely Triassic) age. There are three possible scenarios to explain the presence of a Triassic marble in the otherwise Devonian and older Nome Complex: (1) contamination of the conodont sample; (2) pre-metamorphic fault juxtaposition of Triassic and older rocks; and (3) a pre-metamorphic unconformable relationship between the Triassic and older rocks. The CAI of this conodont is consistent with that of other Nome Complex rocks, and no rocks that could have produced such contamination (that is, rocks of known Permian or Triassic age and high thermal level) were being processed in the laboratory at the time the Seward Peninsula sample was run (A. Harris, written commun., 2005). Thus, the clast-bearing (Triassic?) marble was juxtaposed with Dcs before or during Jurassic metamorphism or may be a rare remnant of rocks that sat unconformably atop protoliths of the Nome Complex before those protoliths were metamorphosed. Because of the uncertainties in the origin of this marble and its uniqueness, we choose not extend the protolith age of the Nome Complex to Triassic
Lithology Metamorphic

Correlated geologic units

Label DSns
Description Nome Complex, pelitic, calcareous, and graphitic schist
Geologic age Silurian to Devonian
Geologic setting Metamorphic, undivided
Lithology Form Importance
Calc-silicate-schist < Schist < Metamorphic Indeterminate, major
Mica-schist < Schist < Metamorphic Indeterminate, major
Quartz-feldspar-schist < Schist < Metamorphic Indeterminate, major
Marble < Metacarbonate < Metasedimentary < Metamorphic Incidental
Quartzite < Metaclastic < Metasedimentary < Metamorphic Incidental