Pelitic and quartzitic schist of the Ruby terrane

Unit symbol: PzPxrqm
Age range early Paleozoic to Proterozoic? (1000 to 419.2 Ma)
Lithology: Metamorphic
Group name: Pelitic and quartzitic schist of the Ruby terrane
Poorly exposed unit of mainly muscovite- and quartz-rich schist, and subordinate calc-schist, quartzofeldspathic schist, biotite granite, and granite gneiss that includes augen gneiss. Also includes amphibolite and quartzite metamorphosed to upper amphibolite to lower granulite facies; unit also includes minor marble and calc-silicate rocks. In the north-central Tanana quadrangle, unit has a polymetamorphic history including metamorphism to greenschist, local blueschist, and amphibolite facies. The apparent grade of metamorphism varies depending on depth of exposure and proximity to zones of ductile deformation and (or) buried plutons or other thermal hot spots (Miyaoka and Dover, 1990; Dover, 1994). Evidence for blueschist-facies metamorphism has been largely obliterated by a greenschist to granulite facies overprint in the Kokrines Hills in the Ruby quadrangle (Roeske and others, 1995). South of the Kaltag Fault, these rocks are even more poorly exposed and little studied. Unit locally is interlayered with carbonate rocks of unit |ls and therefore is, at least in part, Paleozoic in age; however, some of the unit is inferred to be as old as Proterozoic (Patton and others, 2009). Unit also appears to be intruded by orthogneiss of unit MDag. Regional metamorphism predates the widespread intrusion of the Early Cretaceous granitic bodies of unit Kmqm. Metamorphic minerals yielded K/Ar isotopic cooling ages between 144.5±1.3 Ma (white mica) and 108±3 Ma (biotite); the oldest ages were determined on glaucophane-bearing schist (Dillon and others, 1985; Miller and Bundtzen, 1994; Roeske and others, 1995). U/Pb zircon ages from gneiss range between 147.8 and 117.5 Ma (Dillon and others, 1985; Roeske and others, 1995)

Source map information

Source map Patton, W.W., Jr., Wilson, F.H., Labay, K.A., and Shew, Nora, 2009, Geologic map of the Yukon-Koyukuk basin, Alaska: U.S. Geological Survey Scientific Investigations Map SIM-2909, 26 p., 2 sheets, scale 1:500,000.
Symbol PzZsr
Unit name Pelitic schist, calc schist, and quartzite
Description Quartz-mica schist, chlorite schist, quartzofeldspathic schist, quartzite, and micaceous and graphitic schist. Regionally metamorphosed to greenschist-facies, but also locally includes high-pressure greenschist-blueschist-facies (distinguished by the presence of glaucophane) and amphibolite-facies (distinguished by the presence of sillimanite and kyanite). Andalusite-cordierite hornfels and contact schist occur in broad bands around Cretaceous granitic bodies of units Kgr and Ksy. Unit locally is interlayered with carbonate rocks of unit Pzcr and therefore is, at least in part, Paleozoic in age. However, some of the unit may be as old as Proterozoic. The regional metamorphism clearly pre-dates the widespread intrusion of the Early Cretaceous granitic bodies of units Kgr and Ksy. Widely exposed along southeast side of the Yukon-Koyukuk Basin from Ophir quadrangle to the northeast corner of Bettles quadrangle. Also included in this unit are two small areas of schist exposed in windows(?) within the Innoko terrane in the east-central Ophir quadrangle
Lithology Metamorphic

Correlated geologic units

Label PzZrqm
Description Ruby quartz-mica schist
Geologic age Neoproterozoic to Devonian
Geologic setting Metamorphic, undivided
Lithology Form Importance
Mica-schist < Schist < Metamorphic Greenschist Major
Phyllite < Metaclastic < Metasedimentary < Metamorphic Greenschist Indeterminate, major
Quartzite < Metaclastic < Metasedimentary < Metamorphic Greenschist Indeterminate, major
Slate < Metaclastic < Metasedimentary < Metamorphic Greenschist Indeterminate, major