Schist, paragneiss, and marble

Unit symbol: DPxsgm
Age range Devonian to Neoproterozoic (1000 to 358.9 Ma)
Lithology: Metamorphic
Group name: Metasedimentary and metavolcanic rocks of the Central Belt and Northern Thrust assemblage of Till and others (2008a)
Spatially associated with the Arrigetch Peaks orthogneiss, this unit includes pelitic schist, metaquartzite, mafic schist, and rocks originally mapped as the Skajit Limestone and an orange dolomitic marble by (Nelson and Grybeck, 1980) in the Survey Pass quadrangle and banded schist by Dillon and others (1986) in the western Wiseman quadrangle. In the Survey Pass quadrangle the unit consists largely of granoblastic, massive, light-gray weathering, cream to very light-gray, fine- to medium-grained marble and orange-weathering, medium- to coarse-grained dolomitic marble. In the Wiseman quadrangle, unit is interlayered, locally gneissic, coarse-grained quartz-mica schist, quartzite, calcareous schist, marble, graphitic phyllite, and metabasite. “Upper greenschist facies rocks predominate, but includes amphibolite facies paragneiss, and local relict blueschist and eclogite facies metabasite” (Dillon and others, 1986). The informally named Ernie Lake pluton (unit Pxgn), an orthogneiss, intrudes this unit and has yielded a U/Pb zircon age of 971±5 Ma (McClelland and others, 2006), which indicates that at least part of this unit is early Neoproterozoic or older. Till and others (2008a) report that gneissic textures are particularly common where the unit is closest to metaplutonic rocks of the Arrigetch Peaks and Mount Igikpak orthogneiss (unit Dogn). Mafic rocks within the unit contain assemblages of the albite-epidote-amphibolite facies (Dusel-Bacon and others, 1989; Vogl, 2002); exposures west of the major orthogneiss bodies appear to be lower in grade (Nelson and Grybeck, 1980; Toro, 1998). Unit may, in part, represent a higher metamorphic grade equivalent of a number of units of the Brooks Range, including SCbs, Dbf, Degh, DCcs, and PzPxaqm

Source map information

Source map Nelson, S.W., and Grybeck, Donald, 1980, Geologic map of the Survey Pass quadrangle, Brooks Range, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-1176A, 2 sheets, scale 1:250,000.
Symbol Pzsgn
Unit name Undifferentiated Schist and Paragneiss
Description Medium grade schist and gneiss near margins of Mount Igikpak pluton. Common metamorphic minerals (in approximate increasing order of abundance) include garnet, biotite, muscovite, amphibole, feldspar, and quartz.
Lithology Metamorphic

Correlated geologic units

Label DZsg
Description Schist and paragneiss, Arrigetch-Igikpak thermal high
Geologic age Neoproterozoic to Devonian
Geologic setting Metamorphic, undivided
Lithology Form Importance
Paragneiss < Metasedimentary < Metamorphic Indeterminate, major
Schist < Metamorphic Indeterminate, major