Geologic Map of Alaska: Metagranitic rocks

Metagranitic rocks

Unit symbol:	Zngn
Age range	Neoproterozoic (850 to 541 Ma)
Lithology:	Metamorphic
Group name:	Layered sequence (Nome Complex)

Pale-gray to pale-greenish-tan, locally green-weathering, small granitic, granodioritic, and tonalitic orthogneiss bodies in rubble-crop and outcrop (Till and others, 2011). Weakly to well foliated, fine- to coarse-grained, with foliation defined by weak to strong alignment of micas. Coarsest grained varieties may contain lozenge-shaped feldspar grains and thin, millimeter-thick lenses of quartz that also parallel foliation. Plagioclase and quartz, ± microcline, are dominant phases; chlorite, biotite, and white mica are minor phases and are associated with epidote, garnet, and calcite at some localities (Till and others, 2011). Phengitic white mica compositions in microcline-bearing orthogneiss bodies are consistent with crystallization at blueschist-facies conditions (Evans and Patrick, 1987). In outcrop, foliation parallels the surrounding schist; the metagranitic rocks were apparently folded in with metasedimentary rocks of the Nome Complex during the Mesozoic. Detrital zircons from metasedimentary rocks of unit Ocs of Till and others (2011), collected immediately adjacent to part of this unit in east-central Nome quadrangle, include a significant 600-Ma population, which is younger than the metagranitic rock (669±5 Ma; Till and others, 2006b). Late Proterozoic (Neoproterozoic) intrusive age based on several U/Pb zircon analyses that range from approximately 685 to 665 Ma (Patrick and McClelland, 1995; Amato and Wright, 1998; Till and others, 2006b, 2011)

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, 1 pamphlet, 57 p., 2 plates, scale 1:500,000, and database.
Symbol	Zn
Unit name	Metagranitic rocks, Nome Complex
Description	Pale gray to pale greenish tan, locally green-weathering rubble crop and outcrop of small granitic, granodioritic, and tonalitic orthogneiss bodies. Weakly- to well-foliated, fine- to coarse-grained, with foliation defined by weak to strong alignment of micas. Coarsest grained varieties may contain lozenge-shaped feldspar grains and thin, mm-thick lenses of quartz that also parallel foliation. Plagioclase, quartz, ¦ microcline are the dominant phases; chlorite, biotite, and white mica are minor phases, and are associated with epidote, garnet, and calcite at some localities. Accessory phases include opaque oxides, zircon, apatite, allanite, and titanite. In outcrop foliation parallels surrounding schists of the Nome Complex; the metagranitic rocks were apparently folded in with metasedimentary rocks of the Nome Complex during the Mesozoic. Detrital zircons from metasedimentary rocks of unit Ocs collected immediately adjacent to the Basin Creek body (669 ¦ 5 Ma, Table 3; locality 3, Sheet 2) include a significant 600-Ma population, younger than the metagranitic rock (Till and others, 2006). Phengitic white mica compositions in microcline-bearing orthogneiss bodies are consistent with crystallization at blueschist-facies conditions (Evans and Patrick, 1987). Late Proterozoic intrusive age based on several U-Pb zircon analyses that range from approximately 665 to 685 Ma (including errors) (Table 3; Patrick and McClelland, 1995; Amato and Wright, 1998; Till and others, 2006). Equivalent to parts of "pCPzg" of Bundtzen and others (1994) and "pCdog" and parts of unit "pCPzuog" of Amato and Miller (2004)
Lithology	Metamorphic

Correlated geologic units

Label	pCgn
Description	Metagranitic rocks, Nome Complex
Geologic age	Cryogenian to Ediacarian
Geologic setting	Intrusive, granite

Lithology	Form	Importance
Orthogneiss < Metaigneous < Metamorphic		Major