Granite and orthogneiss

Unit symbol: Zgn
Age range Neoproterozoic? (1600 to 541 Ma)
Lithology: Metamorphic
Group name: Gneiss of northern Alaska
Granitic and metagranitic rocks of presumed Proterozoic age are exposed in the southern Baird Mountains and western Wiseman quadrangles and in an isolated exposure in the Black River quadrangle of east-central Alaska. In the Baird Mountains quadrangle, light-gray, medium- to fine-grained, foliated to gneissic metamorphosed granite is associated with unit D<acs. This body is coarsely porphyritic and contains centimeter-sized albite porphyroblasts; Karl and Aleinikoff (1990) reported a fairly discordant upper intercept TIMS U/Pb zircon age of 705±35 Ma. Granodiorite that intrudes the metamorphic complex at Mount Angayukaqsraq, in the northeastern Baird Mountains quadrangle, yielded a nearly concordant TIMS U/Pb zircon age of 750.4±6.3 Ma (Karl and others, 1989b). Other exposures in the Baird Mountains quadrangle are undated and may be of Proterozoic age or of Devonian age, as is relatively common in the Brooks Range. To the east, in the Central belt of Till and others (2008a), Proterozoic metagranitic rocks span the boundary between the Survey Pass and Wiseman quadrangles. The Ernie Lake pluton yielded a SHRIMP U/Pb zircon age of 971±5 Ma (McClelland and others, 2006); Dillon and others (1980) reported TIMS analyses of zircons from both the Ernie Lake orthogneiss and nearby Sixtymile River orthogneiss bodies yielded discordant zircons likely indicating a Proterozoic age. There is no age control for the exposure in the Black River quadrangle; it is a small exposure of granite (Brabb, 1970) intruding phyllite of presumed Precambrian age (shown here as unit CPxt)

Source map information

Source map Dillon, J.T., Brosge, W.P., and Dutro, J.T., Jr., 1986, Generalized geologic map of the Wiseman quadrangle, Alaska: U.S. Geological Survey Open-File Report 86-219, 1 sheet, scale 1:250,000.
Symbol Zg
Unit name Granite gneiss
Description Blastoporphyritic, foliated, coarse-grained, biotite granite orthogneiss. Banded schist, paragneiss, and orthogneiss that may have been regionally metamorphosed three times. Upper greenschist facies rocks predominate, but includes amphibolite facies paragneiss, and local relict blueschist and eclogite facies metabasite.
Lithology Metamorphic

Correlated geologic units

Label Zog
Description Ernie Lake pluton orthogneiss
Geologic age Mesoproterozoic to Proterozoic
Geologic setting Intrusive
Lithology Form Importance
Biotite-gneiss < Gneiss < Metamorphic Major
Orthogneiss < Metaigneous < Metamorphic Major