Syenite, trondhjemite, and granite

Unit symbol: DOgi
Age range Early Devonian to Ordovician? (458.4 to 387.7 Ma)
Lithology: Igneous - Plutonic
Group name: Older plutonic rocks of southeast Alaska
Exposed in southeast Alaska in three areas. On Chichagof Island in the Sitka quadrangle, the unit consists of a plutonic complex of trondhjemite and subordinate syenite, monzonite amd quartz monzonite. The trondhjemite is dominantly biotite trondhjemite and hornblende-biotite trondhjemite, but includes subordinate hornblende trondhjemite. Syenite is predominantly hornblende syenite, but includes subordinate sodalite syenite and sodalite-nepheline syenite. Also present on Chichagof Island is hornblende-bearing biotite monzonite, hornblende-bearing biotite syenodiorite, and biotite quartz monzonite (Loney and others, 1975). U/Pb TIMS-multigrain zircon dates range between 378 and 353 Ma (S.M. Karl, unpub. data), whereas existing K/Ar dates are significantly younger and discordant, at 121.9±5 and 253.1±10 Ma on biotite and hornblende, respectively (Lanphere and others, 1965). The second area, in the Craig quadrangle, rocks mapped by Eberlein and others (1983) as Cretaceous diorite have yielded a 40Ar/39Ar age of 410 Ma (S.M. Karl, unpub. data) and are included here, presuming the age is a minimum age. In the Dixon Entrance quadrangle on southern Prince of Wales Island, leucocratic biotite ± aegirine ± arfvedsonite ± garnet syenite and subordinate leucodiorite yield a number of U/Pb TIMS-multigrain zircon upper intercept ages in the Ordovician (Gehrels and Saleeby, 1987). The plutons in the Craig and Dixon Entrance quadrangles are part of large igneous complex that consists of Ordovician granitic rocks (unit Ogi), the Bokan Mountain complex of Jurassic age (unit Jag), and these Early Devonian to possibly Ordovician plutons. Finally, in the Ketchikan and Prince Rupert quadrangles on Annette and Gravina Islands, rocks ranging from trondhjemite to granite to quartz diorite form another igneous complex (Loney and others, 1975, Berg and others, 1988, Karl, 1992, Brew, 1996) of Silurian age (Gehrels and others, 1987)

Source map information

Source map Gehrels, G.E., 1992, Geologic map of southern Prince of Wales Island, southeastern Alaska: U.S. Geological Survey Miscellaneous Investigations Series Map I-2169, 23 p., 1 sheet, scale 1:63,360.
Symbol SOsy
Unit name Quartz syenite and granite
Description Massive, fine- to medium-grained, gray-, reddish gray-, and maroon-weathering quartz syenite and granite in the Nichols Bay area. These rocks grade into the pyroxenite and hornblendite unit. Rocks have CI less than 20, and consist of interlocking microperthite, quartz, and plagioclase, which occurs as tabular grains to 3 mm long, highly altered to white mica, calcite, and epidote. Ratio of microperthite to plagioclase is generally about 2:1. Quartz constitutes 5-30 % of rock. Also includes glomerocrysts of green hornblende and brown biotite altered to chlorite, opaque minerals, zircon, medium brown garnet, and sphene.
Lithology Igneous

Correlated geologic units

Label SOmi
Description Metamorphosed intermediate and mafic plutonic rocks
Geologic age Silurian to Pragian
Geologic setting Intrusive
Lithology Form Importance
Orthogneiss < Metaigneous < Metamorphic Amphibolite Major
Orthogneiss < Metaigneous < Metamorphic Greenschist Indeterminate, major