Granodiorite and other plutonic rocks

Unit symbol: Keg
Age range Early Cretaceous, Aptian to Hauterivian (145 to 100.5 Ma)
Lithology: Igneous - Plutonic
Group name: Granodiorite and other plutonic rocks
Fine- to medium-grained hornblende granodiorite, hornblende diorite, and biotite-hornblende monzodiorite and minor quartz diorite and tonalite, generally massive but locally foliated. Unit primarily exposed in southeast and eastern south-central Alaska. In eastern south-central Alaska and the southwest Yukon, granodiorite plutons of this unit are largely exposed along the southwestern side of the Denali Fault System. In the Nabesna quadrangle, a few small plutons of the unit are exposed to the northeast of the Denali Fault System and are much more extensively exposed in the Yukon (Gordey and Makepeace, 2003). In southeast Alaska, these plutons are exposed on either side of the Chatham Strait Fault, most commonly west of the fault in the Mount Fairweather, Sitka, and Juneau quadrangles. Tonalite and trondhjemite of this unit are found on both sides of the Chatham Strait Fault in southeast Alaska as well as in the Anchorage quadrangle. Also includes minor quartz-feldspar porphyry in the Nabesna quadrangle. Radiometric dating of the rocks in this unit is sparse and many of the available K/Ar dates, as young as 91 Ma, are cooling ages and are thought to not be reflective of emplacement; other K/Ar and 40Ar/39Ar ages are as old as 128 Ma. Trondhjemite and tonalite within the Border Ranges Fault Zone of the Anchorage quadrangle has yielded multiple K/Ar, 40Ar/39Ar, and U/Pb ages between 132 and 110 Ma (Winkler, 1992; Barnett and others, 1994; Amato and Pavlis, 2010); similar trondhjemite in the Bering Glacier quadrangle is undated, but is spatially associated with rocks of the Jurassic-Cretaceous Saint Elias Suite. The isolated Khotol pluton in the Nulato quadrangle of western Alaska is a large (12 km by 20 km), poorly exposed, coarsely porphyritic granite that has large K-feldspar phenocrysts in a moderately coarse groundmass and has yielded an age of 112 Ma K/Ar (Patton and others, 1984)

Source map information

Source map Nokleberg, W.J., Aleinikoff, J.N., Lange, I.M., Silva, S.R., Miyaoka, R.T., Schwab, C.E., and Zehner, R.E., with contribution for selected areas from Bond, G.C., Richter, D.H., Smith, T.E., and Stout, J.H., 1992, Preliminary geologic map of the Mount Hayes quadrangle, eastern Alaska Range, Alaska: U.S. Geological Survey Open File Report 92-594, scale 1:250,000 [Received as CAD coverage produced in 1996].
Symbol grs2
Unit name Granitic unit 2
Description Chiefly hornblende-biotite granodiorite and hornblende granodiorite and minor granodiorite and granite dikes, stocks, and small plutons. Includes sparse syenodiorite and quartz diorite. Fine- to medium-grained, hyautomorphic granular texture. Locally metamorphosed to lower greenschist facies.
Lithology Igneous

Correlated geologic units

Label Keg
Description Granodiorite, quartz diorite, syenodiorite, and tonalite [>110 Ma]
Geologic age Berriasian to Albian
Geologic setting Intrusive
Lithology Form Importance
Granodiorite < Granitic < Plutonic < Igneous Pluton Major
Granite < Granitic < Plutonic < Igneous Pluton Indeterminate, major
Quartz-monzonite < Syenitic < Plutonic < Igneous Pluton Indeterminate, major
Diorite < Dioritic < Plutonic < Igneous Pluton Minor
Quartz-diorite < Dioritic < Plutonic < Igneous Pluton Minor
Trondhjemite < Tonalite < Granitic < Plutonic < Igneous Pluton Minor
Gabbro < Gabbroic < Plutonic < Igneous Pluton Incidental