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Source map Karl, S.M., and Baichtal, James, 2013, Unpublished data.
Symbol Kd
Unit name Syenite, monzonite, and diorite
Description In Skagway and Mount Fairweather quadrangles, hornblende biotite diorite, hornblende biotite quartz diorite, lesser amounts quartz monzonite, quartz monzonite gneiss, granodiorite, and hornblendite. Color index in diorite is 15-25. Granodiorite contains 15% biotite, 5% hornblende, 15-20% quartz. Plutons west of William Henry Bay mostly sheared augite biotite diorite, augite diorite. At Sullivan Island biotite quartz diorite gneiss, granodiorite gneiss, biotite diorite. Unit includes diabase northwest of William Henry Bay. On Chichagof Island, gray, color index 25-35, fine- to medium-grained, hornblende diorite and biotite-hornblende quartz-diorite. The amphibole is called uralite by Loney and others, (1975), because it clearly surrounds and replaces pyroxene. The diorite also contains lighter colored, younger amphibole.includes uralitized biotite-augite diorite and uralitized biotite-hypersthene diorite, accessory sphene and apatite, very magnetic. On Admiralty Island, biotite-hornblende quartz diorite is inequigranular, hypidiomorphic. On Prince of Wales Island, includes massive, medium to coarse grained diorite,CI 15-40; contains chloritized brown biotite, 5-20 %diopsidic(?) augite, 10-25 % green hornblende, locally in a reaction relationship with augite, magnetite, tabular, zoned andesine plagioclase, interstitial anhedral K-feldspar and quartz, euhedral sphene, and up to 5% anhedral apatite
Lithology Igneous

Correlated geologic units

Label Keg
Description Diorite, quartz diorite and syenodiorite [>110 Ma]
Geologic age Aptian to Albian
Geologic setting Intrusive
Lithology Form Importance
Diorite < Dioritic < Plutonic < Igneous Pluton Major
Quartz-diorite < Dioritic < Plutonic < Igneous Pluton Major
Granodiorite < Granitic < Plutonic < Igneous Pluton Minor