Granodiorite, quartz diorite, and diorite

Unit symbol: Tod
Age range Tertiary, Oligocene and late Eocene (56 to 23.03 Ma)
Lithology: Igneous - Plutonic
Group name: Granitic rocks in southern Alaska
Compositionally variable suite of medium- to coarse-grained, grayish-white, mafic granitic rocks exposed along the southern coastal region of the state. Unit occurs as far west as the Seguam quadrangle to as far east as the Ketchikan quadrangle and as far north as the McCarthy quadrangle. Commonly granodiorite and quartz diorite; also includes diorite and tonalite. Along the west side of Cook Inlet, the plutons are considered part of the Tertiary phase of the Alaska-Aleutian Range batholith of Reed and Lanphere (1969, 1972) and tend to be exposed along its western margin. On the Alaska Peninsula, these plutons are associated with the Meshik magmatic arc and, in the Aleutian Islands, with the “Lower Series” magmatic rocks of Vallier and others (1994). In the eastern Gulf of Alaska area and northern southeast Alaska, associations are not as readily apparent; plutons of this age and composition intrude a variety of terranes. Potassium-argon ages for these plutons range from about 39 to 25 Ma. Hornblende diorite in the Skagway quadrangle is undated but may be Oligocene in age (Gilbert and others, 1987)

Source map information

Source map Plafker, George, 2002, Unpublished data
Symbol Tegd
Unit name Biotite granodiorite
Description Unfoliated light to medium gray, medium-grained hornblende-biotite granodiorite, but ranging from fine- to coarse-grained in the Fairweather Range and east of the Border Ranges Fault zone. Composition ranges from tonalite to quartz monzodiorite. Locally foliated. In the Skagway B-3 quadrangle (Gilbert and others, 1987) the unit includes igneous breccia that is mixed border phases of hornblende diorite, hornblendite, and fine-grained porphyritic diorite in a matrix of granodiorite
Lithology Igneous

Correlated geologic units

Label Togd
Description Granodiorite and quartz diorite
Geologic age Lutetian to Aquitanian
Geologic setting Intrusive, tonalite-granodiorite
Lithology Form Importance
Quartz-diorite < Dioritic < Plutonic < Igneous Pluton Major
Granodiorite < Granitic < Plutonic < Igneous Pluton Major
Tonalite < Granitic < Plutonic < Igneous Pluton Minor