Granitic rocks

Unit symbol: Toegr
Age range Tertiary, early Oligocene and Eocene (56 to 28.1 Ma)
Lithology: Igneous - Plutonic
Group name: Granitic rocks in southern Alaska
Granitic rocks that range in composition from granite to diorite in two belts in southern Alaska. The generally older belt occurs east and southeast from Cordova and typically has U/Pb, K/Ar, and 40Ar/39Ar ages that rangefrom about 53 to 42 Ma. This belt, composed of rocks that range in composition from biotite- and hornblende-bearing granite to tonalite, is distinguished by the almost ubiquitous presence of tonalite phases associated with the plutons. These older plutons intrude Orca Group and Valdez Group rocks and in southeast Alaska are restricted to within a few tens of kilometers of the Gulf of Alaska coast. There is a weak tendency for these plutons to get younger to the southeast. In the Mount Fairweather and Skagway quadrangles, some of the plutons have migmatitic zones. The second, slightly younger belt, exposed in the western Alaska Range and on Adak Island in the Aleutian Islands, ranges in age from about 50 to 32 Ma. Compositionally, plutons in this younger belt range from biotite-bearing granite to biotite- and hornblende-bearing granodiorite; tonalite phases, conspicuously, are not reported. On Adak, the plutons have K/Ar and 40Ar/39Ar ages between 35 and 32 Ma; in the mainland part of the belt, ages range from 55 to 32 Ma and are older to the northeast. An outlier from these belts is found intruding the Ghost Rocks Formation on Kodiak Island; it consists of altered granitic rocks and a fission-track zircon age was 50 Ma (Clendenen, 1991); other plutons intruding the Ghost Rocks Formation, however, have yielded a number of K/Ar ages from 63 to 62.1 Ma (Moore and others, 1983). The plutons yielding these older ages are usually assigned to the Kodiak batholith (unit Tpgi), but that may be inappropriate because these plutons intrude the younger early Tertiary and latest Cretaceous Ghost Rocks Formation (unit TKm, here), whereas the Kodiak batholith is generally restricted to plutons that intrude the Cretaceous Kodiak Formation (included in unit Kaf, here). Another outlier, which intrudes undated peridotite on Saint George Island in the Pribilof Islands, yielded K/Ar ages of 57 to 49.5 Ma (Barth, 1956; Hopkins and Silberman, 1978)

Source map information

Source map Karl, S.M., 2013, Unpublished data
Symbol Toet
Unit name Tonalite
Description Light gray medium-grained hornblende-biotite tonalite, gradational to subordinate granodiorite and trondhjemite. Color index ranges from 15 to 30. Tonalites are not magnetic. Biotite exceeds hornblende, and the pluton contains up to 5 per cent red almandine garnet, as large as 4 mm in diameter, as noted by Loney and others (1975, p. 47). The unit is mainly represented by the Kasnyku Lake pluton, named by Loney and others (1975), which includes several ages of intrusions. Loney and others (1975) report that “trondhjemite is an abundant and characteristic rock type that occurs as dikes and irregular plutons intruding the hornblende tonalite” around Kasnyku Bay and west of Takatz Lake. Pegmatite dikes intrude both phases. Loney and others (1975) also describe a younger phase of tonalite that intrudes the Kasnyku Lake tonalite in the vicinity of Warm Springs Bay. The different phases of the Kasnyku Lake pluton have not been systematically mapped previously or in our study. Unit includes the Vodopod pluton west of Nelson Bay and the small stock in the bay south of Nelson Bay. In Glacier Bay area includes intermediate composition plutons associated with Mt. Crillon- La Perousse mafic and ultramafic bodies. On Yakobi Island includes Yakobi Peak body associated with Bohemia Basin norite. On north Chichagof Island at Idaho Inlet and along Peril Strait, elongate bodies along faults suggest minimum age for faults
Lithology Igneous

Correlated geologic units

Label Toem
Description Granodiorite to tonalite
Geologic age Bartonian to Oligocene
Geologic setting Intrusive, tonalite-granodiorite
Lithology Form Importance
Granodiorite < Granitic < Plutonic < Igneous Pluton Major
Quartz-diorite < Dioritic < Plutonic < Igneous Pluton Indeterminate, major