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., Haeussler, P.J., Himmelberg, G.R., Zumsteg, C.L., Layer, P.W., Friedman, R.M., Roeske, S.M.,and Snee, L.W., 2015, Geologic map of Baranof Island: U.S. Geological Survey Scientific Investigations Map SIM-3335, pamphlet 82 p., 1 sheet, scale 1:250,000.
Symbol Tegd
Unit name Granodiorite
Description Light gray, massive, medium-grained seriate, locally plagioclase porphyritic, biotite granodiorite gradational to hornblende-biotite tonalite. The granodiorite is transitional to border phases of the plutons, contains abundant inclusions of metasedimentary host rock, and is less homogenous in composition and texture than the tonalite phases of the Eocene plutons (Reifenstuhl, 1986). K-feldspar is interstitial to quartz and plagioclase in the granodiorite; orthoclase is common and perthite is rare. Plagioclase phenocrysts range from 2 mm to 8mm, and are zoned and twinned with An contents ranging from An45 to An25 (Loney and others, 1975; Reifenstuhl, 1986). Biotite is consistently dominant over hornblende. Mafic minerals are subhedral and less than 5 mm in dimension. Color index ranges from 10 to 25. The granodiorite is not magnetic. Accessory minerals include zircon, sphene, apatite, opaque minerals, and rare allanite. Minor alteration to sericite, chlorite, and epidote is common.
Lithology Igneous

Correlated geologic units

Label Tegr
Description Granite and granodiorite, Tertiary(?) Eocene?
Geologic age Eocene
Geologic setting Intrusive
Lithology Form Importance
Granitic < Plutonic < Igneous Pluton Major