Plutonic rocks and dikes, granite to diorite

Unit symbol: Kgu
Age range Cretaceous? (145 to 66 Ma)
Lithology: Igneous - Plutonic
Group name: Plutonic rocks and dikes
Consists of a variety of granitic rocks, typically granodiorite, tonalite, and quartz diorite bodies found primarily in two areas of the State. A significant area of these rocks were mapped by Nelson and others (1983) in the eastern Lake Clark quadrangle and extend into the Kenai and Tyonek quadrangles and are considered part of Alaska-Aleutian Range batholith of Reed and Lanphere (1969, 1972). They are medium- to coarse-grained, light-to medium-gray, and contain hornblende, biotite, and, rarely, muscovite; they locally have cataclastic textures. Although mapped as separate plutons by Nelson and others (1983), a number of these bodies may be fault-offset extensions of each other. Largely undated, the sparsely available K/Ar and 40Ar/39Ar dates from the west side of Cook Inlet range from 80.7 to 63.1±1.8 Ma (Reed and Lanphere, 1972; P.J. Haeussler, USGS, written commun., 2008); a number of these ages are either discordant or have disturbed spectra. These rocks are typically exposed between the Jurassic part of the batholith on the east and the early Tertiary to latest Cretaceous plutons of the batholith on the west. The second large exposure area of these plutons is in the Tanacross and Eagle quadrangles, where a significant part of these quadrangles consist of undated granitic rocks (Foster, 1970, 1976; Foster, 1992). In this area, these rocks may range in age from early Tertiary to Jurassic or possibly Triassic; the most likely ages are mid-Cretaceous or early Tertiary to latest Cretaceous. Other undated, but likely Cretaceous, plutons are in the Healy, Kateel River, Hughes, and Bendeleben quadrangles. The Hughes body is a medium- to coarse-grained albite granite, and the Kateel River plutons consist of albite granite and syenite (Patton, 1966; Patton and Miller, 1966)

Source map information

Correlated geologic units

Label Kgu
Description Granitic Rocks
Geologic age Cretaceous
Geologic setting Intrusive
Lithology Form Importance
Granite < Granitic < Plutonic < Igneous Pluton Major
Monzogranite < Granite < Granitic < Plutonic < Igneous Pluton Indeterminate, major
Granodiorite < Granitic < Plutonic < Igneous Pluton Indeterminate, major
Granodiorite < Granitic < Plutonic < Igneous Pluton Indeterminate, major
Quartz-monzonite < Syenitic < Plutonic < Igneous Pluton Indeterminate, major
Syenite < Syenitic < Plutonic < Igneous Pluton Incidental