Granitic rocks of central and southeast Alaska

Unit symbol: Kmgr
Age range Cretaceous, Coniacian to Albian (113 to 86.3 Ma)
Lithology: Igneous - Plutonic
Group name: Granitic rocks of central and southeast Alaska
Primarily granodiorite and lesser quartz diorite, granite, and quartz monzonite that is widely exposed in central and southeastern Alaska. Exposed primarily in four areas: (1) southern southeast Alaska west of the Coast Plutonic Complex; (2) in the Yukon-Tanana Upland of east-central Alaska and northern Alaska Range; (3) on Saint Lawrence Island in western Alaska; and (4) in southwest Alaska in the Bethel and Russian Mission quadrangles. In southeast Alaska, the plutons tend to be more mafic than other areas and consist of granodiorite and quartz diorite and lesser tonalite; most rocks are medium-grained and moderately foliated and lineated parallel to the fabric in country rocks. Some bodies are tabular and oriented parallel to the foliation; epidote is a common accessory mineral, and garnet is less common. Radiometric ages (K/Ar) are commonly discordant and range from 112 Ma to as young as 22.8 Ma, but most age determinations yielded Coniacian (~86 Ma) or older ages. Mid-Cretaceous plutons in the Yukon-Tanana Upland tend to be biotite granite and biotite-hornblende granodiorite with minor diorite phases and are relatively well dated. Some bodies are batholithic in size, like the Goodpaster and Mount Harper batholiths. Analyses, such as those by Wilson and others (1985), have demonstrated that emplacement of these plutons postdate regional metamorphism and reflect relatively slow cooling. Gold mineralization is commonly associated with these plutons, such as at the Fort Knox and Pogo deposits. In the Eagle and Tanacross quadrangles, radiometric dating is sparse and many of the plutons shown as part of this unit are undated. Limited dating in the adjacent northeastern Big Delta quadrangle suggests that at least some of these plutons may be of latest Cretaceous or earliest Tertiary age. A significant part of Saint Lawrence Island is made of these plutons, which consist of fine- to coarse-grained granite and subordinate granodiorite, monzonite, syenite, and alaskite (Patton and others, 2011). In southwest Alaska, this unit includes the Nyac and nearby plutons. Age control on these plutons is somewhat imprecise; K/Ar and 40Ar/39Ar ages vary between 120 and 101 Ma, and the only available U/Pb age was reported as an upper intercept on concordia described as between 129 and 104 Ma (Box and others, 1993). Atypical occurrences of the rocks of this unit are a granodiorite body associated with the Pebble copper deposit in southwest Alaska, dated to 90 Ma, and a coarse-grained biotite granite in the northern Tyonek quadrangle that was dated to 96.9 Ma (Wilson and others, 2009, 2012)

Source map information

Source map Patton, W.W., Jr., Wilson, F.H., and Taylor, T.A., 2011, Geologic map of Saint Lawrence Island, Alaska: U.S. Geological Survey Scientific Investigations Map SIM-3146, pamphlet, 13 p., 1 sheet, scale 1:250,000, and database.
Symbol Kg
Unit name Granite and granodiorite
Description Unit composed of granite and subordinate granodiorite, monzonite, syenite, and alaskite. The rocks vary in texture from coarse to fine-grained and from granitic to porphyritic and seriate. Three broad categories are recognizable: 1) granite, granodiorite, and monzonite that contain intermediate amount of quartz and hornblende as the chief mafic mineral, 2) granite, granodiorite, and alaskite that are characterized by a high quartz content and by biotite as the chief mafic mineral, and 3) monzonite and syenite that contain abundant mafic minerals but little or no quartz. Four biotite samples from this unit yielded K/Ar cooling ages of 93.5 Ma to 108 Ma (Patton and Csejtey, 1980) and two biotite samples gave Ar40/Ar39 ages of 100 Ma and 110 Ma (Amato and others, 2003). Location-Unit composes six large named plutonic bodies that are well exposed in the upland areas in the eastern and western parts of the island. In addition, numerous small exposures are scattered along incised drainages and along the shoreline in the eastern part of the island. Some of these isolated exposures may be parts of much larger plutons that are buried beneath the tundra vegetation cover. On the map we show the possible extent of several of these buried plutons. For location of the exposures, the reader is referred to the 1:250,000-scale geologic map of Patton and Csejtey (1980)
Lithology Igneous

Correlated geologic units

Label Kmg
Description Granite 85-110 Ma
Geologic age Albian to Coniacian
Geologic setting Intrusive
Lithology Form Importance
Granite < Granitic < Plutonic < Igneous Pluton Major