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 Karl, S.M., 2012, Unpublished data.
Symbol Kgd
Unit name Granodiorite
Description Gray, medium-grained, hornblende biotite tonalite and granodiorite, with seriate plagioclase and primary epidote and garnet. Correlated with similar epidote and garnet-bearing plutons that also intrude Gravina belt rocks. Foliated to massive equigranular; average grain size is medium, fine grained near some margins; color index 15 to 30; light to medium gray fresh, weathers medium gray to dark gray; locally distinctively plagioclase porphyritic. Mineralogy includes zoned, complexly twinned plagioclase with minor alteration to sericite; mafic minerals usually hornblende greater than biotite; euhedral and subhedral epidote; and local garnet. Accessory minerals are sphene, apatite, opaque minerals, and allanite. In Petersburg quadrangle, Tonalite, granodiorite, and quartz diorite. Equigranular to sparsely porphyritic. Zoned, seriate plagioclase, interstitial biotite and hornblende, epidote, and rare garnet; porphyritic tonalite , containing zoned and seriate plagioclase up to 1.5 cm, biotite, hornblende, epidote, and garnet, sphene, apatite, and allanite. CI 15-40. Accessory sphene, allanite, apatite. CI 14-52. In Ketchikan quadrangle, massive, medium-grained, plagioclase porphyritic, biotite- and garnet-bearing, hornblende epidote quartz diorite. Typically characterized by crowded centimeter-size subhedral plagioclase phenocrysts separated by a fine to medium grained granular aggregate of the other minerals. Magmatic mineral include essential plagioclase and quartz, accessory epidote and green hornblende, and minor amounts of brown biotite, garnet, potassium feldspar, apatite, and opaque minerals. Typical samples contain conspicuous white plagioclase phenocrysts as large as 3 cm that make up 50 - 60 percent of the rock. The groundmass consists of fine grained quartz (8-20 percent), potassium feldspar (<1-10 percent), biotite (5-25 percent) and hornblende (0-15 percent). Contains magmatic anhedral to euhedral garnet and epidote in groundmass and in cores of plagioclase phenocrysts. Epidote commonly forms 2-5 percent, and locally forms over 10 percent, of the rock. Composition of the epidote varies from sample to sample, but at least some is the iron free variety (clinozoisite). Secondary minerals locally include clinozoisite epidote, white mica, and chlorite. Magmatic foliation most conspicuous near margins. On Revillagigedo Island , dominantly medium- to fine-grained, hypidiomorphic granular granodiorite. Locally it is foliated to gneissic, and has thin muscovite or biotite partings evenly spaced 2-10 mm apart. Color index ranges from 0 to 15, but it is mainly between 0 and 2. The abundance of leucocratic aplite varies from sparse patches to more than 70 percent of some outcrops; aplite contains pink garnet. Near Mount Reid this unit forms two stocks of foliated leucocratic biotite quartz monzonite containing 1 to 2 percent dark red garnet. Biotite, the only mafic mineral, makes up as much as 10 percent of the rock, and forms thin films, layers, streaks, and, locally, clots as large as 2 cm in diameter
Lithology Igneous

Correlated geologic units

Label Kmqd
Description Intermediate granitic rocks, mostly quartz diorite and tonalite, magmatic garnet and epidote 85-110 Ma
Geologic age Albian to Santonian
Geologic setting Intrusive
Lithology Form Importance
Diorite < Dioritic < Plutonic < Igneous Pluton Indeterminate, major
Quartz-diorite < Dioritic < Plutonic < Igneous Pluton Indeterminate, major
Quartz-monzodiorite < Dioritic < Plutonic < Igneous Pluton Indeterminate, major
Tonalite < Granitic < Plutonic < Igneous Indeterminate, major