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 Werdon, M.B., Newberry, R.J., Szumigala, D.J., and Pinney, D.S., 2001, Geologic map of the Eagle A-2 quadrangle, Fortymile mining district, Alaska: Alaska Division of Geological and Geophysical Surveys Preliminary Interpretive Report 2001-3A, 1 sheet, scale 1:63,360.
Symbol Kw
Description Sub-equigranular, medium-grained, hornblende biotite granodiorite and lesser tonalite herein called the Walker Fork pluton. Typical primary modal mineralogy is 25–30 percent quartz, 30–55 percent plagioclase, 10–20 percent poikilitic K-feldspar, 8–15 percent biotite, 0–5 percent hornblende, 1 percent myrmekite, 1 percent opaques, and trace apatite and sphene. Distinguished from nearby Jurassic and Triassic plutons by large, prominent biotite crystals and lack of foliation. Locally cut by garnet-bearing, aplite–pegmatite dikes with secondary pyrite and calcite. The pluton is commonly altered, with 0–20 percent sericite–chlorite–epidote–carbonate–rutile–pyrite, mostly after plagioclase and mafic minerals. Magnetic susceptibility is high, typically 3–30 x 10 SI. Pluton is highly fractured near high-angle faults. As demonstrated by presence and absence of hornfels zones, both intrusive and fault contacts with surrounding gneissic rocks are present. This body is not known to contain or be spatially associated with metallic mineralization or placer gold, although it is cut by chalcedonic quartz veins. Ar/ Ar biotite plateau age of 100 Ma(sample 2; table 1); biotite from nearby gneiss yielded a K-Ar reset age of 99Ma (Wilson and others, 1985).
Lithology Igneous

Correlated geologic units

Label Kmgd
Description Intermediate granitic rocks, mostly granodiorite ( 85-110 Ma)
Geologic age Albian to Santonian
Geologic setting Intrusive
Lithology Form Importance
Dioritic < Plutonic < Igneous Pluton Indeterminate, major
Granodiorite < Granitic < Plutonic < Igneous Pluton Indeterminate, major