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Felsic granitic rocks

Unit symbol: TKg
Age range Tertiary, Paleocene, or Late Cretaceous, Maastrichtian (72.1 to 56 Ma)
Lithology: Igneous - Plutonic
Group name: Granitic rocks of southern and interior Alaska
Fine- to coarse-grained or porphyritic, light- to dark-gray, rarely pink, granitic rocks. Unit ranges in composition from granite to quartz diorite, and includes syenite, granodiorite, and quartz monzonite. Biotite and hornblende are locally common; muscovite is uncommon. K/Ar and 40Ar/39Ar ages range from about 76 Ma to about 57 Ma; the vast majority of samples yielded ages in the range 70 to 59 Ma. Available U/Pb zircon ages fall within the same ranges. Unit includes many of the plutons shown on various source maps for this map that are commonly labeled TKg. These plutons tend to be small and are exposed in a broad belt from southwest Alaska through interior Alaska and into the Yukon. They tend to be potassium-rich, even at lower SiO2 contents, having as much as 6 percent K2O at 60 percent SiO2 in the Dillingham quadrangle (F.H. Wilson, unpub.data). Plutons of this unit are common in the western Dillingham quadrangle (Wilson, 1977) and are unusual in that they tend to have biotite and pyroxene, often orthopyroxene, as their mafic minerals, regardless of the overall pluton composition. Unit consists of hundreds of individual plutons. Many of the hot springs of interior Alaska are spatially associated with these plutons (Motyka and others, 1983). Associated mineralization includes gold, tin, and mercury (see http://ardf.wr.usgs.gov for more information about mineral resources in Alaska)

Source map information

Source map Miller, M.L., and Bundtzen, T.K., 1994, Generalized geologic map of the Iditarod quadrangle, Alaska, showing potassium-argon, major-oxide, trace-element, fossil, paleocurrent, and archaeological sample localities: U.S. Geological Survey Miscellaneous Field Studies Map MF-2219A, 48 p., 1 sheet, scale 1:250,000.
Symbol TKm
Unit name Monzonite, quartz monzonite, syenite, granodiorite, granite, and minor lamprophyre
Description Small stocks and plutons of fine- to coarse-grained, phaneritic to hypidiomorphic, clinopyroxene-biotite +/- olivine monzonite, hornblende-clinopyroxene-biotite quartz monzonite, biotite syenite, biotite-hornblende granodiorite, biotite granite, and rare lampophyric rocks.
Lithology Igneous

Correlated geologic units

Label TKg
Description Granitic rocks
Geologic age Maastrichtian to Paleocene
Geologic setting Intrusive
Lithology Form Importance
Granite < Granitic < Plutonic < Igneous Pluton Indeterminate, major
Dioritic < Plutonic < Igneous Pluton Minor
Quartz-monzonite < Syenitic < Plutonic < Igneous Pluton Minor
Diorite < Dioritic < Plutonic < Igneous Pluton Incidental