Intermediate granitic rocks

Unit symbol: Klgr
Age range Late Cretaceous (83.6 to 72.1 Ma)
Lithology: Igneous - Plutonic
Group name: Intermediate granitic rocks
Granitic rocks including alaskite, granite, quartz monzonite, and dominantly granodiorite of Late Cretaceous age, generally between 85 and 70 Ma. Sparsely distributed in western and south-central Alaska, the largest exposures are the isolated pluton in the Hooper Bay quadrangle and a number of monzogranite to quartz monzodiorite plutons on the Seward Peninsula. Important additional exposures include the so-called tin granites of the Seward Peninsula (Till and others, 2010; 2011). Other areas of significant exposure are in the Talkeetna Mountains north of Anchorage and across the Susitna basin in the Tyonek quadrangle. In the Yukon-Koyukuk Basin (Shungnak, Hughes, and Melozitna quadrangles), large and small plutons of granodiorite and quartz monzonite are spatially associated with the syenitic rocks of unit Ksy, but these rocks are significantly younger than unit Ksy. A small pluton in the Circle quadrangle yields a K/Ar cooling age of 72.8 Ma and is included in this unit; however, given its setting (Wilson and others, 1984), the age may be reset and the pluton may be an older Cretaceous pluton more typical of the Yukon-Tanana Upland. Similarly, other plutons of this unit in the Circle and Big Delta quadrangle yield discordant ages and may also be thermally reset plutons of older Cretaceous age (see, for example, Smith and others, 1994). A number of small, dioritic plutons of Late Cretaceous age also occur in the eastern Taylor Mountains and Lake Clark and Iliamna quadrangles

Source map information

Source map Till, A.B., Dumoulin, J.A., Werdon, M.B., and Bleick, H.A., 2011, Bedrock geologic map of the Seward Peninsula, Alaska, and accompanying conodont data: U.S. Geological Survey Scientific Investigations Map 3131, 2 sheets, scale 1:500,000, 1 pamphlet, 75 p., and database, available at
Symbol Ktg
Unit name Tin-bearing granitic stocks
Description Stocks of biotite granite exposed in outcrop, rubble, and float in northwestern Seward Peninsula. Includes stocks at Cape Mountain, Brooks Mountain, Black Mountain, Ear Mountain, and the Oonatut Granite Complex (Figure 2). The belt also includes bodies not exposed at the surface at Lost River Mine and Kougarok Mountain. Stocks are dominantly biotite granite with lesser aplitic and pegmatitic phases (Hudson and Arth, 1983). Larger stocks, such as the Oonatut, are texturally zoned. Contacts with country rocks are sharply cross-cutting, and contact aureoles are narrow. The epizonal stocks have high 87Sr/86Sr initial ratios and elevated Th, U, Hf, and Ta (Hudson and Arth, 1983). K-Ar biotite ages range from 69.2 ¦ 2 to 80.2 ¦ 3 Ma (Hudson and Arth, 1983). Tin lodes and placers are associated with the stocks (Reed and others, 1989). Tin and tungsten were commercially mined in the Lost River area, western Teller quadrangle (Sainsbury, 1969b)
Lithology Igneous

Correlated geologic units

Label Kgl
Description Granite [< 85 Ma]
Geologic age Campanian to Maastrichtian
Geologic setting Intrusive
Lithology Form Importance
Granitic < Plutonic < Igneous Pluton Major
Quartz-diorite < Dioritic < Plutonic < Igneous Pluton Indeterminate, major
Granodiorite < Granitic < Plutonic < Igneous Pluton Indeterminate, major
Gabbro < Gabbroic < Plutonic < Igneous Pluton Incidental