Granite and granodiorite

Unit symbol: Togr
Age range Tertiary, Oligocene (38 to 23.03 Ma)
Lithology: Igneous - Plutonic
Group name: Granitic rocks in southern Alaska
Largely consists of granite and lesser granodiorite distributed around the Gulf of Alaska, from the Lime Hills quadrangle in the west to as far as the Ketchikan quadrangle in southeast Alaska. Rocks tend to be light-gray, medium- to coarse-grained, leucocratic biotite granite and granodiorite. Larger stocks commonly grade from marginal zones containing more biotite (and locally hornblende) to more leucocratic zones inward; large parts of some intrusions are porphyritic and contain orthoclase phenocrysts in a medium-grained groundmass (Tysdal and Case, 1979; Nelson and others, 1999). Common textures are allotriomorphic granular and hypidiomorphic granular. In southeast Alaska, consists of biotite ± hornblende ± pyroxene granite, alkali granite, quartz monzonite, and subordinate syenite, granodiorite, and diorite (Berg and others, 1988; S.M. Karl, unpub. data). Radiometric ages range from about 37 to 23 Ma

Source map information

Source map Nelson, S.W., Miller, M.L., Haeussler, P.J., Snee, L.W., Phillips, P.J., Huber, Carol, 1999, Preliminary geologic map of the Chugach National Forest Special Study Area, Alaska: U.S. Geological Survey Open-File Report 99-362, 1 sheet, scale 1:63,360.
Symbol Tg
Unit name Leucocratic Granite
Description Light colored, leucocratic biotite granite. Color index 5. Common textures are allotriomorphic granular and hypidiomorphic granular. Comprises several igneous bodies - plutons of Cedar Bay, Terentiev Lake, Granite Cove, and Columbia Glacier stock and an unnamed elongate body northeast of Miners Lake. Also associated with pluton of Miners Bay. Correlated with other 34-Ma plutons in Prince William Sound, however, is more leucocratic and contains greater amounts of silica and alkalis
Lithology Igneous

Correlated geologic units

Label Togr
Description Granitic rocks
Geologic age Priabonian to Chattian
Geologic setting Intrusive, granite
Lithology Form Importance
Alkali-feldspar-granite < Granitic < Plutonic < Igneous Pluton Major
Granite < Granitic < Plutonic < Igneous Pluton Major