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., Dumoulin, J.A., and Miller, M.L., 1985, Geologic map of the Chugach National Forest: U.S. Geological Survey Miscellaneous Field Studies Map MF-1645B, pamphlet, 16 p., 1 sheet, scale 1:250,000 (Chugach NF Arc/Info coverage 1993).
Symbol Tg
Unit name Granite and granodiorite
Description The central part of the plutons are dominantly light-gray, medium- to coarse-grained granite with color index ranging from 3 to 8. The plutons grade outward to fine- to medium-grained, more mafic mineral-rich (color indices of 10 to 20) margins of granitic, granodioritic, and sometimes tonalitic compositions. Hypidiomorphic-granular texture is most common, but equigranular as well as porphyritic varieties also exist. Primary biotite is the most abundant mafic mineral and is sometimes accompanied by hornblende. Towards the pluton margins, however, hornblende commonly dominates over biotite. Optically determined plagioclase compositions range from oligoclase to andesine for all rocks of this unit.
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