Younger granitic rocks

Unit symbol: Tmi
Age range Tertiary, Pliocene to Miocene (23.03 to 2.588 Ma)
Lithology: Igneous - Plutonic
Group name: Younger granitic rocks
Medium- to coarse-grained, equigranular, granodiorite to quartz diorite plutons and stocks that have hornblende, biotite, and pyroxene as mafic minerals typically surrounded by well-developed hornfels zones and sporadic hydrothermal alteration in country rocks. Includes finer grained plutons and phases. Along the Alaska Peninsula and Aleutian Islands, plutons are typically located along Pacific coast and include, but are not limited to, large plutons on Unalaska Island at Captains Bay, and at Moss Cape on the southern Alaska Peninsula, American Bay, Mitrofania Island, Devils Bay (Devils batholith, Detterman and others, 1981), Agripina Bay, Cape Igvak, and Cape Douglas (Wilson and others, 1999). In the McCarthy quadrangle of east-central Alaska, includes fine- to medium-grained hypidiomorphic granular rocks of granodiorite, subordinate granite, and local dioritic or gabbroic border zones (MacKevett, 1978; Richter and others, 2006). In southern southeast Alaska, includes a suite of alkalic granite largely in the Petersburg quadrangle, plutons of varying composition in the Petersburg, Bradfield Canal, Ketchikan, Craig, and Port Alexander quadrangles (Eberlein and others, 1983; Brew and others, 1984; Koch and Berg, 1996; Karl and others, 1999), and small, undated plugs of leucocratic, medium-grained quartz monzonite in the Skagway quadrangle (Redman and others, 1984). Radiometric ages range from about 23.5 Ma to as young as 2.1 Ma (Carr and others, 1970; Marlow and others, 1973; Citron and others, 1980; Wilson and others, 1981; Douglas and others, 1989; Wilson and Shew, 1992)

Source map information

Source map Redman, E.C., Retherford, R.M., and Hickok, B.D., 1984, Geology and geochemistry of the Skagway B-2 quadrangle, southeastern Alaska: Alaska Division of Geological and Geophysical Surveys Report of Investigations 84-31, 34 p., 4 sheets, scale 1:40,000.
Symbol Tlqm
Unit name Leucocratic quartz monzonite
Description Small plugs of leucocratic quartz monzonite, medium-grained, color index 3 or less. Average composition is 25% quartz , 30% K-feldspar, and 45% plagioclase. Contains biotite and red garnets in dikes. Scattered miarolitic cavities. Intrudes Mount Kashagnak granodiorite; Tlqm contains K-feldspar phenocrysts to 2 cm and doubly terminated beta quartz crystals near contact.
Lithology Igneous

Correlated geologic units

Label Tmog
Description Quartz monzonite
Geologic age Miocene to Burdigalian
Geologic setting Intrusive, granite
Lithology Form Importance
Quartz-monzonite < Syenitic < Plutonic < Igneous Pluton Major