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 Koch, R.D., and Berg, H.C., 1996, Reconnaissance geologic map of the Bradfield Canal quadrangle, southeastern Alaska: U.S. Geological Survey Open-File Report 81-728A, scale 1:250,000.
Symbol Tgb
Unit name Gabbro
Description Gabbro - biotite-pyroxene gabbro and biotite-olivine gabbro, massive, medium-grained, and hypidiomorphic granular, intrudes units Kqd and MzPzsp.
Lithology Igneous

Correlated geologic units

Label Tgg
Description Gabbro,
Geologic age Miocene
Geologic setting Intrusive, gabbro
Lithology Form Importance
Gabbro < Gabbroic < Plutonic < Igneous Pluton Major