Young shallow intrusive rocks

Unit symbol: QTi
Age range Quaternary or late Tertiary (5.333 to 0 Ma)
Lithology: Igneous - Plutonic
Group name: Young volcanic and shallow intrusive rocks
Hypabyssal plugs, domes, and subordinate flows that range in composition from rhyolite to basalt at Quaternary volcanic centers along the Aleutian magmatic arc and are associated with the volcanoes of the Wrangell Mountains. In the Aleutian Islands, compositions tend to be dacite to basaltic andesite; along the Alaska Peninsula the full range in composition is present; and in the Wrangell Mountains, compositions more typically range from rhyolite to dacite. Included are quartz keratophyre dikes on widely separated parts of Attu Island. Dikes consist of chalky white euhedral feldspar (albite) crystals and glassy quartz in a bluish-gray to light-gray groundmass of fine-grained albite, quartz and minor sericite, calcite, epidote, chlorite, magnetite, and apatite (Gates and others, 1971, p. 748). Unit also includes rhyolite domes at Mount Edgecumbe in the Sitka quadrangle (Riehle and others, 1989). Unit tends to be best exposed at older volcanic centers

Source map information

Source map Richter, D.H., Waythomas, C.F., McGimsey, R.G., and Stelling, P.L., 1998, Geology of Akutan Island, Alaska: U.S. Geological Survey Open-File Report 98-135, 1 sheet, scale 1:48,000.
Symbol QTg
Unit name Gabbro
Description Small pluton of fine- to medium-grained equigranular gabbro which intrudes subaqueous volcanic rocks (QTv) in Hot Springs Bay. Pluton is highly fractured and hydrothermally altered; disseminated pyrite is locally abundant.
Lithology Igneous

Correlated geologic units

Label Qi
Description Intrusive rocks, undivided
Geologic age Pleistocene to Holocene
Geologic setting Intrusive, undivided
Lithology Form Importance
Hypabyssal-dacite < Felsic-hypabyssal < Hypabyssal < Igneous Dome Major
Hypabyssal-basaltic-andesite < Mafic-hypabyssal < Hypabyssal < Igneous Dike or sill Indeterminate, major
Hypabyssal-rhyolite < Felsic-hypabyssal < Hypabyssal < Igneous Dome Minor