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 Karl, S.M., Haeussler, P.J., and McCafferty, Anne, 1999, Reconnaissance geologic map of the Duncan Canal-Zarembo Island area, southeast Alaska: U.S. Geological Survey Open File Report 99-168, 1 sheet, scale 1:125,000.
Symbol QTd
Unit name Dikes, sills, and intrusive rocks
Description Crosscutting dikes, flows, sills, and hypabyssal intrusive rocks of mafic to felsic composition. Composition of youngest dikes may be either basalt or rhyolite. Shown as unlabeled swarms on Zarembo and Kupreanof Islands
Lithology Igneous

Correlated geologic units

Label QTi
Description Intrusive rocks
Geologic age Pliocene to Pleistocene
Geologic setting Extrusive
Lithology Form Importance
Mafic-hypabyssal < Hypabyssal < Igneous Dome Major