Geologic Map of Alaska: Mafic igneous rocks of Duke Island

Mafic igneous rocks of Duke Island

Unit symbol:	Trdg
Age range	Middle Triassic (242 to 228 Ma)
Lithology:	Igneous - Plutonic
Group name:	Mafic and ultramafic rocks in southern Alaska

“Much of central and eastern Duke Island is underlain by a large body of pyroxene gabbro that consists of varying proportions of olivine, hypersthene, augite, plagioclase, hornblende, and minor opaque minerals (Irvine, 1974). The gabbro intrudes deformed and metamorphosed Ordovician and Silurian sedimentary and igneous rocks [unit SOgi, here] and in turn is intruded by Cretaceous ultramafic rocks [unit Kum, here] * * *” (Berg and others, 1988). Gehrels and others (1987) report a U/Pb age of 226±3 Ma

Source map information

Source map	Berg, H.C., 1973, Geology of Gravina Island: U.S. Geological Survey Bulletin 1373, 41 p., 1 plate, scale 1:63,360.
Symbol	Trci
Unit name	Intrusive rocks of the Chapin Peak Formation
Description	Basic intrusive rocks consist of altered diabase and gabbro interpreted as feeders and hypabyssal equivalents of the Chapin Peak lava flows. Hydrothermally altered, relict textures are intergranular to coarsely ophitic. Minerals include titan(?) augite, plagioclase, chlorite, kaolin (after plagioclase), prehnite, magnetite, green amphibole (after augite), brown biotite (or stilpnomelane), brown hornblende, and traces of quartz and apatite.
Lithology	Igneous

Correlated geologic units

Label	Trdg
Description	Middle Triassic age rocks of intrusive-gabbro association, Duke Island
Geologic age	Ladinian to Carnian
Geologic setting	Intrusive, gabbro

Lithology	Form	Importance
Gabbro < Gabbroic < Plutonic < Igneous		Indeterminate, major