Granitic rocks of southeast Alaska

Unit symbol: Jise
Age range Jurassic (174.1 to 145 Ma)
Lithology: Igneous - Plutonic
Group name: Plutonic rocks
Consists of a variety of intrusive rocks including quartz monzonite, monzonite, tonalite, and quartz diorite on Chichagof and Baranof Islands generally west of or along the Peril Strait Fault (Loney and others, 1967; Karl, 1999). Dominant lithologies are white to medium-gray, medium- to coarse-grained, biotite-hornblende tonalite and garnet-muscovite-biotite tonalite, which have a wide range of quartz-to-plagioclase ratios (from approximately 1:4 to 1:1). Plagioclase-rich samples tend to contain hornblende and have a higher color index; quartz-rich samples tend to be hornblende free and leucocratic (Johnson and Karl, 1985). The tonalite contains local zones of fine-grained, rounded, mafic inclusions and occasional pink pegmatitic dikes. Subordinate to the tonalite is medium- to dark-gray-green, medium- to coarse-grained, foliated biotite-hornblende quartz diorite in which hornblende is always more abundant than biotite. Pyroxene-cored hornblende crystals are locally common (Johnson and Karl, 1985). K/Ar ages range from 182 to 143 Ma (Loney and others, 1967; Karl and others, 1988; S.M. Karl, unpub. data) and are commonly significantly discordant; the youngest ages are from samples collected close to the Peril Strait Fault. A single body is also found north of Tenakee Springs, east of the fault on Chichagof Island; it consists of buff to light-gray hornblende-quartz monzonite, hornblende monzonite, and biotite alaskite (Karl, 1999) and yields a K/Ar hornblende age of 147±7 Ma. S.M. Karl (unpub. data) also reports the presence of small areas of Jurassic plutonic rocks in the Tracy Arm area of the Sumdum quadrangle

Source map information

Source map Johnson, B.R., and Karl, S.M., 1985, Geologic map of western Chichagof and Yakobi Islands, southeastern Alaska: U.S. Geological Survey Miscellaneous Investigations Series Map I-1506, pamphlet, 15 p., 1 sheet, scale 1:125,000.
Symbol KJq
Unit name Quartz diorite
Description A medium to dark gray green (C.I. 25-50), medium to coarse grained, foliated hornblende + or - biotite quartz diorite, in which hornblende is always more abundant than biotite. One to 2 percent magnetite is common in these rocks, some samples contain as much as 5 percent. Pyroxene cored hornblende crystals are common locally.
Lithology Igneous

Correlated geologic units

Label Jsse
Description Sodic granitic rocks of southeast Alaska
Geologic age Middle-Jurassic to Jurassic
Geologic setting Intrusive, tonalite-granodiorite
Lithology Form Importance
Quartz-diorite < Dioritic < Plutonic < Igneous Pluton Major
Tonalite < Granitic < Plutonic < Igneous Pluton Major
Diorite < Dioritic < Plutonic < Igneous Pluton Minor