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Source map Karl, S.M., 2013, Unpublished data.
Symbol Jt
Unit name Tonalite and quartz diorite
Description Fine- to medium-grained, subhedral, hypidiomorphic tonalite and quartz diorite. Primary minerals include quartz, plagioclase (An 35-45, andesine), biotite, hornblende, rare muscovite, garnet, sphene (Loney and others, 1975). At Kennel Creek on east Chichagof Island, body consists of buff to light gray, CI 2-10, hornblende adamellite, hornblende monzonite, and biotite alaskite. Kennel Creek body has a dioritic phase, consisting of medium gray to green , medium-grained, CI 30-50, biotite hornblende diorite, meladiorite, and syenodiorite with accessory magnetite, secondary epidote and pyrite. White to medium gray (C.I. 2-35), medium to coarse grained, biotite + hornblende tonalite and biotite + or - muscovite + or - garnet tonalite which contain a wide range of quartz to plagioclase ratios (approx. 1:4 to 1:1), crop out along the Peril Strait fault and south of Patterson Bay. Plagioclase rich samples tend to contain hornblende and have a higher color index; quartz rich samples tend to be hornblende free and leucocratic. Contains local zones of fine grained, rounded, mafic inclusions, and occasional pink pegmatitic dikes North of Patterson Bay, medium to dark gray (C.I. 20-55), medium grained, foliated, hornblende + biotite quartz diorite and biotite + hornblende tonalite. Commonly contains as much as 2 percent magnetite. Hornblende tends to be most abundant in quartz diorites; biotite most abundant in tonalites. Body at Suloia Bay consists of light to medium gray (C.I. approx. 20), coarse grained, locally porphyritic, foliated, biotite + hornblende tonalite which is generally hypidiomorphic, seriate, well foliated. Biotite is commonly more abundant than hornblende. At Fish Bay and Duffield Peninsula Medium gray, medium grained biotite hornblende tonalite. Commonly altered. Secondary minerals include calcite, albite, prehnite, epidote, chlorite, sericite, magnetite. Color index ranges from 5 to 35. Geochemical element ratios indicate calc-alkaline affinities, suggestive of a magmatic arc source. Not magnetic. Color index 5-35. Contains igneous muscovite and garnet. Duffield Peninsula and Fish Bay
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