Granitic intrusive rocks of the Chugach accretionary complex

Unit symbol: Tpgi
Age range Tertiary, Paleocene (66 to 56 Ma)
Lithology: Igneous - Plutonic
Group name: Granitic intrusive rocks of the Chugach accretionary complex
Medium-grained biotite granodiorite, quartz monzonite, and granite plutons that generally exhibit hypidiomorphic-granular texture and locally contain potassium feldspar phenocrysts as long as 1 cm. Rare muscovite is found in limited areas near contacts in the Shumagin Islands and on Kodiak Island. A minor hornfels zone as wide as 500 m is mapped at contact with Shumagin Formation (included in unit Kaf, here) (Moore, 1974a). Plutons of this unit form a belt from Sanak Island in the False Pass quadrangle at the southwest end of the Alaska Peninsula, through the outer Shumagin Islands, Semidi Islands, and Kodiak Island to the Kenai Peninsula. These plutons form an erosion-resistant core of the islands offshore of the Alaska Peninsula and form an extensive batholith on Kodiak Island, as well as many satellite plutons on Kodiak and adjacent islands. On the Kenai Peninsula, unit is primarily “medium- to dark-gray foliated medium- to coarse-grained biotite-muscovite-(hornblende) granite and granodiorite; marginal phases are locally biotite-muscovite-(hornblende) tonalite” (Tysdal and Case, 1979). Consists of a large batholith that extends from Nuka and Aialik Bays and offshore islands northward more than 60 km into the Harding Icefield, where it is exposed in many nunataks, and westward into the Seldovia quadrangle around Harris Bay. Potassium-argon and 40Ar/39Ar ages reported by Burk (1965), Moore (1974a, b), Kienle and Turner (1976), Wilson (1980), Hill and others (1981), Shew and Wilson (1981), Moore and others (1983), Haeussler and others (1995), Bradley and others (1999; 2000), and (Lytwyn and others, 2000) range from 65.6±3.3 to 49.6±1.7 Ma and U/Pb age determinations range between 61.1±0.5 to 56.0±0.5 Ma (Bradley and others, 2000, Farris and others, 2006). The most reliable of both types of age determinations show a trend of slight younging to the northeast from Sanak to Kodiak Island (750 km) and then a more significant decrease in age on the Kenai Peninsula (250 km more). These plutons, as well as some of the plutons in map unit Toegr are thought to represent a belt of near-trench, “slab-window” intrusions related to the subduction of a spreading center as the spreading center passed along the continental margin (Marshak and Karig, 1977; Bradley and others, 1992, 2000; Farris and others, 2006). Lytwyn and others (2000) described a suite of dikes (part of map unit Tehi, here) that they associated with the same ridge-subduction-related system thought responsible for these plutons

Source map information

Source map Tysdal, R.G., and Case, J.E., 1979, Geologic map of the Seward and Blying Sound quadrangles, Alaska: U.S. Geological Survey Miscellaneous Investigations Series Map I-1150, scale 1:250,000.
Symbol Tgh
Unit name Granite of Harding Icefield region
Description Medium- to dark-gray foliated medium- to coarse-grained biotite-muscovite-(hornblende) granite and granodiorite; marginal phases are locally biotite-muscovite-(hornblende) tonalite; mafic minerals, almost wholly biotite, constitute 20 to 30 percent of minerals in tonalitic rocks, 15 to 20 percent of minerals in tonalitic rocks, 15 to 20 percent of granodiorite, and 5 to 10 percent of granite; muscovite, a late but primary mineral, commonly makes up less than 2 percent.
Lithology Igneous

Correlated geologic units

Label Tg
Description Granitic rocks, including early Tertiary granitic intrusive rocks of the Kodiak, Shumagin, Semidi, and Sanak Islands
Geologic age Paleocene
Geologic setting Intrusive
Lithology Form Importance
Granitic < Plutonic < Igneous Pluton Major
Granite < Granitic < Plutonic < Igneous Pluton Major
Quartz-diorite < Dioritic < Plutonic < Igneous Pluton Minor
Granodiorite < Granitic < Plutonic < Igneous Pluton Minor
Tonalite < Granitic < Plutonic < Igneous Pluton Minor
Quartz-monzonite < Syenitic < Plutonic < Igneous Pluton Minor
Gabbro < Gabbroic < Plutonic < Igneous Pluton Incidental
Syenite < Syenitic < Plutonic < Igneous Pluton Incidental