Metamorphic rocks of Admiralty Island, undivided

Unit symbol: MzPza
Age range Mesozoic to Paleozoic (541 to 145 Ma)
Lithology: Metamorphic
Group name: Metamorphic rocks of Admiralty Island, undivided
Clastic sedimentary rocks, subordinate mafic to felsic volcanic rocks, thin- to thick-bedded gray carbonate, chert, and minor ultramafic rocks that have been regionally metamorphosed to slate, phyllite, greenschist, schist, gneiss, and marble in many areas. Age and grades of metamorphism have yet to be reliably determined because these rocks have undergone multiple metamorphic and deformational events. Rocks assigned to unit on Admiralty Island belong to the “undifferentiated metamorphic rocks” and the “migmatite, gneiss, and feldspathic schist” units of Lathram and others (1965) and may belong to the Gambier Bay Formation (unit Dgb) and the Retreat Group, Cannery Formation, Hyd Group, and Gravina-Nutzotin sequence (units |<rg, PDcf, ^hg, and KJgn, respectively). Lathram and others (1965) stated that “the most widespread rock types are hornblende-albite-epidote hornfels, micaceous schist, metamorphosed chert, coarse-grained marble, slate, and phyllite.” The unit, as shown here, also includes areas of migmatite, gneiss, feldspathic schist, and amphibolite mapped near exposures of plutonic rocks (Lathram and others, 1965). These rocks are primarily gray, green, and white banded hornblende-plagioclase orthogneiss (S.M. Karl, unpub. data). Also includes migmatite, schist that contains large feldspar porphyroblasts, and amphibolite. “Chloritization and sericitization are widespread in the higher grade rocks. Granite pegmatite dikes are common in the feldspathic schist * * *” (Lathram and others, 1965). S.M. Karl (unpub. data) reports a U/Pb zircon age of 227.3 ± 2.0 Ma and an 40Ar/39Ar hornblende age of 236.3±3.8 Ma on intrusive rocks that provide a minimum age for the metamorphism

Source map information

Source map Karl, S.M., Haeussler, P.J., Himmelberg, G.R., and Brew, D.A., 2002, Unpublished data.
Symbol Trgn
Unit name Gneiss
Description Gray/green and white banded hornblende-plagioclase orthogneiss. Subordinate quartz-biotite schist, gneiss, marble, and garnet-pyroxene plagioclase gneiss. Metamorphic minerals include plagioclase, hornblende, actinolite, epidote, biotite, quartz, potassium feldspar, clinopyroxene, garnet, muscovite, calcite.
Lithology Metamorphic

Correlated geologic units

Label Trgn
Description Orthogneiss, southeast Alaska
Geologic age Carnian
Geologic setting Intrusive
Lithology Form Importance
Orthogneiss < Metaigneous < Metamorphic Major
Marble < Metacarbonate < Metasedimentary < Metamorphic Indeterminate, major
Mica-schist < Schist < Metamorphic Minor