Heterogeneous metamorphic rocks, southeast Alaska

Unit symbol: SOmi
Age range Silurian and Ordovician (485.4 to 419.2 Ma)
Lithology: Metamorphic
Group name: Heterogeneous metamorphic rocks, southeast Alaska
Unit consists of unnamed rocks on Revillagigedo, Gravina, Annette, Mary, and Duke Islands. This heterogeneous assemblage includes metamorphosed mafic, intermediate, and subordinate felsic volcanic and volcaniclastic rocks; clastic and carbonate sedimentary rocks; and subordinate intermediate and mafic intrusive rocks. Extrusive protoliths, which are partly altered to spilite and keratophyre, include basaltic tuff, agglomerate, and pillow flows and intermediate and felsic tuff. Sedimentary protoliths include argillite, graywacke-siltstone, limestone, conglomerate, and, in the matrix of some of the pillow flows, red chert. Intrusive protoliths include medium- and coarse-grained diorite and quartz diorite. Metamorphic minerals include plagioclase, hornblende, biotite, epidote, chlorite, actinolite, sericite, almandine garnet, and possibly staurolite. Amphibolite-facies minerals define a foliation that predates randomly oriented greenschist facies minerals (Berg and others, 1988). Zircon populations do not include any inherited or entrained zircons that have ages older than Phanerozoic (Saleeby, 2000). U/Pb zircon ages ranging between 436 and 404 Ma are inferred crystallization ages for intrusive dioritic rocks (Saleeby, 2000)

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 PzSm north
Unit name Undivided metamorphosed bedded and intrusive rocks, northern area
Description Massive to locally schistose metavolcanic and metasedimentary rocks retain primary sedimentary textures. Pillow basalt and volcanic breccia, greenstone, greenschist, phyllite, marble, and metadioritic rocks. Greenschist facies metamorphic mineral assemblages include quartz, albite, k-feldspar, chlorite, colorless amphibole pseudomorphing pyroxene, sericite, epidote-clinozoisite, calcite, dolomite, prehnite, magnetite, and pyrite. Hydrothermally altered and cut by fissure veins containing dolomite, calcite, quartz, barite, hematite, and sulfide minerals.
Lithology Metamorphic

Correlated geologic units

Label SOsv
Description Metamorphosed extrusive, intrusive, and sedimentary rocks, undivided
Geologic age Ordovician to Silurian
Geologic setting Undivided
Lithology Form Importance
Amphibolite < Metamorphic Amphibolite Indeterminate, major
Metavolcanic < Metaigneous < Metamorphic Amphibolite Indeterminate, major
Metasedimentary < Metamorphic Amphibolite Indeterminate, major
Metacarbonate < Metasedimentary < Metamorphic Indeterminate, major