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 Karl, S.M., 1992, Map and table of mineral deposits on Annette Island, Alaska: U.S. Geological Survey Open-File Report 92-690, 57 p., 1 map sheet, scale 1:63,360.
Symbol SOu
Unit name Volcanic, sedimentary and intrusive rocks, undivided
Description Heterogenous assemblage of metamorphosed and highly deformed mafic to intermediate and minor felsic volcanic rocks, clastic and carbonate rocks, and subordinate mafic to intermediate intrusive rocks. Volcanic rocks are dominant, and include calc-alkaline pillow basalt flows with interpillow red chert, pillow breccia, , agglomerate, breccia, and tuff. Volcanic rocks are interbedded with tuffaceous carbonate rocks, massive to bedded marble, calcareous conglomerate, graywacke, and dark gray phyllite. Ubiquitous minor disseminated sulfides. Greenschist facies mineral assemblages include chlorite, epidote-clinozoisite, albite, actinolite,sericite, calcite, dolomite, quartz, hematite, and pyrite.
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