Retreat Group

Unit symbol: PzPxrg
Age range Paleozoic and Proterozoic (1000 to 443.4 Ma)
Lithology: Metamorphic
Group name: Four Winds complex of Gilbert and others (1987), Retreat Group, and orthogneiss
Dominantly quartz-rich schist and semischist, as well as marble; at Point Retreat in the Juneau quadrangle, unit also includes a distinctive, very fine-grained, black graphitic quartzite associated with albite-chlorite-white mica schist and semischist that locally contain garnet, calcite and (or) biotite. First named by Barker (1957) for rocks on the Mansfield Peninsula of northern Admiralty Island, Lathram and others (1965) extended the Retreat Group to include rocks southward to the contact with a Cretaceous pluton of unit Keg on central Admiralty Island. More recent mapping and analysis by S.M. Karl (unpub. data) indicates that the Retreat Group, as originally delineated by Barker (1957), should be restricted to rocks similar in age and lithology to the rocks at Point Retreat on the Mansfield Peninsula. Karl’s analyses suggest that at least some of these rocks are intruded by several metamorphosed and highly strained plutons of intermediate composition, which have U/Pb zircon ages that range between 546.9±3.2 and 544.08±0.8 Ma (S.M. Karl, unpub. data). The Retreat Group is considered pre-Silurian, as it is apparently intruded by Silurian plutons. The rocks south of the Mansfield Peninsula are compositionally different and yield younger ages. Mineral assemblages indicate that metamorphic grade ranges from greenschist facies to lower amphibolite facies with no systematic spatial distribution, suggesting that conditions were in the general range of the greenschist-amphibolite facies transition. On Kupreanof Island, greenschist-facies schist, including graphitic schist and subordinate marble, are tentatively included here also. No age control is available for these rocks on Kupreanof Island other than being intruded by foliated granodiorite of presumed Cretaceous age (unit Ksfg). The rocks of the Retreat Group may be correlative with rocks of the Wales Group on Prince of Wales Island and with the informally defined Four Winds complex of northern southeast Alaska, both of which are also apparently intruded by plutons that yield late Neoproterozoic ages. No pre-Cretaceous metamorphic mineral ages are available for the Retreat Group. The Four Winds complex (unit SPxfw, here) may be the same unit structurally offset along the Lynn Canal

Source map information

Source map Karl, S.M., Haeussler, P.J., Himmelberg, G.R., and Brew, D.A., 1998, Unpublished data.
Symbol Pza
Unit name Amphibolite
Description Dark green semischist, chlorite schist, subordinate metadiorite, marble, metachert. Dominantly igneous protoliths. Metamorphic minerals include chlorite, biotite, muscovite, hornblende, actinolite, epidote, clinopyroxene, garnet, plagioclase, calcite, graphite, quartz. Pyrite, magnetite, rutile, titanite, apatite, staurolite, anchorite, tourmaline, and possible sillimanite also identified. Amphibolite facies metamorphism predates greenschist facies metamorphism.
Lithology Metamorphic

Correlated geologic units

Label PzZrg
Description Retreat Group (restricted)
Geologic age Neoproterozoic to Ordovician
Geologic setting Metamorphic, undivided
Lithology Form Importance
Marble < Metacarbonate < Metasedimentary < Metamorphic Indeterminate, major
Quartzite < Metaclastic < Metasedimentary < Metamorphic Indeterminate, major
Mica-schist < Schist < Metamorphic Indeterminate, major