Geologic Map of Alaska: Nikolai and Goon Dip Greenstones and equivalent rocks

Nikolai and Goon Dip Greenstones and equivalent rocks

Unit symbol:	Trn
Age range	Late and Middle Triassic (247.2 to 201.3 Ma)
Lithology:	Igneous - Volcanic
Group name:	Massive basalt and greenstone

Massive, dark-gray-green, dark-gray-brown, and maroon-gray, subaerial and submarine basalt flows and minor interbedded volcaniclastic sedimentary rocks, aquagene and epiclastic tuff, breccia, argillite, and radiolarian chert (Nokleberg and others, 1992a), commonly metamorphosed to lower greenschist facies. Widely distributed and several thousands of meters thick. Includes unnamed Triassic greenstone units in Talkeetna Mountains quadrangle (Csejtey and others, 1978). Commonly associated with Late Triassic carbonate and cherty carbonate rocks. Together with Chitistone and Nizina Limestones, this is one of the diagnostic units of the Wrangellia terrane (Jones and others, 1977). Plafker and others (1976) and Jones and others (1977) correlated the Goon Dip Greenstone of southeast Alaska with the Nikolai Greenstone based on similar lithology and stratigraphic position relative to the overlying Whitestripe Marble, which they correlated with the Upper Triassic Chitistone Limestone. Goon Dip Greenstone is dominantly massive greenstone and minor greenschist and marble. The greenstone also commonly contains sparsely distributed copper-bearing sulfides; the Nikolai was the host rock for the Kennecott group of mines. Similar and possibly correlative units, the Cottonwood Bay and Chilikradrotna Greenstones of the Alaska Peninsula, are described below as unit Trcb

Source map information

Source map	Karl, S.M., 2013, Unpublished data.
Symbol	Trg
Unit name	Goon Dip Greenstone
Description	Massive mafic metavolcanic rocks locally contains vesicles and calcite-filled amygdules. Unit consists mostly of massive greenstone; and includes pillow breccia, rare lenses of lapilli tuff, and feeder dikes. Greenstone is several 100 meters thick east of Patterson Bay. No marble was observed associated with the greenstone east of Patterson Bay. The greenstone contains sparse phenocrysts of plagioclase, hornblende, and augite. Secondary minerals include actinolite, chlorite, epidote, prehnite, pyrite, and chalcopyrite. Chemical analyses of volcanic rocks in the Patterson Bay area indicates they are low Ti, low Mg, low Al, low Nb, tholeiitic basalts, and they plot consistently in mid-ocean ridge basalt (MORB) and enriched-MORB fields on element discriminant diagrams of Pearce (1982), Wood and others (1979), and Mullen (1982) (figs. 3a-3e). On the same diagrams, they plot very much like The Goon Dip Greenstone on Chichagof Island, the Karmutsen basalts on Vancouver and the Queen Charlotte Islands, except some Karmutsen basalts also plot in island arc tholeiite (IAT) fields (see Barker and others, 1989). Greenstone in Nakwasina sound has relict pillow and pillow breccia structures. It is thermally recrystallized adjacent to the Jurassic diorite. The greenstone east of Patterson Bay locally contains relict pillow structures, and contains no fabric, no flattening of vesicles or amygdules, contains low greenschist facies minerals including chlorite and epidote, and is thermally recrystallized adjacent to Tertiary plutons
Lithology	Igneous

Correlated geologic units

Label	Trgn
Description	Goon Dip Greenstone and related rocks, southeast Alaska
Geologic age	Triassic
Geologic setting	Extrusive, mafic

Lithology	Form	Importance
Basalt < Mafic-volcanic < Volcanic < Igneous	Flow, pillows	Major
Marble < Metacarbonate < Metasedimentary < Metamorphic		Minor
Schist < Metamorphic		Minor
Limestone < Carbonate < Sedimentary	Calcareous	Incidental