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Gabbro and quartz gabbro

Unit symbol: Trgb
Age range Late Triassic? (235 to 201.3 Ma)
Lithology: Igneous - Plutonic
Group name: Mafic and ultramafic rocks in southern Alaska
Consists of a variety of amphibole- and pyroxene-bearing gabbro and sparse quartz gabbro and quartz diorite, along with other mafic and ultramafic rocks. Unit has rare porphyritic phases. Crops out in small plutons, dikes and sills cutting, predominantly, the Hasen Creek Formation with a few small dikes within the Nikolai Greenstone and is possibly magmatically related to the Nikolai Greenstone and its related units (MacKevett and others, 1978). Occurs in the McCarthy quadrangle (MacKevett and others, 1978), the northern Talkeetna Mountains (Kline and others, 1990) and Gulkana quadrangles (Wilson and others, 1998; W.J. Nokleberg, written commun., 1997), and the Mount Hayes quadrangle (Nokleberg and others, 1992a). Also present in the Healy quadrangle as part of map units TrIPam and Trbd of Csejtey and others (1992), though not mapped separately

Source map information

Source map Nokleberg, W.J., Aleinikoff, J.N., Lange, I.M., Silva, S.R., Miyaoka, R.T., Schwab, C.E., and Zehner, R.E., with contribution for selected areas from Bond, G.C., Richter, D.H., Smith, T.E., and Stout, J.H., 1992, Preliminary geologic map of the Mount Hayes quadrangle, eastern Alaska Range, Alaska: U.S. Geological Survey Open File Report 92-594, scale 1:250,000 [Received as CAD coverage produced in 1996].
Symbol ga
Unit name Gabbro, diabase, and metagabbro
Description Chiefly hornblende gabbro, clinopyroxene gabbro, and hornblende-clinopyroxene gabbro and sparse quartz gabbro and hornblende quartz diorite. Dark-gray, fine- to medium-grained, and mainly relict hypautomorphic granular texture, but locally porphyritic, ophitic to subophitic, and diabasic. Locally intensely metamorphosed and weakly schistose. In some areas, may be late Paleozoic in age and part of the Slana Spur Formation and Tetelna Volcanics.
Lithology Igneous

Correlated geologic units

Label Trgb
Description Gabbro, diabase, and metagabbro, Late Triassic(?)
Geologic age Late-Triassic to Norian
Geologic setting Intrusive
Lithology Form Importance
Gabbro < Gabbroic < Plutonic < Igneous Pluton Major
Ultramafic < Plutonic < Igneous Indeterminate, major
Quartz-diorite < Dioritic < Plutonic < Igneous Pluton Incidental