Mafic and ultramafic rocks in the vicinity of the Denali Fault System

Unit symbol: Trum
Age range Triassic? (252.2 to 201.3 Ma)
Lithology: Igneous - Plutonic
Group name: Mafic and ultramafic rocks in southern Alaska
Gabbro of Mount Moffit of the Mount Hayes quadrangle and a correlative unit of gabbro, diorite, metagabbro, metadiabase, and amphibolite dikes, sills and small plutons (units gbm and mgb of Nokleberg and others, 1992a) are distributed along the Denali Fault System in the Mount Hayes quadrangle and also in the adjacent Tanacross and Nabesna quadrangles. In south-central Alaska, we correlate the country rocks of these intrusive rocks with the Yanert Fork sequence of Csejtey and others (1992 (unit Dmvs here) in the Mount Hayes quadrangle, but such correlation is increasingly speculative east of the Mount Hayes quadrangle. Unit also includes medium- to coarse-grained hornblende gabbro, hornblendite, plagioclase-hornblende pegmatite, and tectonized peridotite, informally named the Katzehin ophiolite complex; it is associated with the Coast Plutonic Complex of Brew and Morrell (1979b) in the Atlin quadrangle (Brew and others, 2009) and small peridotite bodies in the Juneau and Taku River quadrangles (Brew and Ford, 1985). In the Atlin quadrangle, these rocks are associated with greenstone and amphibolite. These rocks have an inferred Early Triassic age because they are close to, and may be stratigraphically overlain by, bedded sedimentary rocks of Early and Middle Triassic age (Brew and others, 2009). We suggest these rocks may correlate with other Jurassic or Triassic mafic and intermediate plutons associated with the Coast Plutonic complex of Brew and Morrell (1979b) such as the Texas Creek granodiorite of unit Trqd

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 mgb
Unit name Gabbro, diorite, metagabbro, metadiorite, metadiabase, and amphibolite
Description Chiefly hornblende metagabbro, metadiabase, and minor amphibolite. Undifferentiated dikes, sills, and small plutons. Metagabbro and metadiabase generally fine- to medium-grained with hypautomorphic granular to diabasic texture, locally porphyritic. Amphibolite is fine- to medium-grained with granoblastic to porphyroblastic texture, containing hornblende porphyroblasts up to 2 mm in size. Correlative with unit gbm.
Lithology Metamorphic

Correlated geologic units

Label Trms
Description Triassic mafic igneous rocks of south central Alaska. Possibly Stikine
Geologic age Triassic
Geologic setting Intrusive, gabbro
Lithology Form Importance
Metadiorite < Metaintrusive < Metaigneous < Metamorphic Major
Metagabbro < Metaintrusive < Metaigneous < Metamorphic Major