|Name||Gabbroic rocks of Keweenawan affinity|
|Geologic age||Middle Proterozoic|
Igneous > Plutonic > Gabbroic > Gabbro (Stock or pipe)
Igneous > Plutonic > Anorthosite (Stock or pipe)
Igneous > Plutonic > Ultramafic > Peridotite (Stock or pipe)
Igneous > Plutonic > Granitic > Granite (Stock or pipe)
|Comments||This unit was originally called "gabbros of uncertain affinity" on the Sims (1992) map. Since that map was published, several of these bodies have been dated and appear to be of Keweenawan affinity. This unit includes gabbros and anorthosites in the Amnicon Intrusion, the Mellen complex, and the peridotite at Upson, MI as well as individual bodies (the unexposed Clam Lake and Round Lake intrusions). These units are now known to be of Keweenawan affinity. The gabbro associated with the Mellen complex has been dated by U-Pb zircon dating yielding a date of 1102.0 +/- 2.8 Ma and the Mellen granite has a U-Pb zircon date of 1100.9 +/- 1.4 Ma (Zartman and others,1997, U-Th-Pb zircon ages of some Keweenawan Supergroup rocks from the south shore of Lake Superior: Canadian Journal of Earth Sciences, v. 34, p. 549-561). A rhyolite associated with the Amnicon pluton has a U-Pb zircon date of 1101.8 +/- 6.7 Ma (C. Isachsen, Univ. of Arizona, unpublished data, 2003)|
Zartman and others, 1997, U-Th-Pb zircon ages of some Keweenawan Supergroup rocks from the south shore of Lake Superior: Canadian Journal of Earth Sciences, v. 34, p. 549-561.
C. Isachsen, Univ. of Arizona, unpublished data, 2003.
Sims, P.K., 1992, Geologic map of Precambrian rocks, southern Lake Superior region, Wisconsin and northern Michigan: U.S. Geological Survey Miscellaneous Investigations Map I-2185, scale 1:500,000.
USGS Geologic Names lexicon found at: http://ngmdb.usgs.gov/Geolex/