| State | Wisconsin |
|---|---|
| Name | Gabbroic rocks of Keweenawan affinity |
| Geologic age | Middle Proterozoic |
| Lithologic constituents |
Major
Igneous > Plutonic > Gabbroic > Gabbro (Stock or pipe)
Minor
Igneous > Plutonic > Anorthosite (Stock or pipe)
Igneous > Plutonic > Granitic > Granite (Stock or pipe)
Igneous > Plutonic > Ultramafic > Peridotite (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 peridotiite at Upson, MI as well as individual bodies (the unexposed Clam Lake, 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 of Arizona, unpublished data, 2003) |
| References | 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.USGS Geologic Names lexicon found at: http://ngmdb.usgs.gov/Geolex/https://ngmdb.usgs.gov/Geolex/search |
| NGMDB product | |
| Counties | Ashland - Bayfield - Douglas - Iron - Sawyer |