Geology and Rb-Sr age of lower Proterozoic granitic rocks, northern Wisconsin
Geology and Rb-Sr age of lower Proterozoic granitic rocks, northern Wisconsin (in Selected studies of Archean gneisses and lower Proterozoic rocks, southern Canadian Shield, G. B. Morey (editor) and Gilbert N. Hanson (editor))
Special Paper - Geological Society of America (1980) (182): 139-146
- absolute age
- age
- alkali metals
- Ashland County Wisconsin
- Bayfield County Wisconsin
- Canadian Shield
- chemical composition
- dates
- diorites
- geochronology
- granites
- Great Lakes region
- igneous rocks
- intrusions
- Iron County Wisconsin
- isotopes
- K/Ar
- metals
- North America
- Paleoproterozoic
- plutonic rocks
- plutons
- potassium
- Precambrian
- Proterozoic
- Rb/Sr
- Southern Province
- tonalite
- United States
- upper Precambrian
- Wisconsin
- Mercer
- Butternut
Granitic rocks ranging in composition from granite to tonalite and associated metavolcanic-metasedimentary rocks compose an east-trending belt as much as 180 km wide and 300 km long in N Wisconsin. The granitic rocks have an initial (super 87) Sr/ (super 86) Sr ratio of 0.7025 + or - 0.0005 and a Rb/Sr isochron age of 1885 + or - 65 m.y., which is interpreted as the time of crystallization of the granitic rocks. Rb/Sr whole-rock mineral secondary isochrons for two samples give ages of 1655 + or - 55 m.y. and 1545 + or - 55 m.y.; K/Ar ages of biotite from these samples are 1615 + or - 55 m.y. and 1598 + or - 54 m.y., respectively. These mineral ages are interpreted as resulting from isotopic resetting caused by a thermal event about 1600 m.y. ago.
