(super 40) Ar/ (super 39) Ar evidence for ca. 1800 Ma tectonothermal activity along the Great Falls tectonic zone, central Montana
(super 40) Ar/ (super 39) Ar evidence for ca. 1800 Ma tectonothermal activity along the Great Falls tectonic zone, central Montana
Canadian Journal of Earth Sciences = Revue Canadienne des Sciences de la Terre (December 2002) 39 (12): 1719-1728
- absolute age
- amphibole group
- Ar/Ar
- biotite
- Canadian Shield
- chain silicates
- Churchill Province
- clinoamphibole
- dates
- Hearne Province
- hornblende
- Laurentia
- Little Belt Mountains
- mica group
- Montana
- nesosilicates
- North America
- orthosilicates
- Paleoproterozoic
- Precambrian
- Proterozoic
- Rocky Mountains
- sheet silicates
- silicates
- suture zones
- tectonics
- thermal history
- U. S. Rocky Mountains
- U/Pb
- United States
- upper Precambrian
- Wyoming Province
- zircon
- zircon group
- Neihart Montana
- Great Falls tectonic zone
Late Paleoproterozoic (1900-1600 Ma) tectonothermal activity on all borders of the Archean Wyoming Province has long been established by low-precision K-Ar and Rb-Sr studies. However, recent tectonic models advanced for supercontinent aggregation require improved constraints on the timing of tectonothermal activity along major boundaries. On its northwestern boundary, the Great Falls tectonic zone separates the Archean Wyoming and Hearne provinces. Recently published U/Pb ages and geochemical data reveal the presence of a Paleoproterozoic (ca. 1860 Ma) marginal-arc magmatic complex along a portion of the Great Falls tectonic zone in central Montana. We present nine new (super 40) Ar/ (super 39) Ar mineral ages (on hornblende and biotite) from these same arc rocks, which indicate ca. 1800 Ma thermal activity (>500 degrees C) and subsequent rapid cooling to below 300 degrees C by 1775 Ma. This new data set constrains the timing of Wyoming-Hearne collision to between 1860 and 1800 Ma and the timing of last significant tectonothermal activity of this portion of the Great Falls tectonic zone (1800-1775 Ma.) We note that our data add to a growing geochronologic database indicating ca. 1800 Ma tectonothermal activity (via either initial suturing or continued tectonic activity) associated with Paleoproterozoic docking of the Wyoming Province with Laurentia.