Newly discovered 1550 Ma mafic magmatism in the Tobacco Root Mountains, MT USA: An end to the North American Magmatic Gap?

English Abstract:

A new baddeleyite U-Pb age of 1552 ± 6 Ma for a dyke in the Tobacco Root Mountains (TRM) falls in the middle of a “magmatic gap” that was previously noted to exist between 1500 and 1600 Ma in western North America (Ross et al. 2003) based on the detrital zircon record of Proterozoic sedimentary rocks (Finney et al. 2005). Dykes of this age offer a potential local source for ca. 1550 Ma detrital zircons found within the Belt-Purcell Basin. The 1552 Ma age is significantly older than 1470-1430 Ma U-Pb mafic sills within the Belt-Purcell Supergroup and is unknown elsewhere in the Wyoming craton. The new U-Pb age for these intra-plate mafic dykes indicate that this magmatic event may have preceded 1470 to 1400 Ma extension and deposition of the Belt-Purcell Supergroup.

In the TRM, Proterozoic mafic dykes are subvertical, strike NW, and cut foliation of the Archean basement. Geochemical analyses of mafic dykes from the southern TRM and the Ruby Range (RR) by Wooden et al. (1978) identified distinct geochemical groups that they termed A, B, and C. We concur that there are three distinct groups of dykes within the TRM and RR. However, we revised the original groups based on more extensive and modern geochemical analysis. We propose A, B and C group dykes be renamed the Ramshorn Creek Group (RCG; 1552 Ma date applies), 780 Ma Gunbarrel dykes, and the yet undated Mammoth Group (MG), respectively. The very similar geochemical signatures of the RCG and MG dykes indicate deeper melting, more lithospheric contribution and different source(s) than the Gunbarrel dykes.

Paleomagnetic studies by Harlan and others (2005, 2008) yielded dual polarity magnetizations for both the RCG and MG dykes that are supported by both normal and reverse polarity baked contact tests. The definition of a 1550 Ma RCG pole from the southern TRM dykes would contribute to enhanced definition of the Laurentian apparent polar wander path during the Mesoproterozoic.