New 207Pb–206Pb and 40Ar–39Ar ages from SW Montana, USA: constraints on the Proterozoic and Archæan tectonic and depositional history of the Wyoming Province

This paper addresses the Precambrian depositional and thermotectonic history of a prominent suite of metasupracrustal rocks exposed within Laramide uplifts of the northern Wyoming province, southwestern Montana. This suite consists of highly metamorphosed carbonates, pelites, sandstones and banded iron formations. New isotopic data supporting 430 40Ar–39Ar biotite ages and four 207Pb–206Pb step-leach garnet ages, not only constrain the timing of Early Proterozoic thermotectonism to between 1820 and 1740 Ma, but also challenge the conventional view that the supracrustal rocks were deposited and metamorphosed during the Late Archæan (pre-2750 Ma). No mineral-isotopic evidence has been found for a ≈2750 Ma thermotectonic event which, based upon a published but highly equivocal whole rock Rb–Sr errorchron, is widely assumed to have affected the metasupracrustal rocks. Therefore, this metamorphic age and a corresponding pre-2750 Ma age of deposition are discounted pending further data. Instead, it is proposed that the supracrustal rocks were derived from gneisses now exposed in the Beartooth Mountains to the east and deposited between the ≈2800 Ma metamorphism age of these gneisses and the 1820 Ma age of the oldest garnets in the Ruby Range. An older cryptic thermotectonic event is also potentially recognised beginning at ≈2470 Ma, based upon the age of monazite-dominated garnet determined in one sample of paragneiss. The nature of this cryptic event is obscured by the pervasive ≈1820–1785 Ma overprint, but its timing coincides with global superplume activity previously associated with the incipient break-up of supercontinent Kenorland and may be related in some way. We propose that the metasupracrustal sequence was deposited in late Archæan to early Proterozoic time, between ≈2750 and ≈2470 Ma and then underwent two episodes of Early Proterozoic thermotectonism. The first metamorphic event is the ≈2470 Ma cryptic event. The rocks were then metamorphosed for the second time at ≈1820 Ma producing the main NE-trending fabric and was followed by ≈1780–1740 Ma post-tectonic cooling through ≈300 °C. Most likely, this event reflects the timing of Wyoming–Medicine Hat continental collision along the Great Falls tectonic zone during the assembly of supercontinent Laurentia. This proposal may constrain the role of the Archæan Wyoming province during Early Proterozoic supercontinent break-up (Kenorland) and reassembly (Laurentia) in what is now central North America.