Paper No. 8
Presentation Time: 10:15 AM
LATE ARCHEAN TO PALEOPROTEROZOIC EVOLUTION OF THE MONTANA METASEDIMENTARY SUBPROVINCE AND ITS IMPLICATIONS FOR THE GROWTH AND AMALGAMATION OF THE WYOMING PROVINCE
The tectonic histories of metasupracrustal sequences that characterize the Montana Metasedimentary subprovince (MM) constrain the Precambrian evolution of the northern Wyoming Province (WP) and contribute to understanding the growth and consolidation of Laurentia. When did these sequences exist as sedimentary strata and what was the tectonic setting of deposition? North of Giletti’s Line - a NE-trending geoochronologic front that transects the MM - Ar/Ar cooling ages and ion microprobe Pb/Pb monazite ages record the regional, high-grade affects of the 1.71-1.78 Ga Big Sky orogeny (BSO) and collision of a Paleoproterozoic arc now partly exposed in the Little Belt Mts (Vogl et al. 2004), but there is no evidence of a classically-understood passive continental margin in the northern WP prior to the BSO. South of Giletti’s Line, metasupracrustal rocks retain 2.4-2.5 Ga Ar/Ar cooling ages and ion microprobe Pb/Pb monazite ages. North of Giletti’s line, relict monazites of that age are also preserved in all of the carbonate-hosting metasupracrustal sequences, whereas metasupracrustals lacking 2.4 Ga monazites also lack carbonates and probably represent juvenile oceanic basins. Thus none of these sequences could have represented a mature (1.78-2.4 Ga age range) stable WP continental margin at the time of the BSO. Despite the well-documented presence of >3.0 Ga zircons (cf. Mueller et al. 1998), the metasupracrustals of the MM do not contain compelling evidence of a mid-Archean history as sedimentary rocks. If deposited in the latest Archean, in a basin accumulating carbonate, clastic, and mafic to intermediate volcanic rocks - without detritus from known 2.7-2.8 Ga sources in the WP - the MM is an unlikely candidate for a passive northern WP continental margin at 2.5-2.7 Ga. Geochronologically, the MM appears to have more in common with the allochthonous Selway terrane (Foster et al. 2006) to the NW than with the WP to its S. These observations lead to a recharacterization of the northern WP in which the MM was a tectonically-active accretionary domain in the Paleoproterozoic, punctuated by rifting at 2.1 Ga and collision and collapse at 2.4-2.5 and 1.71-1.78 Ga in an arc/backarc basin setting. Thick, refractory lithosphere of the WP’s Archean core did not prevent S-directed subduction along the Paleoproterozoic MM margin.