THE NATURE OF ARCHEAN TERRANE BOUNDARIES REVISITED: EVIDENCE FOR TWO PALEOPROTEROZOIC THERMOTECTONIC EVENTS RECORDING CRUSTAL ASSEMBLY, MELT PRODUCTION, AND MODIFICATION IN THE RUBY RANGE, SOUTHWESTERN MONTANA

The Ruby Range in SW Montana is within the Montana Metasedimentary Terrane (MMT) at the NW margin of the Wyoming Province. Once thought to be dominantly Archean in age, this range is now known to have experienced a profound tectonothermal event termed the Big Sky orogeny (1.79 -1.72 Ga). A transect along Stone and Cottonwood Creeks was selected for U-Pb zircon, LASS monazite, and Lu-Hf garnet dating. The section includes three major lithotectonic units: the Christensen Ranch Metasedimentary Suite (CRMS), the Dillon gneiss, and the Pre-Cherry Creek Suite. Three detrital zircon samples from quartzites within the CRMS record distinct depositional signatures. Two samples record atypical MMT signatures with peaks at 2.6-2.7 Ga and grains as young as 2.5 Ga. The presence of late Archean and earliest Paleoproterozoic ages requires both a younger depositional age for these rocks than was previously inferred as well as heterogeneity in source regions. Pelitic gneisses interlayered with the quartzites record monazite ages of 1.76-1.79 Ga. The exception is in one sequence north of the Stone Creek fault where garnet leucogneiss is intrusive with respect to the CRMS pelites. Here, monazite ages from the pelite and leucogneiss are 2.43 Ga and 2.48 Ga respectively. Garnet from the leucogneiss has been dated by Lu-Hf and is 2.43 Ga. A pelitic gneiss that occurs as a screen in the Dillon unit yields mixed ages of 1.78 and 2.44 Ga. The younger population occurs almost entirely as inclusions in garnet. Garnet leucogneiss from the Dillon unit records an identical 2.48 Ga age as that of the CRMS sample. Two samples of migmatitic Pre-Cherry Creek gneiss record ages of 1.76-1.77 Ga with scarce 2.4 Ga cores. A sample of garnet leucosome from the migmatitic gneiss contains a dominant population 1.77 Ga grains. In contrast, zircon from the leucosome yields 2.43 Ga ages, but REE patterns indicate that zircon grew prior to garnet so could be inherited. These results indicate that the Big Sky orogeny occurred at 1.79-1.76 Ga in the Ruby Range and profoundly affected all of the lithotectonic units. However, ages from the garnet leucogneiss place constraints on an earlier period of widespread crustal melting at 2.43-2.48 Ga, a period that may reflect the penultimate stage of crustal assembly that was subsequently followed by rifting and collision during the Big Sky orogeny.