Foreland-directed propagation of high-grade tectonism in the deep roots of a Paleoproterozoic collisional orogen, SW Montana, USA

Abstract

The study of deeply exhumed ancient collisional belts offers important constraints on geologic processes and properties complementary to inaccessible portions of the crustal column in active orogens. The ca. 1.8-1.7 Ga Big Sky orogeny in southwest Montana is a major convergent belt associated with the Proterozoic amalgamation of Laurentia. New structural, petrologic, and geochronologic data from the Northern Madison Range, crossing the NE-SW trend of the belt, record key information about the internal dynamics of the orogen. At least two phases of Big Sky-related deformation are preserved, both nearly coeval with peak metamorphic conditions of ~0.9-0.8 GPa and >700 °C. Metamorphic zircon grains from a deformed mafic dike yield a weighted mean ion probe U-Pb date of 1737 ± 28 Ma (2s). Monazite grains from a metapelite yield electron microprobe U-Th total-Pb dates of ca. 1750-1705 Ma, spanning prograde, peak, and retrograde intervals. Exposed Proterozoic paleodepths range from deeper levels (~45-40 km; 1.2 GPa) in the northwestern end of the range to shallower levels (~30-25 km) in the central-southeast area.

Dating Metasomatism: Monazite and Zircon Growth during Amphibolite Facies Albitization

“…There is evidence of multiple metamorphic events [3], and at least three phases of high-grade deformation in and nearby the northern Gallatin Peak terrane [20,33,35]. The oldest deformation fabrics (D 1 ) are compositional layered surfaces that are subsequently folded and overprinted by the more pervasive second and third deformational phases.…”

“…These structures developed between~1740-1720 Ma at metamorphic conditions of~0.9 GPa and 700 • C [20]. The rocks of Bear Basin and the surrounding area preserve field and textural evidence for pervasive fluid flow, including coarse-grained orthoamphibole, garbenschiefer textures within multiple units, and the presence of pervasive hydrous phases.…”

“…The oldest deformation fabrics (D 1 ) are compositional layered surfaces that are subsequently folded and overprinted by the more pervasive second and third deformational phases. This younger deformation includes regional D 2 and locally occurring D 3 events, both occurring during the late Paleoproterozoic Big Sky orogeny [20]. Major D 2 structures in the region, including map scale folds of earlier D 1 surfaces ( Figure 1C), are Northeast-Southwest striking and moderately to steeply southeast dipping foliations, and strong southeast plunging mineral lineations.…”

“…These consist of Archean to Paleoproterozoic orthogneiss units with several suites of deformed intrusive mafic dikes and interleaved supracrustal metamorphic rocks [21][22][23][24]. The majority of these lithologies have Archean protoliths, and along the northwestern margin of the Wyoming province, they contain evidence for at least two high temperature thermotectonic events: the enigmatic 2500-2450 Ma Tobacco Roots-Tendoy orogeny [20,[25][26][27] and the 1800-1710 Ma Big Sky orogeny [20,22], interpreted as the results of the Wyoming province docking with the rest of the Archean core of Laurentia during the amalgamation of supercontinent Nuna [28,29]. To the southeast of the rocks overprinted by Paleoproterozoic thermotectonism, K-Ar and 40 Ar- 39 Ar thermochronology indicates that the Archean rocks have not been thermally overprinted since the Neoarchean (Figure 1A,B; [30][31][32]).…”

“…Rocks in the study area of Bear Basin, located in the Gallatin Peak terrane of the Northern Madison Range ( Figure 1B), are composed of quartzofeldspathic orthogneisses, foliated granitoids, amphibolite, quartzite, leucogranite sheet dikes, and intercalated supracrustal schist ( Figure 1C; [20,22,33]). Foliated granitoids and orthogneiss units have U-Pb zircon crystallization ages between~2.8-3.2 Ga [34].…”