NEW EVIDENCE FOR AN EXHUMED CRUSTAL SECTION FROM THE PALEOPROTEROZOIC BIG SKY OROGENY, N. MADISON RANGE, SW MONTANA

Exhumed crustal sections are fruitful natural laboratories for studying deep crustal properties and processes. One of the more extensive crustal sections in North America, in terms of exposed paleodepths, may occur in the N. Madison Range in SW Montana. The range is a ~50 km long NW-SE trending uplift of Archean rocks modified by significant Paleoproterozoic (~1.75-1.72Ga) thermotectonism, which is consistent with “Big Sky” orogenic activity recognized in adjacent ranges to the west. From previously published data, rocks in the NW part of the range were metamorphosed up to conditions of 1.2 GPa and 800 °C at 1753 ±9 Ma (SIMS zircon). Published K-Ar mica data from SE of Gallatin Canyon indicates this part of the range has not experienced significant thermal perturbation since the Archean and may represent Big Sky upper crust. We report new structural, petrologic, and geochronological data from the east-central area of the range. Near Bear Basin, rocks contain evidence for at least three phases of deformation. D1 structures include variable preservation of compositional layering and penetrative foliation (S1) that strike NW and dip shallowly to the NE. D2 structures include the dominant penetrative foliation (mean: 051,51) and moderately inclined commonly isoclinal F2 folds whose axes are subparallel to down-dip L2 mineral lineations (~140, 61). D3 structures are commonly open to tight moderately plunging Z-folds whose dextral asymmetry suggests they may be coeval with a dextral transpressional high strain zone to the SE. Bt-schists in Bear Basin yield P-T conditions of 0.85 GPa and 700 °C. Monazite from two schist samples, one dominated by S1, one by S2, crystallized at 1740 ±7 Ma and 1745 ±5 Ma (U-Th-total Pb), suggesting that both fabric generations are due to Big Sky deformation. Nearby, a deformed mafic dike yields P-T conditions of 0.92 GPa and 725 °C linked to a matrix zircon population of 1737 ±14 Ma. Collectively, the data indicate that Big Sky deformation and metamorphism affected most of the N. Madison Range, and the spatial pattern of metamorphic conditions and timing indicates shallowing from lower crustal paleodepths of ~45 km to the NW to <10 km to the SE. Continued work is focused on the tectonic significance of high strain zones and more thorough understanding of the nature of the transition from deep to upper crust.