Evidence for Proterozoic mylonitization in the northwestern Wyoming province

Abstract

The 3-km-thick Madison mylonite zone encompasses the contact between the Cherry Creek Metamorphic Suite and the pre-Cherry Creek metamorphic complex in the southern Madison Range of southwest Montana. In the shear zone, greenschist- and epidote-amphibolite-facies assemblages overprint earlier amphibolite- and granulite-facies assemblages. Large-scale metasomatic conversion of granitic gneiss to mylonite gneiss increased siderophile (Fe, Mn, Mg, P, and Ca) elements and decreased K, Al, and Si in the mylonite gneiss, suggesting extensive fluid flux in the Madison mylonite zone. Southeast-dipping foliations adjacent to the Madison mylonite zone are rotated about their northeastern strike through the vertical to northwest-dipping orientations, indicating southeast-directed thrusting of the pre-Cherry Creek metamorphic complex over the Cherry Creek Metamorphic Suite. Complete stratigraphic continuity of Cherry Creek units within the mylonite zone shows the predominance of ductile shearing. Ductile thrusting is also indicated by asymmetric fabrics, stretching lineations, finite strain, and metamorphic gradients within the shear zone. If the foliations were passively rotated during simple shear, a minimum of 10 km of distributed ductile slip occurred in the Madison mylonite zone. 40Ar/39Ar thermochronology of hornblende and muscovite adjacent to the Madison mylonite zone records a Late Archean (2.5 Ga) cooling event. Within the shear zone, a discordant 40Ar/39Ar muscovite spectrum is consistent with initial cooling to argon closure at about 1.9 Ga with subsequent disturbance at about 1.6 Ga. Hornblendes from the Madison mylonite zone show highly discordant argon-loss spectra with temperature steps starting at 1.8 Ga and ending at 2.5 Ga. These argon age spectra indicate Early Proterozoic heating and argon loss in the Madison mylonite zone, resetting and subsequent retrogressing of muscovite but only partial resetting of hornblende due to its higher argon closure temperature. This study strongly suggests Early Proterozoic (1.80-1.90 Ga and possibly later) thrusting in the Madison mylonite zone, which may be correlative with retrograde metamorphism and Early Proterozoic isotopic resetting northwest of the Madison Range. The Madison mylonite zone is probably a foreland thrust zone on the margin of a major compressional orogen of Early Proterozoic age that reworked the Archean basement of the northwestern Wyoming province.