Structural Analysis of Archean Metasedimentary Rocks, Jardine, Montana

Abstract

The structural history of the Precambrian deformational events in the vicinity of Jardine, Montana, has been reconstructed in some detail. Three phases of folding, as evidenced by minor planar and linear structures, have been recognized in the metamorphosed greywacke and mudstone sequences. The folds developed during the first phase of folding (F1), were isoclinal and recumbent and likely reached nappe-like scale. A pervasive S1 schistosity, found in all rock types throughout the region, was formed during, and axial planar to, F1 folding. S1 schistosity is a continuous type 1 schistosity that is subparallel to bedding (So). The folds developed during the second phase of folding (F2) were close and nearly upright, with gently to moderately plunging fold axes that trend northeast-southwest. An S2 zonal crenulation cleavage (symmetric or asymmetric) was formed axial planar to F2 folds. The folds developed during the third phase of folding (F3) were open and nearly upright, with gently to moderately plunging fold axes that trend northwest-southeast. A weak S3 zonal crenulation cleavage (symmetric or asymmetric) was formed axial planar to F3 folds. Metamorphic mineral growth did not occur during the F2 and F3 fold events. The S2 and S3 crenulation cleavages are defined by the axial planar alignment of microfold hinges or mica-rich domains. S2 and S 3 axial surfaces are nearly orthogonal to each other and their intersection is nearly normal to S1 schistosity. Rare, post-F3 kink bands were observed. The superposition of the three phases of folding resulted in the formation of a dome and basin (Ramsay type 1) fold interference pattern, observed in the S1 foliation surface. So displays the same dome and basin pattern, but in addition, small domains near the F 1 fold hinges display a combination of Ramsay type 2 and type 3 fold interference patterns. Calculations from electron microprobe analyses of metamorphic mineral assemblages indicate a peak metamorphic pressure and temperature of approximately 2.9 kb and 560° C. This pressure corresponds to a depth of burial of approximately 10 km. Microfabric observations show that peak metamorphic conditions occurred during or shortly after the first phase of folding. Brittle deformation features (e.g. fault zones, shear zones, joints) observed in the region were formed during the Laramide Orogeny (Late Cretaceous to Early Tertiary).