Petrographic study of metamorphic rocks associated with the Penokean orogeny in Michigan
The Early Proterozoic Fern Creek formation has been recognized as originally glacial sediments that were accumulated and preserved in topographic lows of a highly eroded 2.75 Ga Carney Lake Gneiss. This gneiss is part of the basement of the Superior Craton in Michigan’s Upper Peninsula. The Fern Creek formation is overlaid by the shallow marine deposits of the Sturgeon River Quartzite and the Randville Dolomite. All these units are part of the Chocolay Group, a continental margin sequence that was deposited following rifting and development of a passive margin along the south edge of the Superior Craton between 2.3 and 2.2 Ga. The Chocolay Group was deformed and metamorphosed during the Penokean orogeny, a tectonic event that began at about 1880 Ma when an oceanic arc (Pembine-Wausau terrane) collided with the southern margin of the Archean craton along the Niagara shear zone. This was followed by northward directed subduction that led to the eventual collision of the Marshfield terrane to the Pembine-Wasau terrane along the Eau Pleine shear zone between 1860 and 1835 Ma. This collision likely resulted in the major period of foreland deformation. Carney Lake Gneiss and Fern Creek rock samples were collected for petrographic and microstructural analysis at the Sturgeon River Dam (about 3 km northeast of Loretto, Michigan), where the contact between these two units is exposed. At this site, the Fern Creek formation includes mostly slate, conglomerate, arkose and diamictite. The matrix-supported diamictite contains abundant sub-angular to sub-rounded milimetric to centimetric-scale fragments of the local Carney Lake Gneiss, granite, and quartz. These small grains, pebbles and boulders, are embedded in a sandy to silty matrix. Plagioclase and microcline clasts and grains are also present. Quartzofeldspathic and quartz fragments show brittle deformation, and some dissolution (pressure-solution seams are present). Elongated clasts are preferentially oriented. The matrix is composed of small quartz and feldspar grains, but also of clays and small white micas (that are, in part, alteration products of feldspars) that are also preferentially oriented, defining a compaction cleavage. An incipient anastomosing cleavage defined by small white micas (mm-long) wrapping around asymmetric clasts suggests that the diamictite is locally sheared. In the same locality, samples of the red variety of the Carney Lake Gneiss are characterized by a gneissic banding defined by quartzofeldspathic and polycrystalline quartz ribbons. The ribbons wrap around sericitiziced symmetric and asymmetric winged feldspar porphyroclasts (that are 2 to 3 mm in diameter). The quartz in the ribbons exhibits grain boundary migration and locally subgrain rotation recrystallization. Some quartz grains also show deformation bands. Ribbons alternate with mm-thick layers of intensely sericitized feldspar and abundant small white micas. These preliminary petrographic descriptions are part of an on-going project that will include a combination of petrographic, scanning electron microscope (SEM), and microstructural analyses. The main goal is to determine the metamorphic conditions and characterize the deformation that affected these Proterozoic rocks during the Penokean orogeny.