Montevideo Gneiss
John Huggenvik Stop #4
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Location: The Montevideo Gneiss outcrops across western and northern Minnesota. The outcrop we looked at is located in Section 20, Township 117N, Range 40W. The outcrop was 2 miles southwest of Montevideo, Minnesota on county road 212 in the Chippewa County.
Brief Description: The Montevideo gneiss is a high-grade metamorphic gneiss. The main minerals are quartz, feldspar, and amphibole, with various other minerals appearing in accessory throughout the unit. The feldspar is responsible for the discoloration that is seen when the rock weathers, taking on a grayish color. The feldspars consist mainly of alkali feldspar; orthoclase is probable. The amphibole is a hornblende.
In Depth: The Montevideo Gneiss is a high grade metamorphic gneiss. The grading comes from the level of heat that was involved in the deformation that occurred (Perkins, 2002). The Montevideo displays less deformation than the Morton, appearing pinstriped with lineations. The Montevideo has bee dated at 3.680 +/- 70 billion years old (Morey, 1996). There are several faults, dipping northward, which Dr. Allard informed us were caused by an anticline that this section of gneiss was part of. The Yellow Medicine Shear Zone is relatively near by, which the metamorphism can be attributed to. The Montevideo was a felsic granite body that was part of a proto-continent during the Achaean era. The Yellow Medicine Shear Zone is the evidence that the small continent collided into another small continent, deforming the granite into the gneiss we see today during the period in which the pre-Cambrian shields were forming. The original minerals within the gneiss were dated, not the deformation, giving us a good indication of when the unit was originally formed.
Activities: We stopped at the Montevideo gneiss to compare different grades of metamorphism. The Morton, which we looked at earlier, was also a high-grade gneiss, but was much more deformed than the Montevideo. The Morton swirls and individual layers terminating into one another. The Montevideo, while at a high grade of deformation, remains relatively planar throughout the unit. As you can see in figure 5A the unit exhibits a high level of metamorphism, while remaining lineated.
(Figure 5A: Montevideo Gneiss with rock hammer for scale)
The key difference in the two metamorphisms could be the pressure at which the units were metamorphosed and the vector in which pressure was applied. This could account for the difference in appearance, with and without foliation.
References:
Perkins, Dexter. 2002. Mineralogy. Prentice-Hall, Upper Saddle River, New Jersey.
Morey, G.B. 1996. Geologic Map of Minnesota Bedrock Geology. Minnesota Geological Survey. Scale 1:1,000,000, I sheet.