Contact of Morton Gneiss and Sacred Heart Granite

Memorial Park - Montevideo Gneiss

Lisa Clifford and Katie Miller Konen

Stop #3

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 Location: From the city of Granite Falls, Minnesota, take highway 67 south away from town for one and a half miles to the entrance road to Memorial Park.  The outcrop is located just past the gate to the park on the right side of the road. (NW ¼,Quad3, TW115N, Range 39W). 

Significance of Locality:

            The location of this outcrop is just north of the Yellow Medicine Shear Zone which separates the Montevideo Gneiss from the Morton Gneiss and Sacred Heart Granite (figure 3a). The Yellow Medicine Shear Zone cuts through the Sacred Heart Granite showing that it is younger than this 2.6 billion year old rock. (Southwick, 2002 cites Doe and Delevaux, 1980).  There are also various dikes located throughout this region aged at 2 billion years old (Southwick, 2002  cites Wirth and others, 1995; Buchan and others, 1996), therefore the Yellow Medicine Shear Zone must be between the ages of 2 and 2.6 billion years old. 

Figure 3a (Southwick, 2002)

Description of site: The outcrop is located within the Montevideo Gneiss, estimated at 3.6 billion years old (whole-rock Rb-Sr determination, Golrich and others, 1980, after Himmelberg, 1968).  The specific section we focused on was an amphibolite facies mafic rock formed within the Montevideo Gneiss.  It is located in a fold plunging to the northeast lined by a biotite garnet gneiss, both inclusions within the Montevideo Gneiss (Southwick, 2002).  (Figure 3a)  The mafic rock contained amphibolite facies.  The amphibolite facies must be formed at a temperature between 550-750 degrees Celsius and under a pressure of 5 to 10 kbars (Osman, 2003 cites Eskola, 1920). The rock contained the minerals pyrite, garnet, and horneblende.  The presence of garnet shows that this amphibolite facies was formed under high pressure.  In order for garnet to form in an amphibolite facies a pressure of 6-10 kbar is needed otherwise garnet would not be seen in this type of rock (Spear, 1993, Frost, Frost 2002). (Figure 3b and Figure 3c) Thus being the reason we did not observe any garnet in the rocks located south of the Yellow Medicine Shear Zone. 

Figure 3b (Spear, 1995 cites Ringwood, 1975)                  Figure 3c (Frost and Frost 2000)

References:

Frost, B. R., Frost, C. D., 2002,  An Intoduction to Igneous and Metamorphic Petrology Version 1.2:  Department of Geology and Geophysics University of Wyoming, 697p.

Osman, Ali Farrag, “Facies Concept of Metamorphism” 2003.  Department of Geology United Arab Emirates University.http://faculty.uaeu.ac.ae/~afarrag/Igneous/Facies.htm. 

Southwick, D.L., 2002, Geologic Map of Pre- Cretaceious Bedrock in Southwest Minnesota:  Minnesota Geiological Survey, scale 1:250,000, 1 sheet.

Spear, F. S., 1993, Metamorphic Phase Equilibria and Pressure – Temperature – Time Paths: Washington, D.C.,  Mineralogical Society of America,  799p.