Sacred Heart Granite
Caleb Stroup and Doug Portis Stop#5
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Location: The Sacred Heart Granite can be found at various locations throughout the Minnesota River Valley, predominantly south of the Yellow Medicine Shear Zone. The location where the Sacred Heart granite described below outcrops is near a rural residence; address 43512 hwy cty 7 Granite Falls, MN (SE1/4, NE1/4, SEC 19, T114N, R37W Iverson Lake Quadrangle). This unit is a result of an igneous intrusion into the Morton Gneiss, the surrounding basement rock.
Brief Description: The Sacred Heart granite is generally a grayish-pink color. However, color does vary locally as mineral compositions change and weathering processes vary. The color ranges from a very deep pink to a grayish pink. At the south section of this stop the granite is very angular and blocky. Mafic minerals are more abundant in the lower third of the section and feldspars and quartz dominate the upper part. The north section, which is higher topographically, is slightly more rounded and the same trend in mineral distribution is present. Evidence of metamorphism is subtle but present. Foliation seen locally is near-gneissic. Mineral modes estimated by visual inspection using compositional percentage diagrams (Compton, 1985) are 65% feldspars, 32% quartz, and 3% mafic minerals. Point count estimations verify this, yielding compositions as follows: 65% feldspars, 30% quartz, and 5% mafic minerals. Further investigation with a macroscope confirm that the majority of the feldspar grains are plagioclase, however the percentage varied with the overall color of the rock viewed at outcrop being a function of the plagioclase-K-spar percentage (excluding weathering alterations). The mafic minerals identified in hand sample of the Sacred Heart granite consist of biotite and amphibole (± pyroxene), with biotite being the most abundant. There are also trace amounts of pyrite or chalcopyrite (or both) present (Fig 5-B). Pyrite is indistinguishable from chalcopyrite without the use of thin sections because of the small size of the crystals and similarities in mineral properties.
Figure 5-B
The Sacred Heart granite yields a Pb-Pb age of 2.605 ± 0.006 b.y. (Doe and Delevaux 1980 who cite Lund 1956). This age is consistent with Rb-Sr isochron age of 2.7 b.y. (Sims and Morey 1972, who cite Goldich and others 1970 p. 3687). Another complimentary age is given by the K-Ar age of 2.74 ± 1.4b.y. (Sims and Morey 1972) determined by Thomas 1963).
References:
Doe, BR, Deleaux, MH, 1980. Lead-Isotopes Investigations in The Minnesota River Valley- Late Tectonic and Post-Tectonic Granites. Pgs 105-107. Edited by Morey, GB, Hanson, GH, in Selected Studies of Archean Gneisses and Lower Proterozoic Rocks, Southern Canadian Shield, GSA Special Paper 182.
Compton, RR, 1985. Geology in the Field. United States. John Wiley and Sons Inc. Pgs 366-367.
Sims, PK, Morey GB, 1971. Geology of Minnesota: A Centennial Volume. St. Paul, Minnesota. Minnesota Geological Survey.