Structural State of Plagioclase Phenocrysts in Midcontinent Rift Porphyritic Rocks, Minnesota

1. STRUCTURAL STATE OF PLAGIOCLASE PHENOCRYSTS IN PORPHYRITIC ROCKS OF THE MIDCONTINENT RIFT, NORTHEASTERN MINNESOTA Damion Knudsen and Bernhardt Saini-Eidukat Department of Geosciences, North Dakota State University, Fargo, ND James D. Miller, Jr. Minnesota Geological Survey Peter Daniels Markstrasse 123, 44803 Bochum, Germany Steven Dutch, U W - Green Bay

2. N Area of study

3. BEAVER BAY AREA N

5. Can we use the structural state of plagioclase to determine if there is a petrogenetic relationship between anorthosite inclusions in Beaver Bay Complex intrusives and plagioclase phenocrysts in NSVG basalts?

6. Separation between 131 and 131 peaks 131 131

7. What do we mean by structural state? 2.50 (Smith and Yoder, 1956) Split Rock 2.00 Δ131= 2θ(131)-2θ(131) + Samples from anorthositic masses near the tops of gabbroic sills, Northern Minnesota 1.50 1.00 60 40 80 0 20 100 Mol Percent Anorthite

8. Kroll and Ribbe (1980)

9. Kroll and Ribbe’s (1980) g method

10. Plagioclase crystal structure An0 = 1:3 Al:Si C1 An100 = 1:1 Al:Si I1 R J. Harrison, University of Cambridge, England The feldspar “crankshaft” structure Steven Dutch, Dept. of Natural and Applied Sciences, University of Wisconsin - Green Bay In feldspars, both Al and Si occur in tetrahedrally coordinated sites, known as T sites. With the value t10-<t1m>, t1O = the probability that an Al atom will be found on the t1O site. <t1m> is the average probability of finding an Al atom at any one of these three sites and t10-<t1m> is simply the difference between the two probablities.

11. Phase diagram of plagioclase An0 = 1:3 Al:Si C1 An100 = 1:1 Al:Si I1 R J. Harrison, University of Cambridge, England

12. 5 mm 5 mm 5 mm (Smith and Brown, 1988) High Intermediate Low Diagrams illustrating High, Intermediate, and Low disorder.

13. An0 = 1:3 Al:Si C1 An100 = 1:1 Al:Si I1 ? Temperature and order (Smith and Brown, 1988) Looking at these two different graphs may allow one to interpret a relationship between the two. My idea is that the area between the high and low plagioclase lines on this graph is essentially a solid solution. Once you drop below this line you start to encounter exsolution lamallae and higher states of order. Kroll and Ribbe’s (1980) g method

14. Methods used in this study Anorthite contents determined by electron microprobe at the Department of Geology and Geophysics, University of Minnesota-Twin Cities Philips X’pert PW 3040-MPD diffractometer at NDSU

15. Which 2θ reflection(s) should be used here? Assigning reflections to peaks

16. Profile Fitting using WinPlotr

17. Unit Cell Refinement using “Unit Cell”

18. Croftville Cabin Creek Scott Creek Katydid Lake PDD

19. Results using  method of Kroll and Ribbe, 1980

20. t1O-<t1m> Kroll and Ribbe’s (1980) g method A new method: t10-<t1m> vs. An γ-2.011-0.2471*(An % /100)-0.35*0.2012|(An % /100)-0.33| -0.86*0.2012|(An % /100)-0.33| t10-<t1m> =

21. Results using our method of t10-<t1m>

22. Conclusions • Plagioclase phenocrysts in basalts exhibit intermediate to high structural state • The Leveaux Porphyry phenocrysts have intermediate structural states and plot near Duluth Complex anorthositic series samples, while the Cabin Creek phenocrysts have distinctly more disordered structures. • Plagioclase phenocrysts from the Scott Creek leucogabbro and the Katydid Lake gabbroic anorthosite are intermediate to ordered and plot within the field of Duluth Complex anorthositic series samples. • IN ADDITION: • Large plagioclase rich xenoliths in the Beaver Bay Complex vary in their structural state with texture type, and Anorthositic series rocks of the Duluth Complex are generally in low structural state.

23. Acknowledgements Paul Albers University of Minnesota Duluth Ellery Frahm Electron microprobe laboratory at the Department of Geology and Geophysics, University of Minnesota-Twin Cities.