Crystal Size Distribution of Saganaga Tonalite, MN ALEXANDER REIMERS PETROLOGY PROJECT 2018 Regional Setting Saganaga Tonalite is situated to the north plex of the Duluth com within the Archean Superior Craton Named a er Saganaga Lake Archean in Age: m.y.a ~2700 ± 50 ◦ Rb-‐Sr, K-‐Ar, and U-‐Pb da ng methods were used Originated from the Algoman Orogeny (Kenoran Orogeny in Canada) before the Mid-‐con nental ri Saganaga Tonalite
Map of Achean terranes of Minnesota
Map: usgeologymorphology.com
Saganaga Tonalite Extent (USA) Algoman Orogeny Repeated episodes of Late Archean con nental collisions, compressions, and subduc ons Saganaga Tonalite Homogenous, composite Intrusive body Intrudes Greenstone and Northern Light Gneiss and overlain by metasedimentary rocks of Knife Lake Group (Hanson,1962) Composi on ◦ Felsic ◦ 20-‐60% Quartz ◦ 50-‐90% Plagioclase (Oligoclase or Andesine) ◦ <10% Alkali Feldspar Mineral Approximation
Sample Plagioclase (%) K-‐spar (%) Quartz (%) Other (%)
55 5 35 5 ACR18 -‐ 1 Saganaga Tonalite 65 5 25 5 ACR18 -‐ 2
65 0 30 5 ACR18 -‐ 3
61.67 3.33 30.00 5.00 Average • Mineral Approxima ons were made via petrographic microscope for three different thin sec ons. • Average mineral percentage for the bulk rock sample is shown. Guiding Question
Determine if the cooling rate of the Saganaga Tonalite can be es mated based on 2-‐D crystal size distribu on of Plagioclase Methods
Produced 3 thin sec ons Analyzed general mineral composi on and iden fied lase Plagioc crystals Import image, taken on Dr. Tacke ’s microscope, of thin sec ons into an image analysis so ware (Digimizer) for crystal size distribu on analysis Photo courtesy of Eidukat Dr. Crystal Size Distribution (CSD) Crystal size and number density are intricately o linked t the rates of crystal growth and nuclea on, which change in response to a varia ons in magm temperature, vapor pressure, and melt composi on. 3-‐D analysis must be done to get truly ver, accurate data, howe this study will analyze a 2-‐D surface, so there is error built it The focus of this study is to see if CSD will work on a Tonalite using a 2-‐D analysis
(Brugger & Hammer, 2010) Digimizer: Image Analysis Program Digimizer allows you to: ◦ Manipulate an Image Make Manual Measurements • Image Analysis ◦ Contrast & brightness • ◦ Background correc on • Define unit of measurement • Binariza on ◦ Despeckle • Distances • Overlay binary Layer ◦ Convert to color or grayscale • Perimeters • Noise reduc on ◦ Sharpen • Areas • Object detec on ◦ Nega ve • Measure Angles ◦ Filters • Mark and Count Objects ◦ Etc. • Etc.
www.digimizer.com Analysis of PlagioclaseLength Vs. Width Minerals 3.5
3 Measured Data Same Length & Width 2.5
2 y = 0.628x + 0.0191 1.5 Width (mm) (mm) Width 1
0.5
9.5 0 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 Length (mm) Crystal Size Vs. Amount of Crystals (Plagioclase) 14 13 12 Interpretation 11 10 CSD analysis shows that the majority of plag. grains 9 2 8 are between 0.75 – 3 mm in area 7 6 Distribu on shows some outliers and has a high 5 4 variance 3 ◦ Could be misiden fied minerals (quartz ) and not plag Crystals of Amount 2 1 ◦ Whole rock could have been slightly metamorphosed 0 and some crystals didn’t melt en rely 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5 Crystal Size Area (mm2) Conclusion Saganaga Tonalite is composed of approximately % 61% Plag, 3 K-‐spar, 30% quartz, 5% Other Plag. crystals are approximately 30% longer ide than they are w Plag. crystal size distribu on is highly variable (0.43 – 9mm2) with the majority of crystals between (0.75 2 – 3mm ) CSD to determine cooling rate in 2-‐D analysis was not determined because of extreme error in size distribu on because of the cut-‐sec on effect