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Geol 2312 Igneous and Metamorphic Petrology

Geol 2312 Igneous and Metamorphic Petrology. Lecture 19 Anorthosites and the Anorthosite Problem. March 11, 2009. Global Occurrences of Anorthosite. 1. Archean anorthosite 2. Proterozoic “massif-type” anorthosite plutons 3. Centimeter-to-100m thick layers in layered mafic intrusions

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Geol 2312 Igneous and Metamorphic Petrology

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  1. Geol 2312 Igneous and Metamorphic Petrology Lecture 19 Anorthosites and the Anorthosite Problem March 11, 2009

  2. Global Occurrences of Anorthosite • 1. Archean anorthosite • 2. Proterozoic “massif-type” anorthosite plutons • 3. Centimeter-to-100m thick layers in layered mafic intrusions • 4. Thin cumulate layers in ophiolites/oceanic crust • 5. Small inclusions in other rock types (xenoliths and cognate inclusions) • 6. Lunar highland anorthosites (after Ashwal, 1993) Streikeisen (1991)

  3. Archean Anorthosites • Hi-Ca plagioclase (An >80) • Equant crystal habit • Pl megacrysts in a gabbroic • matrix • Sill-like bodies • Associated with gabbroic • intrusions in greenstone • belts • Commonly metamorphosed • and deformed

  4. Massif-type Anorthosites • Proterozoic age (1.7-0.9 Ga) • Intermediate Pl composition (An 65-40) • Tabular crystal habit • Pl-adcumulates common (minor interstitial mafics) • Large pluton-scale bodies • Anorogenic setting • Associated with K-/Fe-rich gabbros (jonuites) and “dry” felsic rocks (charnokites) Grenville Province

  5. Anorthosite Layers/Inclusions in Mafic Layered Intrusions Bushveld Complex Skaergaard Intrusion Stillwater Complex

  6. Anorthositic Rocks of the Duluth Complex and Related Rocks of NE MN Anorthositic Rock–bearing Units

  7. Anorthosite Inclusions of the Beaver Bay Complex • 90-100% Plagioclase • An 70-90 • Mafic phases – Ol, Opx • Single grains to 500m block • Isotopes indicate older crustal component • Enclosing diabase NOT chilled against inclusions

  8. Lake Superior AnorthositesA 150 Year Puzzle Norwood (1852) Lawson (1893) Winchell (1899)

  9. Beaver River DiabaseHost of Anorthosite Inclusions Leveaux Porphyry Lutsen Tofte Silver Bay Carlton Peak Split Rock

  10. Porphyritic Fe-Diorite SillsLeveaux Porphyry & Cabin Creek Porphyritic Diorite

  11. troctolitic anorthosite Anorthositic Series of the Duluth Complex poikilitic olivine anorthosite olivine leucogabbro gabbroic anorthosite olivine-oxide leucogabbro

  12. “An Igneous Breccia” R. Taylor, 1964 pOGA (PPocf) TA (PpO) An (mPP) mTA (mPpO) GA (cPpcf) TA (cPPOc)

  13. Well developed, but erratic foliation

  14. Summary of Lithologic and Structural Relationships of Anorthositic Series rocks in the Snowbank Lake Quadrangle

  15. Fo-An Relationships

  16. Plagioclase Zonation Patterns Complex Patterns Outer jump in An – Decompression? Hi-P Lo-P

  17. Models of Anorthosite Petrogenesis

  18. Weiblen and Morey (1980) Model for the formation of the Anorthositic and Layered (Troctolitic) Series

  19. Major Problem: No Evidence for Significant Upper Crustal Ultramafic Rocks

  20. AS Parent Magma = Plagioclase Crystal Mush

  21. Geochronology of MCR IntrusionsAnorthositic Series ≈ Layered Series

  22. General Model of Anorthosite PetrogenesisAshwal (1993)

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