G EOL 2312 I GNEOUS AND M ETAMORPHIC P ETROLOGY Lecture 19 Anorthosites and the Anorthosite Problem March 11, 2009.

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G EOL 2312 I GNEOUS AND M ETAMORPHIC P ETROLOGY Lecture 19 Anorthosites and the Anorthosite Problem March 11, 2009

G LOBAL O CCURRENCES OF A NORTHOSITE 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)

A RCHEAN A NORTHOSITES 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

M ASSIF - TYPE A NORTHOSITES Proterozoic age ( 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

A NORTHOSITE L AYERS /I NCLUSIONS IN M AFIC L AYERED I NTRUSIONS Skaergaard Intrusion Bushveld Complex Stillwater Complex

A NORTHOSITIC R OCKS OF THE D ULUTH C OMPLEX AND R ELATED R OCKS OF NE MN Anorthositic Rock–bearing Units

A NORTHOSITE I NCLUSIONS OF THE B EAVER B AY C OMPLEX % Plagioclase An Mafic phases – Ol, Opx Single grains to 500m block Isotopes indicate older crustal component Enclosing diabase NOT chilled against inclusions

Norwood (1852) Winchell (1899)Lawson (1893) L AKE S UPERIOR A NORTHOSITES A 150 Y EAR P UZZLE

B EAVER R IVER D IABASE H OST OF A NORTHOSITE I NCLUSIONS Carlton Peak Lutsen Tofte Split Rock Silver Bay Leveaux Porphyry

P ORPHYRITIC F E -D IORITE S ILLS L EVEAUX P ORPHYRY & C ABIN C REEK P ORPHYRITIC D IORITE

troctolitic anorthosite olivine leucogabbro poikilitic olivine anorthosite gabbroic anorthosite olivine-oxide leucogabbro A NORTHOSITIC S ERIES OF THE D ULUTH C OMPLEX

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

Well developed, but erratic foliation

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

Fo-An Relationships

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

M ODELS OF A NORTHOSITE P ETROGENESIS

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

Major Problem: No Evidence for Significant Upper Crustal Ultramafic Rocks

AS Parent Magma = Plagioclase Crystal Mush

G EOCHRONOLOGY OF MCR I NTRUSIONS A NORTHOSITIC S ERIES ≈ L AYERED S ERIES

G ENERAL M ODEL OF A NORTHOSITE P ETROGENESIS A SHWAL (1993)