GEOL 2312 IGNEOUS AND METAMORPHIC PETROLOGY

Lecture 19Anorthositesand 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 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)

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

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

ANORTHOSITE LAYERS/INCLUSIONS IN MAFIC LAYERED INTRUSIONS

Skaergaard IntrusionBushveld Complex

Stillwater Complex

ANORTHOSITIC ROCKS OF THE DULUTH COMPLEX AND RELATED ROCKS OF NE MN

Anorthositic Rock–bearing Units

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

Norwood (1852)

Winchell (1899)Lawson (1893)

LAKE SUPERIOR ANORTHOSITESA 150 YEAR PUZZLE

BEAVER RIVER DIABASEHOST OF ANORTHOSITE INCLUSIONS

Carlton Peak

Lutsen

Tofte

SplitRock

Silver Bay

Leveaux Porphyry

PORPHYRITIC FE-DIORITE SILLSLEVEAUX PORPHYRY & CABIN CREEK PORPHYRITIC

DIORITE

troctolitic anorthosite

olivine leucogabbro

poikilitic olivine anorthosite

gabbroic anorthosite

olivine-oxide leucogabbro

ANORTHOSITIC SERIES OF THE DULUTH COMPLEX

mTA (mPpO)

TA (cPPOc)

An (mPP)

GA (cPpcf)

TA(PpO)

pOGA(PPocf)

““An Igneous Breccia”An Igneous Breccia”R. Taylor, 1964

Well developed, but erratic foliationWell 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?

MODELS OF ANORTHOSITE PETROGENESIS

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

Major Problem:Major Problem:

No Evidence for Significant Upper Crustal Ultramafic Rocks

AS Parent Magma =Plagioclase Crystal Mush

GEOCHRONOLOGY OF MCR INTRUSIONSANORTHOSITIC SERIES ≈ LAYERED SERIES

GENERAL MODEL OF ANORTHOSITE PETROGENESISASHWAL (1993)