Transcript GEOL 2312 IGNEOUS AND METAMORPHIC PETROLOGY Lecture 19 Anorthosites and the Anorthosite Problem March 11, 2009

GEOL 2312
IGNEOUS AND METAMORPHIC
PETROLOGY
Lecture 19
Anorthosites
and the Anorthosite Problem
March 11, 2009
GLOBAL OCCURRENCES
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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
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Proterozoic age (1.7-0.9 Ga)
Intermediate Pl
composition (An 65-40)
Tabular crystal habit
Pl-adcumulates common
(minor interstitial mafics)
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Large pluton-scale bodies
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Anorogenic setting
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Associated with K-/Fe-rich
gabbros (jonuites) and “dry”
felsic rocks (charnokites)
Grenville Province
ANORTHOSITE LAYERS/INCLUSIONS IN
MAFIC LAYERED INTRUSIONS
Bushveld Complex
Skaergaard Intrusion
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
LAKE SUPERIOR ANORTHOSITES
A 150 YEAR PUZZLE
Norwood (1852)
Lawson (1893)
Winchell (1899)
BEAVER RIVER DIABASE
HOST OF ANORTHOSITE INCLUSIONS
Leveaux Porphyry
Lutsen
Tofte
Silver Bay
Carlton Peak
Split
Rock
PORPHYRITIC FE-DIORITE SILLS
LEVEAUX PORPHYRY & CABIN CREEK PORPHYRITIC DIORITE
ANORTHOSITIC SERIES
OF THE DULUTH COMPLEX
troctolitic anorthosite
poikilitic olivine anorthosite
olivine leucogabbro
gabbroic anorthosite
olivine-oxide leucogabbro
pOGA
(PPocf)
TA
(PpO)
“An Igneous Breccia”
R. Taylor, 1964
An (mPP)
mTA (mPpO)
GA (cPpcf)
TA (cPPOc)
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
Complex Patterns
Outer jump in An –
Decompression?
Hi-P
Lo-P
MODELS OF ANORTHOSITE PETROGENESIS
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
GEOCHRONOLOGY OF MCR INTRUSIONS
ANORTHOSITIC SERIES ≈ LAYERED SERIES
GENERAL MODEL OF ANORTHOSITE PETROGENESIS
ASHWAL (1993)