James D. Miller
Department of Geological Sciences
Precambrian Research Center
University of Minnesota Duluth
“ A layered intrusion may be likened to the
crucible, greatly magnified, of the
experimental petrologist. However, because
of its vastly greater scale, new phenomena
not seen in the crucible become important.
Among these are convection currents in the
liquid, effects due to different hydrostatic
pressures at different levels in the liquid, the
variable sites of nucleation and growth of the
crystals in the magma, and special processes
connected with the crystallization of intersitial
liquid between the cumulus crystals.
Fundamentally, however, the varied
phenomena of layered intrusions are to be
traced to back to the results of gravity acting
on a crystallizing magma as heat is lost slowly
to the surroundings.”
Wager and Brown (1968)
Layered Igneous Rocks, p. 7
Phenomenon
Dynamic/Synmagmatic
Static/Postmagmatic
Style of Crystallization
Magmatic Sedimentation
In Situ Boundary Layer Crystallization
Magmatic
Differentiation
Cumulate Processes
Crystal Fractionation
Liquid Fractionation
Primocryst Accumulation/Trapped Liquid
Cementation
In Situ Crystallization
Compositional Zone Refining
Convection
Vigorous Whole Intrusion Convection
Double Diffusive Convection
Weak Whole Intrusion Convection
Liquid Line of Descent
Extreme Fe Enrichment (Fenner Trend)
Fe and Si Enrichment (Bowen Trend)
Openness to Recharge
and Venting
Igneous Lamination
Closed
Open
Flow Alignment
Rotation, Dissolution & Reprecipitation
During Compaction
Modally Graded
Layering
Density Stratification of Cumulus Minerals
Diffusion-Controlled Nucleation & Growth
Solution & Reprecipitation of Weak Layering
Trough Layers
Channelized Magmatic Flow
Compaction-induced Layering
Anorthositic Autoliths
Pre-Detachment Plagioclase Enrichment &
Layering
Post-Detatchment Recrystallization &
Secondary Layering
25-50%
7-25%
<7%
Postcumulus Assemblage = Intercumulus Magma, Porosity of Cumulate (based on Irvine, 1987)
Primocrysts
suhedral to anhedral
granular phases that
form the structural
framework of the rock
and evidently
crystallized early.
Interstitial Phases
anhedral granular to
poikilitic phases that
occur between
primocrysts and
evidently crystallized
late.
A Complete Rock Description should include:
• Alteration/Metamorphism (intensity, extent, type)
• Homogeneity/Relative Grain Size
• Absolute Grain Size (range)
• Internal Structures (layering, igneous foliation)
• Bulk Rock Texture (pyroxene-based in gabbroic rocks)
• Accessory/Minor Mineral Modifier
• Modal Rock Name
• Special Features
For Example:
Moderately serpentinized, homogenous, medium-grained, well foliated,
subophitic to ophitic, Cr spinel-bearing, augite melatroctolite with
sub-mm olivine grains
Intensity: Complete, Strong, Moderate, Weak
Extent: even, irregular, patchy, layered, along fractures, mafic phases
only, plagioclase only, olivine only, pyroxene only,...
Type:
Olivine  serpentine, talc, magnetite, iddingsite
Plagioclase  sericite, sausserite (ser+ epid + calc), albite,
hematite, prehnite, pumpellyite, zeolite
Pyroxene  uralite, actinolite, chlorite
Ilmenite  leucoxene
See Alteration.pdf in CD
Generalized scale:
Fine (<1 mm)Medium (1-5 mm) Coarse (5-12 mm)
Very Coarse/Pegmatitic (>12 mm)
Detailed scale:
Very Fine (<0.2 mm) Fine (0.2-0.8 mm)
Medium Fine (0.8-1.5 mm)
Medium (1.5-3 mm) Medium Coarse (3-7 mm)
Coarse (7-12 mm) Very Coarse (12-30 mm)
Pegmatitic (>30 mm)
Equigranular – generally equal sizes for all granular phases
Seriate – gradational range in grain size of all granular phases
Hiatial – bimodal range in grain size of all granular phases
Porphyritic – bimodal range in grain size of one (or rarely two) primary phase types
modifiers based on contrast in grain size– weakly, moderately, strongly
alternate terminology – "phase"-phyric (e.g., plagioclase-phyric)
Foliation - Alignment of elongate or tabular mineral
phases (igneous lamination, fluxion structure)
Degree - % aligned within 10 of a common plane
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non-foliated (<25%)
poorly foliated (25-50%)
moderately foliated (50-75%)
well foliated (75-90%)
very well foliated (>90%)
Layering - Not typically observed at the scale of a thin section)
Type: modal, isomodal, graded modal, grain size, textural, phase
Contrast (or demarcation): strong, moderate, weak, subtle
Frequency: single layer, rhythmic, intermittent, irregular
Scale: centimeter, decimeter, meter, inconsistent, variable
Lateral continuity: continuous, discontinuous, lenticular, wispy
Other descriptors: wavy, cross-bedded, schlerien, colloform, trough,
corrugated, deformed, slumped, convoluted
For Gabbroic Rocks
 ophitic – multiple lath-shaped
crystals of plagioclase totally
enclosed in crystals of pyroxene
 subophitic – multiple lath-shaped
crystals of plagioclase partially
enclosed in crystals of pyroxene
 intergranular – generally
equigranular euhedral to anhedral
primary minerals (need not be augite
and plagioclase), none enclosing the
others
Terms for other primary non-granular phases
poikilitic – one phase completely envelops many other more
See Textures.pfd in CD
granular phases (e.g., plagioclase-poikilitic)
subpoikilitic – one phase partially envelops other more granular
phases (e.g., olivine-subpoikilitic)
Mineral Textures
Ophitic
Subophitic
Intergranular
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uses all five major essential mineral phases (Pl, Ol, Cpx, Opx, and
FeOx)
defines modal boundaries that bracket natural modal populations
and cotectic proportions determined from experimental data
uses simple mafic mineral ratios (3:1 to 1:1) which are easy to
estimate in the field.
Cotectic Proportions (volume %)
Plagioclase
Olivine
70-75
60-64
65-70
55-60
55-60
25-30
Clinopyx
Orthopyx
36-40
10-15
30-35
25-30
15-20
20-25
Rock Type
Troctolite
Norite
Gabbro
Ol Gabbro
Gabbronorite
From McCallum, Raedke, and Mathez (1980)
Pl+Cpx+Ol+FeOx
~58 : 25 : 10 : 7
Ol Gabbro (PCOF)
Ol Ox Gabbro (PCFO)
Ox Gabbronorite (PCFIP)
Pl
T
G
Pl+Ol cumulate
~ 70 : 30
D
Tholeiitic
Parent Magma
Ol
Troctolite (PO)
Melatroctolite (OP)
Augite Troctolite (POcf)
Ol Gabbro (PcOf)
Ol cumulate
Dunite (O)
Feldspathic Dunite (Op)
Plagioclase 10-30% - “Feldspathic”
Ol
Peridotite Group
Duluth Complex, Minnesota
Dunite from Layered Series
Opx
Pyroxenite Group
Pl
Bushveld Complex, South Africa
Feldspathic orthopyroxenite above
Merensky Reef, Upper Critical Zone
Modified from LeMaitre (1989)
-Cotectic Troctolite
72%Pl 28%Ol
Streckeisen (1976)
LeMaitre (2002)
Duluth Complex Schemes
Phinney (1972)
Davidson (1969)
Severson and Hauck (1990)
Modal Data compiled by Miller (1986)
Cp+Ox
Fails to take into account the
mode and texture of interstial
(postcumulus) phases
Main Attributes:
 Applied to rocks that show some igneous foliation or modal layering and
therefore solidified under conditions in which mineral phases segregated
from their parent magma by either crystal or liquid fractionation
 Lists abbreviations of all minerals composing greater than about 2 modal
percent in decreasing order of abundance regardless of their cumulus
status
 Denotes granular (cumulus) mineral phases with upper case letter
abbreviations and interstitial (intercumulus) mineral phases with lower case
abbreviations
Cumulus/Intercumulus Mineral Codes
PP*/P/p - plagioclase
F/f
O/o
- olivine
A/a
C/c
- clinopyroxene (augite) -/b
I/i
- inverted pigeonite
-/a
H/h
- hypersthene, bronzite
-/g
* used with anorthositic group rocks
-
Fe-Ti oxide
apatite
biotite
amphibole
granophyre
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Cumulate Code Translation of Some Common Rock Types in
the Duluth Complex
Ophitic augite troctolite POcf
Augite-bearing oxide troctolite POFc
Olivine gabbroic anorthosite with poikilitic olivine
PPoc
Ophitic olivine gabbro PcOf
Biotitic, feldspathic dunite with poikilitic plagioclase Opb
Intergranular, apatitic oxide olivine gabbro
PCFOA
Ophitic biotitic augite leucotroctolite POcb
Possible modifications of the code:
- preface with grain size designator (e.g. mPOcf, mcPPcfab)
-designate well developed foliation of plagioclase with a bar over the P or PP
-designate poikilitic texture with a ^ over the phase abbreviation (e.g. cPPĥ)
-designate subpoikilitic texture with a ´ over the phase abbreviation (e.g mfPCó)
O
PO
PCFO
PcOf
Lithostratigraphic Classification for Keweenawan Rocks of NE Minnesota