Large or particularly well-studied LMIs exposed in continents (many in flood basalt provinces)

1
Chapter 12 Layered Mafic Intrusions
Table 12.1
. Some Principal Layered Mafic Intrusions

• Large or particularly well-studied LMIs exposed
  in continents (many in flood basalt provinces)

2
The form of a typical LMI
Figure 12.1. From Irvine and Smith (1967), In P.
J. Wyllie (ed.), Ultramafic and Related Rocks.
Wiley. New York, pp. 38-49.
The Muskox Intrusion
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Layering

• layer any sheet-like cumulate unit distinguished
  by its compositional and/or textural features
• uniform mineralogically and texturally homogeneous

4
Uniform Layering
Figure 12.3b. Uniform chromite layers alternate
with plagioclase-rich layers, Bushveld Complex,
S. Africa. From McBirney and Noyes (1979) J.
Petrol., 20, 487-554.
5
Layering

• layer any sheet-like cumulate unit distinguished
  by its compositional and/or textural features
• uniform mineralogically and texturally
  homogeneous
• non-uniform vary either along or across the
  layering

• graded gradual variation in either
• mineralogy
• grain size - quite rare in gabbroic LMIs

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Graded Layers
Figure 12.2. Modal and size graded layers. From
McBirney and Noyes (1979) J. Petrol., 20,
487-554.
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Layering (or stratification)

• Addresses the structure and fabric of sequences
  of multiple layers
• 1) Modal Layering characterized by variation in
  the relative proportions of constituent minerals
• may contain uniform layers, graded layers, or a
  combination of both

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Layering (or stratification)

• 2) Phase layering the appearance or
  disappearance of minerals in the crystallization
  sequence developed in modal layers
• Phase layering transgresses modal layering

9

• 3) Cryptic Layering (not obvious to the eye)
• Systematic variation in the chemical composition
  of certain minerals with stratigraphic height in
  a layered sequence

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• The regularity of layering
• Rhythmic layers systematically repeat
• Macrorhythmic several meters thick
• Microrhythmic only a few cm thick
• Intermittent less regular patterns
• A common type consists of rhythmic graded layers
  punctuated by occasional uniform layers

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Rythmic and Intermittent Layering
Figure 12.3a. Vertically tilted cm-scale rhythmic
layering of plagioclase and pyroxene in the
Stillwater Complex, Montana.
Figure 12.4. Intermittent layering showing graded
layers separated by non-graded gabbroic layers.
Skaergård Intrusion, E. Greenland. From McBirney
(1993) Igneous Petrology (2nd ed.), Jones and
Bartlett. Boston.
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The Bushveld Complex, South Africa
The biggest 300-400 km x 9 km
Lebowa granitics intruded 5 Ma afterward
Simplified geologic Map and cross section of the
Bushveld complex. From The Story of Earth Life
McCarthy and Rubidge
13

• Marginal Zone the lowest unit, is a chill zone
  about 150 m thick
• Fine-grained norites from the margin correspond
  to a high-alumina tholeiitic basalt

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Stratigraphy

• Basal Series
• Thin uniform dunite cumulates alternating with
  orthopyroxenite and harzburgite layers
• The top defined as the Main Chromite Layer

Figure 12.6. Stratigraphic sequence of layering
in the Eastern Lobe of the Bushveld Complex.
After Wager and Brown (1968) Layered Igneous
Rocks. Freeman. San Francisco.
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• Critical Series
• Plagioclase forms as a cumulate phase (phase
  layering)
• Norite, orthopyroxenite, and anorthosite layers
  etc

Figure 12.6. Stratigraphic sequence of layering
in the Eastern Lobe of the Bushveld Complex.
After Wager and Brown (1968) Layered Igneous
Rocks. Freeman. San Francisco.
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The Merensky Reef 150 m thick sequence of
rhythmic units with cumulus plagioclase,
orthopyroxene, olivine, and chromite
Figure 12.6. Stratigraphic sequence of layering
in the Eastern Lobe of the Bushveld Complex.
After Wager and Brown (1968) Layered Igneous
Rocks. Freeman. San Francisco.
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Main Zone the thickest zone and contains thick
monotonous sequences of hypersthene gabbro,
norite, and anorthosite
Figure 12.6. Stratigraphic sequence of layering
in the Eastern Lobe of the Bushveld Complex.
After Wager and Brown (1968) Layered Igneous
Rocks. Freeman. San Francisco.
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Upper Zone Appearance of cumulus magnetite
(Fe-rich) Well layered anorthosite, gabbro, and
ferrodiorite Numerous felsic rock types late
differentiates
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Also note Cryptic layering systematic change
in mineral compositions Reappearance of Fe-rich
olivine in the Upper Zone
Figure 12.6. Stratigraphic sequence of layering
in the Eastern Lobe of the Bushveld Complex.
After Wager and Brown (1968) Layered Igneous
Rocks. Freeman. San Francisco.
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Figure 12.7. The Fo-Fa-SiO2 portion of the
FeO-MgO-SiO2 system, after Bowen and Schairer
(1935) Amer. J. Sci., 29, 151-217.
21
How can we explain the conspicuous development of
rhythmic layering of often sharply-defined
uniform or graded layers?
22
The Stillwater Complex, Montana
Figure 12.8. After Wager and Brown (1968) Layered
Igneous Rocks. Freeman. San Francisco.
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Stratigraphy

• Basal Series
• a thin (50-150 m) layer of norites and gabbros
• Ultramafic Series base first appearance of
  copious olivine cumulates (phase layering)
• Lower Peridotite Zone
• 20 cycles (20-150 m thick) of macrorhythmic
  layering with a distinctive sequence of
  lithologies
• The series begins with dunite (plus chromite),
  followed by harzburgite and then orthopyroxenite
• Upper Orthopyroxenite Zone
• is a single, thick (up to 1070 m), rather
  monotonous layer of cumulate orthopyroxenite

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• The crystallization sequence within each rhythmic
  unit (with rare exception) is
• olivine chromite ?
• olivine orthopyroxene ?
• orthopyroxene ?
• orthopyroxene plagioclase ?
• orthopyroxene plagioclase augite

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Stratigraphy

• The Banded Series
• Sudden cumulus plagioclase significant change
  from ultramafic rock types (phase layering again)
• The most common lithologies are anorthosite,
  norite, gabbro, and troctolite (olivine-rich and
  pyroxene-poor gabbro)

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The Skaergård Intrusion E. Greenland
Figure 12.10. After Stewart and DePaolo (1990)
Contrib. Mineral. Petrol., 104, 125-141.
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• Magma intruded in a single surge (premier natural
  example of the crystallization of a mafic pluton
  in a single-stage process)
• Fine-grained chill margin

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Stratigraphy

• Skaergård subdivided into three major units
• Layered Series
• Upper Border Series
• Marginal Border Series
• Upper Border Series and the Layered Series meet
  at the Sandwich Horizon (most differentiated
  liquids)

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• Cross section looking down dip.

Figure 12.11. After After Hoover (1978) Carnegie
Inst. Wash., Yearb., 77, 732-739.
31

• Upper Border Series thinner, but mirrors the
  2500 m Layered Series in many respects
• Cooled from the top down, so the top of the Upper
  Border Series crystallized first
• The most Mg-rich olivines and Ca-rich
  plagioclases occur at the top, and grade to more
  Fe-rich and Na-rich compositions downward
• Major element trends also reverse in the Upper
  Border Series as compared to the LBS

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• Sandwich Horizon, where the latest, most
  differentiated liquids crystallized
• Ferrogabbros with sodic plagioclase (An30), plus
  Fe-rich olivine and Opx
• Granophyric segregations of quartz and feldspar
• F G immiscible liquids that evolve in the
  late stages of differentiation?

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Stratigraphy, Modal, and Cryptic
Layering(cryptic determined for intercumulus
phases)
Figure 12.12. After Wager and Brown (1968)
Layered Igneous Rocks. Freeman. and Naslund
(1983) J. Petrol., 25, 185-212.
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Chemistry of the Skaergård
Figure 12-13. After McBirney (1973) Igneous
Petrology. Jones and Bartlett.
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The Processes of Crystallization,
Differentiation, and Layering in LMIs

• LMIs are the simplest possible case
• More complex than anticipated
• Still incompletely understood after a half
  century of intensive study

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• Rhythmic modal layering most easily explained by
  crystal settling interrupted by periodic
  large-scale convective overturn of the entire
  cooling unit
• Reinjection of more primitive magma may explain
  major compositional shifts and cases of irregular
  cryptic variations

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• Problems with the crystal settling process.
• Many minerals found at a particular horizon are
  not hydraulically equivalent
• Size is more important than density in Stokes
  Law, but size grading is rare in most LMIs
• Dense olivine in the Upper Border Series of the
  Skaergård
• Plagioclase is in the lower layers of the
  Skaergård

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• Inverted cryptic variations in the Upper Border
  Series suggests that the early-formed minerals
  settled upward
• The Marginal Border Series shows vertical
  layering
• Basaltic magmas develop a high yield strength,
  slightly below liquidus temperatures

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In-Situ Processes

• Nucleation and growth of minerals in a thin
  stagnant boundary layer along the margins of the
  chamber
• Differential motion of crystals and liquid is
  still required for fractionation
• Dominant motion migration of depleted liquid
  from the growing crystals
• Crystals settle (or float) a short distance
  within the boundary layer as the melt migrates
  away
• Boundary layer interface inhibits material motion

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• Systems with gradients in two or more properties
  (chemical or thermal) with different rates of
  diffusion
• Especially if have opposing effects on density in
  a vertical direction
• Compositional Convection

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• One gradient (in this case rtemp) is
  destabilizing (although the total density
  gradient is stable)
• The diffusivity of the destabilizing component
  (heat) is faster than the diffusivity of the salt

Figure 12.14. After Turner and Campbell (1986)
Earth-Sci. Rev., 23, 255-352.
42

• Double-diffusive convection situation
• A series of convecting layers

Figure 12.14. After Turner and Campbell (1986)
Earth-Sci. Rev., 23, 255-352.
43

• Density currents
• Cooler, heavy-element-enriched, and/or
  crystal-laden liquid descends and moves across
  the floor of a magma chamber
• Dense crystals held in suspension by agitation
• Light crystals like plagioclase also trapped and
  carried downward

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Figure 12.15a. Cross-bedding in cumulate layers.
Duke Island, Alaska. Note also the layering
caused by different size and proportion of
olivine and pyroxene. From McBirney (1993)
Igneous Petrology. Jones and Bartlett
Figure 12.15b. Cross-bedding in cumulate layers.
Skaergård Intrusion, E. Greenland. Layering
caused by different proportions of mafics and
plagioclase. From McBirney and Noyes (1979) J.
Petrol., 20, 487-554.
45

• Neil Irvings Vortex model

Figure 12.16. After Irvine et al. (1998) Geol.
Soc. Amer. Bull., 110, 1398-1447.
Black flow lines and arrows indicate motion
relative to the cell
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Figure 12-17. After Irvine et al. (1998) Geol.
Soc. Amer. Bull., 110, 1398-1447.
47
Figure 12.18. Cold plumes descending from a
cooled upper boundary layer in a tank of silicone
oil. Photo courtesy Claude Jaupart.
48
Figure 12.19. Schematic illustration of the
density variation in tholeiitic and calc-alkaline
magma series (after Sparks et al., 1984) Phil.
Trans. R. Soc. Lond., A310, 511-534.
49
Figure 12.20. Schematic illustration of a model
for the development of a cyclic unit in the
Ultramafic Zone of the Stillwater Complex by
influx of hot primitive magma into cooler, more
evolved magma. From Raedeke and McCallum (1984)
J. Petrol., 25, 395-420.