Layered Mafic Intrusions
(Chapter 12)
last update:10/04/06
These are commonly referred to as Large Igneous Provinces (LIPs)
These may flattened plutons, or lopoliths or funnel-shaped with common chill margins.
They are generally gabbroic, usually tholeiitic with general Fe-enrichment - Skaergard trend.
Crystallization of these take 10,000 to >1 Ma, depending on the size
Layering
layer: any sheet-like cumulate unit distinguished by its compositional and/or textural features
uniform mineralogically and texturally homogeneous
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Uniform chromite layers alternate with plagioclase-rich layers, Bushveld Complex, S. Africa. From McBirney and Noyes (1979) J. Petrol., 20, 487-554. |
non-uniform vary either along or across the layering
graded = gradual variation in either
mineralogy or grain size (rare in gabbroic LMIs)
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| Modal and size graded layers. From McBirney and Noyes (1979) J. Petrol., 20, 487-554. |
Layering (or stratification)
Addresses the structure and fabric of sequences of multiple layers
1) Modal Layering: variation in the relative proportions of constituent minerals
i.e. may contain uniform layers, graded layers, or a combination of both
2) Phase layering: the appearance or disappearance of minerals in the crystallization sequence developed in modal layers
3) Cryptic Layering (not obvious to the eye)
Systematic variation in the chemical composition of certain minerals with stratigraphic height in a layered sequence
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
Rhythmic and Intermittent Layering
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| Vertically tilted cm-scale rhythmic layering of plagioclase and pyroxene in the Stillwater Complex, Montana. | Intermittent layering showing graded layers separated by non-graded gabbroic layers. Skaergård Intrusion, E. Greenland. |
The Stillwater Complex, Montana
2.7 Ga complex in the Beartooth Mtns, MT
(see Kurt Hollocher field trip page)
Two major magma types and times of intrusion:
Base: mafic and olivine saturated magma
Banded Zone: gabbroic or noritic magma
these are likely punctuated by local new contrasting injections of magma that undergo fractionation
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The Banded
Series Notable appearance of 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) Probable multiple reintroduction of magmas J-M Reef (Pt-Pd sulfide layer) in Lower Banded Zone
Ultramafic Series Upper Orthopyroxenite Zone is a single, thick (up to 1070 m), rather monotonous layer of cumulate orthopyroxenite 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 The crystallization sequence within each rhythmic unit (with rare exception) is: olivine + chromite ® olivine + orthopyroxene ® orthopyroxene ® orthopyroxene + plagioclase ® orthopyroxene + plagioclase + augite Basal Series a thin (50-150 m) layer of norites and gabbros - possibly the general bulk composition of the initial magma injection |
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The Skaergård Intrusion E. Greenland
Likely single injection magma
Magma intruded in a single surge (premier natural example of the crystallization of a mafic pluton in a single-stage process)
Eocene (56 Ma) intrusion in Precambrian, Cretaceous and Tertiary rocks
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Skaergård subdivided into three major units: Layered Series Crystallization from the floor upward
Upper Border Series Crystallization from the roof downward
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Marginal Border Series Crystallization from the wall inward
Sandwich Horizon, where the latest, most differentiated liquids crystallized
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The Processes of Crystallization, Differentiation, and Layering in LMIs
LMIs are the simplest possible case, however, they are more complex than anticipated
Still incompletely understood after a half century of intensive study
Rhythmic modal layering and sedimentary-like features 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
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, but it should float |
| Inverted cryptic variations in the Upper
Border Series suggests that the early-formed minerals settled upward? |
| The Marginal Border Series shows vertical
layering |
Compositional Convection Model
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
Double-diffusive convection situation
A series of convecting layers
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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| 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 | 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. |
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| Schematic synthesis of processes possibly affecting the Skaergard Middle Zone |
