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

Table 12.1Table 12.1. Some Principal Layered Mafic Intrusions. Some Principal Layered Mafic Intrusions

NameName AgeAge LocationLocation AreaArea (km(km 22))

BushveldBushveld PrecambrianPrecambrian S. AfricaS. Africa 66,00066,000

DufekDufek JurassicJurassic AntarcticaAntarctica 50,00050,000

DuluthDuluth PrecambrianPrecambrian Minnesota, USAMinnesota, USA 4,7004,700

StillwaterStillwater PrecambrianPrecambrian Montana, USAMontana, USA 4,4004,400

MuskoxMuskox PrecambrianPrecambrian NW Terr. CanadaNW Terr. Canada 3,5003,500

Great DikeGreat Dike PrecambrianPrecambrian ZimbabweZimbabwe 3,3003,300

KiglapaitKiglapait PrecambrianPrecambrian LabradorLabrador 560560

SkaergårdSkaergård EoceneEocene East GreenlandEast Greenland 100100

Chapter 12: Layered Mafic IntrusionsChapter 12: Layered Mafic Intrusions

The form of a typical LMI

The Muskox Intrusion

Figure 12.1. From Irvine and Smith (1967), In P. J. Wyllie (ed.), Ultramafic and Related Rocks. Wiley. New York, pp. 38-49.

LayeringLayeringlayer: any sheet-like cumulate unit distinguished by

its compositional and/or textural features uniform mineralogically and texturally

homogeneous

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

LayeringLayeringlayer: 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

layeringgraded = gradual variation in either

mineralogy grain size - quite rare in gabbroic LMIs

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

Layering (or stratification)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

Layering (or stratification)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

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 layeringThe 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

Rythmic and Intermittent LayeringRythmic and Intermittent Layering

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.

Figure 12.3a. Vertically tilted cm-scale rhythmic layering of plagioclase and pyroxene in the Stillwater Complex, Montana.

The Bushveld Complex, South AfricaThe Bushveld Complex, South Africa

The biggest: 300-400 km x 9 km

Lebowa graniticsintruded 5 Maafterward

Simplified geologic Map and cross section of the Bushveld complex. From The Story of Earth & Life McCarthy and Rubidge

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

StratigraphyStratigraphyBasal 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.

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.

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.

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.

Upper Zone

Appearance of cumulus magnetite (Fe-rich)

Well layered: anorthosite, gabbro, and ferrodiorite

Numerous felsic rock types = late differentiates

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.

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.

How can we explain the conspicuous development of rhythmic layering of often sharply-defined uniform or graded layers?

The Stillwater Complex, MontanaThe Stillwater Complex, Montana

Figure 12.8. After Wager and Brown (1968) Layered Igneous Rocks. Freeman. San Francisco.

StratigraphyStratigraphy 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

The crystallization sequence within each rhythmic unit (with rare exception) is:

olivine + chromite olivine + orthopyroxene orthopyroxene orthopyroxene + plagioclase orthopyroxene + plagioclase + augite

StratigraphyStratigraphyThe 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)

The Skaergård Intrusion E. GreenlandThe Skaergård Intrusion E. Greenland

Figure 12.10. After Stewart and DePaolo (1990) Contrib. Mineral. Petrol., 104, 125-141.

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

StratigraphyStratigraphySkaergård subdivided into three major units:

Layered SeriesUpper Border SeriesMarginal Border Series

Upper Border Series and the Layered Series meet at the Sandwich Horizon (most differentiated liquids)

Cross section looking down dip. Figure 12.11. After After Hoover (1978) Carnegie Inst. Wash., Yearb., 77, 732-739.

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

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?

Stratigraphy, Stratigraphy, Modal, and Modal, and

Cryptic Cryptic LayeringLayering

(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.

Chemistry Chemistry of the of the

SkaergårdSkaergård

Figure 12-13. After McBirney (1973) Igneous Petrology. Jones and Bartlett.

The Processes of Crystallization, The Processes of Crystallization, Differentiation, and Layering in LMIsDifferentiation, and Layering in LMIs

LMIs are the simplest possible case More complex than anticipated Still incompletely understood after a half

century of intensive study

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

Problems with the crystal settling process.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

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

In-SituIn-Situ Processes 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

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

• One gradient (in this casetemp) 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.

Double-diffusive convection situation A series of convecting layers

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

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

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.

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

Neil Irving’s Vortex model

V S

R

progradedeposition

Black flow lines and arrows indicate motion relative to the cell

Figure 12.16. After Irvine et al. (1998) Geol. Soc. Amer. Bull., 110, 1398-1447.

Figure 12-17. After Irvine et al. (1998) Geol. Soc. Amer.

Bull., 110, 1398-1447.

Figure 12.18. Cold plumes descending from a cooled upper boundary layer in a tank of silicone oil. Photo courtesy Claude Jaupart.

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.

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.