Super Laboratories for the study of Super Laboratories for the study of fractionationfractionation• Initially hotInitially hot• Initially low viscosityInitially low viscosity• Slow cooling - deepSlow cooling - deep

Mafic intrusions come in all sizes Mafic intrusions come in all sizes – from thin dikes and sills from thin dikes and sills – up to the huge 66,000 kmup to the huge 66,000 km22 x 9 km thick Bushveld x 9 km thick Bushveld

intrusion of South Africaintrusion of South Africa

Can occur in any tectonic environment Can occur in any tectonic environment where basaltic magma is generatedwhere basaltic magma is generated

Chapter 12 Layered Mafic IntrusionsChapter 12 Layered Mafic Intrusions

Small, shallow intrusives such as the Palisades Sill, show mineral layers consistent with gravity settling and fractionation to silica enrichment.

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

Large magma chambers are not so simple

The funnel shape of a typical LMI (lopolith,

etc.)

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.

LayeringLayering

layer:layer: any sheet-like cumulate unit any sheet-like cumulate unit distinguished by its compositional distinguished by its compositional and/or textural featuresand/or textural features– 1. uniform1. uniform mineralogically and mineralogically and

texturally homogeneoustexturally homogeneous

Uniform Uniform LayeringLayering

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.

LayeringLayering– 2. non-uniform2. non-uniform vary either along or across vary either along or across

the layeringthe layering

gradedgraded = gradual variation in either = gradual variation in either mineralogymineralogy , sometimes with different , sometimes with different

densitiesdensities Example: SkaergårdExample: Skaergård grain sizegrain size - Rare in gabbroic LMIs - Rare in gabbroic LMIs

Example: Duke IslandExample: Duke Island

Graded Graded LayersLayers

Figure 12-2. Modal and size graded layers. From McBirney and Noyes (1979) J. Petrol., 20, 487-554.

Modal (Minereralogical) layering of olivine at base and plagioclase higher, Skaergaard

Size layering Opx and Plag, Duke Island. Larger crystals at the bottom.

The regularity of layeringThe regularity of layering Rhythmic:Rhythmic: layers systematically layers systematically repeat repeat

– Macrorhythmic:Macrorhythmic: several meters thick several meters thick– Microrhythmic:Microrhythmic: only a few cm thick only a few cm thick

Example: StillwaterExample: Stillwater Intermittent:Intermittent: less regular patterns less regular patterns

– A common type consists of rhythmic A common type consists of rhythmic graded layers punctuated by graded layers punctuated by occasional uniform layers: Skaergårdoccasional uniform layers: Skaergård

Rythmic and Intermittent Rythmic and Intermittent LayeringLayering

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 Microrhythmic layering of plagioclase and pyroxene in the Stillwater Complex, Montana.

Plag – pyroxene, StillwaterPlag – pyroxene, Stillwater

Intermittent graded and Intermittent graded and non-graded layers, non-graded layers, SkaergårdSkaergård

Ex. 1: Bushveld Complex, South Ex. 1: Bushveld Complex, South AfricaAfrica

The biggest: 300-400 km x 9 km

The Red Graniteintruded 50-100 Maafterwards

Figure 12-5. Simplified geologic Map and cross section of the Bushveld complex. After Willemse (1964), Wager and Brown (1968), and Irvine et al. (1983).

These and other examples show These and other examples show conspicuous development of rhythmic conspicuous development of rhythmic layering of often sharply-defined uniform layering of often sharply-defined uniform or graded layers?or graded layers?

The repetition requires either some impressively periodic reinjection of fresh magma, or cyclic variation in one or more physical properties if it is to be produced by gravitational crystal settling alone. The pattern of cryptic layering, however, indicates a progressive differentiation that spans the full vertical height of the intrusion, precluding any model based solely on replenishment

Bushveld layering with dark chromite

The Stillwater Complex, MontanaThe Stillwater Complex, Montana

Figure 12-8. After Wager and Brown (1968) Layered Igneous Rocks. Freeman. San Francisco.

Convenient cross section

Stillwater StratigraphyStillwater Stratigraphy Basal Series Basal Series NoriteNorite is a mafic intrusive igneous rock composed largely of is a mafic intrusive igneous rock composed largely of

the calcium-rich plagioclase labradorite and hypersthene (enstatite) with olivinethe calcium-rich plagioclase labradorite and hypersthene (enstatite) with olivine

– a thin (50-150 m) layer of norites and gabbros a thin (50-150 m) layer of norites and gabbros Ultramafic SeriesUltramafic Series base = first appearance of base = first appearance of

copious olivine cumulates (phase layering)copious olivine cumulates (phase layering)– Lower Lower Peridotite ZonePeridotite Zone

20 cycles (20-150 m thick) of macrorhythmic layering 20 cycles (20-150 m thick) of macrorhythmic layering with a distinctive sequence of lithologies with a distinctive sequence of lithologies

The series begins with dunite (plus chromite), followed The series begins with dunite (plus chromite), followed by harzburgite (both depleted mantle) and then by harzburgite (both depleted mantle) and then orthopyroxeniteorthopyroxenite

– Upper Upper Orthopyroxenite ZoneOrthopyroxenite Zone is a single, thick (up to 1070 m), rather monotonous is a single, thick (up to 1070 m), rather monotonous

layer of cumulate orthopyroxenitelayer of cumulate orthopyroxenite

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

olivine + chromite FeCrolivine + chromite FeCr22OO44 olivine + orthopyroxene olivine + orthopyroxene orthopyroxene orthopyroxene orthopyroxene + plagioclase orthopyroxene + plagioclase orthopyroxene + plagioclase + the orthopyroxene + plagioclase + the

Cpx AugiteCpx AugiteCommon basaltic crystallization sequence, which Common basaltic crystallization sequence, which suggests that each sequence is initiated by some suggests that each sequence is initiated by some major change in the crystallization conditions followed major change in the crystallization conditions followed by a period of cooling and crystal accumulationby a period of cooling and crystal accumulationWe still must explain the repetition of the cyclesWe still must explain the repetition of the cycles

Stillwater Stratigraphy (cont.)

Stillwater (cont)Stillwater (cont)TheThe Banded SeriesBanded Series

– Sudden cumulus plagioclase Sudden cumulus plagioclase significant significant change from ultramafic rock types (phase change from ultramafic rock types (phase layering again)layering again)

– The most common lithologies are The most common lithologies are anorthosite, norite, gabbro, and troctolite anorthosite, norite, gabbro, and troctolite (olivine-rich and pyroxene-poor gabbro)(olivine-rich and pyroxene-poor gabbro)

– Winter thinks such a sudden and dramatic Winter thinks such a sudden and dramatic change suggests the change suggests the introduction of a introduction of a second principal magma typesecond principal magma type into the into the Stillwater magma chamberStillwater magma chamber

– Let’s take a lookLet’s take a look

The Skaergård Intrusion E. The Skaergård Intrusion E. GreenlandGreenland

Figure 12-10. After Stewart and DePaolo (1990) Contrib. Mineral. Petrol., 104, 125-141.

The “type locality” for LMIs. It is now perhaps the most intensely studied igneous body in the world

Convenient Cross Section

– Skaergård is layered BUTSkaergård is layered BUT– Magma intruded in a Magma intruded in a

single surge. Example of single surge. Example of the crystallization of a the crystallization of a mafic pluton in a single-mafic pluton in a single-stage processstage process

– Fine-grained chill marginFine-grained chill margin

Skaergård

StratigraphyStratigraphySkaergård subdivided into three Skaergård subdivided into three major units:major units:

– Layered SeriesLayered Series– Upper Border SeriesUpper Border Series– Marginal Border SeriesMarginal Border Series

Upper Border Series and the Layered Upper Border Series and the Layered Series meet at the Series meet at the Sandwich HorizonSandwich Horizon (most differentiated liquids)(most differentiated liquids)It is generally agreed that the Layered Series crystallized from the floor upward, the Upper Border Series from the roof downward, and the Marginal Border Series from the walls inward

Cross section looking down dip, so as to get a Cross section looking down dip, so as to get a better view.better view.

Figure 12-11. After After Hoover (1978) Carnegie Inst. Wash., Yearb., 77, 732-739.

Recall (Mineralogy Lecture 7, Winter Chapter Recall (Mineralogy Lecture 7, Winter Chapter 6) that in the system MgO-FeO- Fe6) that in the system MgO-FeO- Fe22OO33-SiO-SiO4 4 (left) Olivines becomes increasingly Fe(left) Olivines becomes increasingly Fe22SiOSiO4 4 (Fayalite) as temperatures fall.(Fayalite) as temperatures fall.

Plagioclase becomes increasingly Albite , Plagioclase becomes increasingly Albite , NaAlSiNaAlSi33OO88 the Sodium-rich end member the Sodium-rich end member

: : Upper Border Series: Upper Border Series: thinner, but mirrors thinner, but mirrors the 2500 m Layered Series in many respectsthe 2500 m Layered Series in many respects

– Cooled from the top down, so the top of Cooled from the top down, so the top of the Upper Border Series crystallized firstthe Upper Border Series crystallized first

– Evidence: Evidence: The most Mg-rich olivines The most Mg-rich olivines and Ca-rich plagioclases occur at the and Ca-rich plagioclases occur at the top, and grade to more Fe-rich and top, and grade to more Fe-rich and Na-rich compositions downward Na-rich compositions downward

– Major element trends also reverse in Major element trends also reverse in the Upper Border Series as the Upper Border Series as compared to the LBS compared to the LBS

Sandwich Horizon,Sandwich Horizon, where the latest, most where the latest, most differentiated liquids crystallizeddifferentiated liquids crystallized

– FerrogabbrosFerrogabbros with sodic plagioclase with sodic plagioclase (An(An3030), plus Fe-rich olivine and Opx), plus Fe-rich olivine and Opx

– GranophyricGranophyric segregations of quartz segregations of quartz and alkali feldsparand alkali feldspar Simultaneous crystallizationSimultaneous crystallization

in the late stages ofin the late stages of

differentiation?differentiation?

– Granophyric Texture: the groundmass minerals, usually quartz and alkali Granophyric Texture: the groundmass minerals, usually quartz and alkali feldspar, penetrate each other as feathery irregular intergrowths.feldspar, penetrate each other as feathery irregular intergrowths.

StratigraphStratigraphy and y and Modal Modal

LayeringLayering

Figure 12-12. After Wager and Brown (1968) Layered Igneous Rocks. Freeman. and Naslund (1983) J. Petrol., 25, 185-212.

Top down

Bottom up

• Compatible (Cr and Ni) and incompatible (Rb and Zr) trace elements for the complete section. Trends are compatible with differentiation of a single surge of magma• No evidence for any cyclic variations suggestive of repeated injections of fresh magma• Layering must be explained by otheer changes

Wager and Deer (1939)Wager and Deer (1939) “… “… postulated … convection… down the postulated … convection… down the

walls, across the floor, … up the center walls, across the floor, … up the center [and along the roof.]” “Currents stirred [and along the roof.]” “Currents stirred the magma .. to keep it … homogeneous the magma .. to keep it … homogeneous … and fractionation uniform.”… and fractionation uniform.”

““As the liquid flowed horizontally [across] As the liquid flowed horizontally [across] the floor, … crystals settled through a … the floor, … crystals settled through a … small thickness of liquid.”small thickness of liquid.”

““Currents … fluctuated in velocity,… Currents … fluctuated in velocity,… producing changes in the proportions of producing changes in the proportions of [different] minerals reaching the floor….”[different] minerals reaching the floor….”

Note the analogy to competence Note the analogy to competence (largest/densest particle carried) as a (largest/densest particle carried) as a function of stream velocity.function of stream velocity.

Quotes from Young, Davis A. (2003) Quotes from Young, Davis A. (2003) Mind Mind Over Magma, Over Magma, page 324. Princeton page 324. Princeton University Press, Princeton and Oxford.University Press, Princeton and Oxford.

Lawrence Richard Wager

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

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

Bull., 110, 1398-1447.

Density currents initiate at cool roof and descend along walls to cross floor

Create layering along whole path- roof, walls, floorPeriodic slumps and drops of accumulated material (autoliths) -> scour/fill and craters

Skaergård magma evolves toward high Skaergård magma evolves toward high iron, following the theories of Kennedy, iron, following the theories of Kennedy, and Fenner. and Fenner.

AFM diagram for Skaergård AFM diagram for Skaergård compared to Crater Lake compared to Crater Lake volcanics, Oregon Cascades. volcanics, Oregon Cascades.

and contrary to the predictions of Bowen’s early work.Later Bowen and Schairer (1935) studied the system FeO-MgO-SiO2 to explain high iron fractionates.