Layered Mafic Intrusions Intrusions

Layered Layered MaficMafic IntrusionsIntrusions(Ch t 12)(Ch t 12)(Chapter 12)(Chapter 12)

Skaergaard layered mafic intrusion (Greenland): Fine graded beds with intervening homogeneous rock.. Image: Kurt Hollocher - http://www.union.edu/PUBLIC/GEODEPT/hollocher/skaergaard/travellogs/homestead_site.htm

Layered Mafic IntrusionsLayered Mafic Intrusions

Table 12Table 12--11. . Some Principal Layered Mafic Intrusions (LMI)Some Principal Layered Mafic Intrusions (LMI)

NameName AgeAge LocationLocation AreaArea (km(km 22))NameName AgeAge LocationLocation AreaArea (km(km ))BushveldBushveld PrecambrianPrecambrian S. AfricaS. Africa 66,00066,000DufekDufek JurassicJurassic AntarcticaAntarctica 50,00050,000DuluthDuluth PrecambrianPrecambrian Minnesota, USAMinnesota, USA 4,7004,700StillwaterStillwater PrecambrianPrecambrian Montana, USAMontana, USA 4,4004,400MuskoxMuskox PrecambrianPrecambrian NW Terr. CanadaNW Terr. Canada 3,5003,500Great DikeGreat Dike PrecambrianPrecambrian ZimbabweZimbabwe 3,3003,300KiglapaitKiglapait PrecambrianPrecambrian LabradorLabrador 560560SkaergårdSkaergård EoceneEocene East GreenlandEast Greenland 100100

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

SkaergårdSkaergård EoceneEocene East GreenlandEast Greenland 100100

Image source: Winter, 2001

(many in flood basalt provinces)(many in flood basalt provinces)

Layered Mafic IntrusionsLayered Mafic IntrusionsTypical shape of an LMI (Muskox Intrusion)Typical shape of an LMI (Muskox Intrusion)


L r h t lik l t nit h in nif r

Layered Mafic IntrusionsLayered Mafic IntrusionsLayer: sheet-like cumulate unit having uniform

compositional and/or textural features

Uniform chromite layers l i halternate with

plagioclase-rich layers, Bushveld y ,Complex, S. Africa.

McBirney and Noyes (1979) J. Petrol., 20, 487-554.


Graded bedding: any gradual variation in mineralogy or Layered Mafic IntrusionsLayered Mafic Intrusions

g y g gygrain size

M d l ndModal and size graded layers.

McBirney and Noyes (1979) J. Petrol 20 487Petrol., 20, 487-554


Graded bedding: any gradual variation in mineralogy or Layered Mafic IntrusionsLayered Mafic Intrusions

g y g gygrain size

Graded beds between homogeneous layers –ySkaergard intrusion (Greenland)(Greenland)

Image source: Kurt Hollocher:

http://www.union.edu/PUBLIC/GEODEPT/hollocher/skaergaard/geologic_features/layering_types.htm#Modally%20graded%20layering

Layering (stratification): the structure and fabric of Layered Mafic IntrusionsLayered Mafic Intrusions

y g ( )sequences of multiple layers

1) Modal Layering: variation in relative proportions of constituent minerals

i e may containi.e. may contain uniform layers, graded layers, or combination of both

Image source: Kurt Hollocher: microrhythmic layering of mt and plagioclase (easy to discern) and olivine and pyroxenes (rusty patches)

http://www.union.edu/PUBLIC/GEODEPT/hollocher/skaergaard/geologic_features/layering_types.htm#Modally%20graded%20layering



2) Phase layering: appearance orappearance or disappearanceof minerals in

lli icrystallization sequence developed in pmodal layers

Image source: Kurt Hollocher: Stillwater Complex (MT) – troctolite (left) – anorthosite (right) contacthttp://www.union.edu/PUBLIC/GEODEPT/hollocher/teaching_petrology/stillwater.htm



3) Cryptic layering (not obvious to the ) yp y g (eye):

Systematic variation in chemical composition of certain minerals withcomposition of certain minerals with stratigraphic height in layered sequence

Cumulate orthopyroxenite. -Cumulus orthopyroxeneorthopyroxene (brown), intracumulus plagioclase (white) and the large, poikilitic intracumulus augite

Image source: Kurt Hollocher: Stillwater Complex (MT)http://www.union.edu/PUBLIC/GEODEPT/hollocher/teaching_petrology/stillwater.htm

intracumulus augite (green)



Regularity of layering

Rhythmic: layersi llsystematically

repeat

• Macrorhythmic:Macrorhythmic: several meters thickMi h h i• Microrhythmic: only a few cm thick

Image source: Kurt Hollocher: Stillwater Complex (MT) – Vertically tilted cm-scale rhythmic layering of plagioclase and pyroxenehttp://www.union.edu/PUBLIC/GEODEPT/hollocher/teaching_petrology/stillwater.htm



Regularity of layering

Intermittent: less regular patterns

Common typeCommon type consists of rhythmic

d d lgraded layers punctuated by uniform layersy

Skaergaard layered mafic intrusion (Greenland): Fine graded beds with intervening homogeneous rock.. Image: Kurt Hollocher - http://www.union.edu/PUBLIC/GEODEPT/hollocher/skaergaard/travellogs/homestead_site.htm

2 7 G pl in th B rt th Mtn MT

Stillwater Intrusion (MT)Stillwater Intrusion (MT)2.7 Ga complex in the Beartooth Mtns, MT

Image: Darrell Henry

Stillwater Intrusion (MT)Stillwater Intrusion (MT)Two major magma types and times of intrusion:j g yp

• Base: mafic and olivine-saturated magma• Banded Zone: gabbroic or noritic magma punctuated by local

new contrasting injections of magma that fractionatednew contrasting injections of magma that fractionated

Image source: Winter (2001)

Stillwater Intrusion (MT)Stillwater Intrusion (MT)• Base: mafic and olivine-saturated magmag

Ultramafic Seriesbase = first appearance of olivine cumulatesbase first appearance of olivine cumulates (phase layering)

B l S iBasal Seriesthin (50-150 m) layer of norites and gabbros - possibly general bulk


composition of initial magma injection

Stillwater Intrusion (MT)Stillwater Intrusion (MT)• Base: mafic and olivine-saturated magmaBase: mafic and olivine saturated magma

Ultramafic Seriesbase = first appearance of olivine cumulates (phase layering)

Upper Orthopyroxenite Zonesingle, thick (up to 1070 m), rather monotonous layer of cumulate orthopyroxenitepy

Lower Peridotite Zone20 cycles (20-150 m thick) of


macrorhythmic layering with distinctive sequence of lithologies

Stillwater Intrusion (MT)Stillwater Intrusion (MT)• Banded Zone: gabbroic or noritic magma punctuated by local g g p y

new contrasting injections of magma that undergo fractionation

The Banded SeriesNotable appearance of cumulus plagioclase -significant change from ultramafic rock types (phase layering again)

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

Probable multiple reintroduction of magmas

J-M Reef (Pt-Pd sulfide layer) in Lower Banded Zone


Skaergaard Intrusion (E. Greenland)Skaergaard Intrusion (E. Greenland)Likely single injection magma

• Magma intruded in single surge (best natural example of crystallization of mafic pluton in single-stage process)

• Eocene (56 Ma) intr sion in Precambrian Cretaceo s and Tertiar rocks• Eocene (56 Ma) intrusion in Precambrian, Cretaceous and Tertiary rocks

Skaergaard layered mafic intrusion geologic map: Image: Kurt Hollocher -http://www.union.edu/PUBLIC/GEODEPT/hollocher/skaergaard/geologic_features/geologic_map.htm

Skaergaard Intrusion (E. Greenland)Skaergaard Intrusion (E. Greenland)(1) L d S i(1) Layered SeriesCrystallization from the floor upward

(2) Upper Border ( ) ppSeriesCrystallization from the roof

Skaergaard layered interpretatve cross-section: John Winter (2001)

from the roof downward• thinner (960 m), but

mirrors the 2500 mmirrors the 2500 m Layered Series

• Cooled from top d f U

(3) Marginal Border Series• Crystallization from the wall inward

down, so top of Upper Border Series crystallized first

(4) Sandwich Horizon• the latest, most differentiated liquids crystallized

Skaergaard Intrusion (E. Greenland)Skaergaard Intrusion (E. Greenland)

(1) Layered Series

•Upper Zone - appearance of Fe-rich olivine with late K-feldspar + quartz

•Middle Zone olivine disappears•Middle Zone - olivine disappears

•Lower Zone - cumulates of olivine + plagioclase later joined by augite

John Winter (2001)

Skaergaard Intrusion (E. Greenland)Skaergaard Intrusion (E. Greenland)(2) U B d S i(2) Upper Border Series

• Most Mg-rich olivines and Ca-rich plagioclases occur at top, and gradeplagioclases occur at top, and grade to more Fe-rich and Na-rich compositions downward

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

John Winter (2001)

Skaergaard Intrusion (E. Greenland)Skaergaard Intrusion (E. Greenland)3) M i l B d S i(3) Marginal Border Series• compressed version of other series

(4) S d i h H i(4) Sandwich Horizon• Ferrogabbros with sodic plagioclase

(An30), plus Fe-rich olivine and Opx

• Granophyric segregations of quartz and feldspar immiscible liquids that evolve in late stages ofevolve in late stages of differentiation.

John Winter (2001)

Process of crystallization, differentiation and Process of crystallization, differentiation and layeringlayering

LMIs are simplest possible case, but more complex than anticipated

• Still incompletely understood after half century of intensive study

• Rhythmic modal layering and sedimentary-like features most easily explained by crystal settling interrupted p y y g pby periodic large-scale convective overturn of entire cooling unit

• Reinjection of more primitive magma may explain major compositional shifts and cases of irregular cryptic variationsg yp

Image source: Kurt Hollocher


Problems with the crystal settling process

• Many minerals found at particular horizon are y pnot hydraulically equivalent

• Size is more important than density in Stokes’ Law, but size grading is rare

• Dense olivine in Upper Border Series of the SkaergårdSkaergård

• Plagioclase is in lower layers of Skaergård, but it should float

• Inverted cryptic variations in Upper Border Series suggests early-formed minerals settled

d(?)upward(?)

• Marginal Border Series shows vertical layeringImage source: Kurt Hollocher


Compositional pConvection Model

On r di nt (in thiOne gradient (in this case ρtemp) is destabilizing (although total density gradient is stable)

Diffusivity of destabilizing y gcomponent (heat) is faster than diffusivity of salt



Compositional Convection ModelSeries of convection cells


Layer formation dynamicsLayered Mafic IntrusionsLayered Mafic Intrusions

y y


Cold plumes descending from a cooled upper boundary layer in a tank of silicone oil


Density currents• Cooler, heavy-element-enriched, and/or crystal-laden liquid

d d d fl f h bdescends and moves across floor of magma chamber• Dense crystals held in suspension by agitation• Light crystals like plagioclase also trapped and carried

downward

Cross-bedding in cumulate layers. Duke Island, Alaska. Note also the layering caused by different size and proportion ofdifferent size and proportion of olivine and pyroxene.

From McBirney (1993) Igneous Petrology. Jones and Bartlett


Layer formation

Process of crystallization, differentiation and Process of crystallization, differentiation and layeringlayering Layer formation

dynamics

Density currents i i i l finitiate at cool roof and descend along walls to cross floor.

• Create layering along whole path-roof, walls, floorroof, walls, floor

• Periodic slumps and drops of paccumulated material (autoliths) ->


scour/fill and craters

Layered Mafic Intrusions Intrusions

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