slide1 l.
Download
Skip this Video
Loading SlideShow in 5 Seconds..
Large or particularly well-studied LMIs exposed in continents (many in flood basalt provinces) PowerPoint Presentation
Download Presentation
Large or particularly well-studied LMIs exposed in continents (many in flood basalt provinces)

Loading in 2 Seconds...

play fullscreen
1 / 49

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


  • 171 Views
  • Uploaded on

Chapter 12: Layered Mafic Intrusions. 2. (km. ). Area. Name. Age. Location. Bushveld. Precambrian. S. Africa. 66,000. Dufek. Jurassic. Antarctica. 50,000. Duluth. Precambrian. Minnesota, USA. 4,700. Stillwater. Precambrian. Montana, USA. 4,400. Muskox. Precambrian.

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'Large or particularly well-studied LMIs exposed in continents (many in flood basalt provinces)' - Antony


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
slide1

Chapter 12: Layered Mafic Intrusions

2

(km

)

Area

Name

Age

Location

Bushveld

Precambrian

S. Africa

66,000

Dufek

Jurassic

Antarctica

50,000

Duluth

Precambrian

Minnesota, USA

4,700

Stillwater

Precambrian

Montana, USA

4,400

Muskox

Precambrian

NW Terr. Canada

3,500

Great Dike

Precambrian

Zimbabwe

3,300

Kiglapait

Precambrian

Labrador

560

Skaergård

Eocene

East Greenland

100

Table 12.1

. Some Principal Layered Mafic Intrusions

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

the form of a typical lmi
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

layering
Layering

layer: any sheet-like cumulate unit distinguished by its compositional and/or textural features

  • uniform mineralogically and texturally homogeneous
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.

Uniform Layering
layering5
Layering

layer:any sheet-like cumulate unit distinguished by its compositional and/or textural features

  • uniformmineralogically and texturally homogeneous
  • non-uniformvary either along or across the layering
  • graded= gradual variation in either
    • mineralogy
    • grain size- quite rare in gabbroic LMIs
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 stratification8
Layering (or stratification)

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

  • Phase layeringtransgressesmodal layering
slide9
3) Cryptic Layering (not obvious to the eye)
  • Systematic variation in the chemical composition of certain minerals with stratigraphic height in a layered sequence
slide10
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
rythmic and intermittent layering
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.

the bushveld complex south africa
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

slide13
Marginal Zone: the lowest unit, is a chill zoneabout 150 m thick

Fine-grained norites from the margin correspond to a high-alumina tholeiitic basalt

stratigraphy
Stratigraphy

Basal Series

Thin uniform dunitecumulates 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.

slide15
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.

slide16

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.

slide17

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.

slide18

Upper Zone

Appearance of cumulusmagnetite (Fe-rich)

Well layered: anorthosite, gabbro,andferrodiorite

Numerous felsic rock types = late differentiates

slide19

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.

slide20

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.

slide21

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

slide22

The Stillwater Complex, Montana

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

stratigraphy23
Stratigraphy
  • Basal Series
    • a thin (50-150 m) layer of norites and gabbros
  • Ultramafic Seriesbase = 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
slide24
The crystallization sequence within each rhythmic unit (with rare exception) is:
  • olivine + chromite 
  • olivine + orthopyroxene 
  • orthopyroxene 
  • orthopyroxene + plagioclase 
  • orthopyroxene + plagioclase + augite
stratigraphy25
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)
the skaerg rd intrusion e greenland
The Skaergård Intrusion E. Greenland

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

slide28
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
stratigraphy29
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)

slide30

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

Cross section looking down dip.

slide31
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
slide32
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 modal and cryptic layering cryptic determined for intercumulus phases
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.

chemistry of the skaerg rd
Chemistry of the Skaergård

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

the processes of crystallization differentiation and layering in lmis
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
slide36
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
slide37
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
slide38
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 situ processes
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
slide40
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

slide41

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.

slide42

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

Double-diffusive convection situation

  • A series of convecting layers
slide43
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
slide44

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.

slide45
Neil Irving’s Vortex model

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

Black flow lines and arrows indicate motionrelative to the cell

slide47

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

slide48

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.

slide49

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.