ESS 439: Layered Mafic Intrusions (LMI)What are they and why are they important?

•Represent basaltic magma that has crystallized at depth in large magma chambers•Show the effects of slow cooling and strong fractionation•Natural laboratories to study differentiation in all its aspects: complementary to

volcanics in the sense that they preserve the crystalline products of fractionation.•Repositories of the some of the largest ore bodies on earth Cu, Ni, PGE, Cr, V…

Name Age Location Area (km 2 )

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

Some well-studied LMIs (not an exhaustive list)

a. Crystals accumulate by crystal settling or simply form in place near the margins of the magma chamber (known as in situcrystallization). In this case plagioclase crystals (white) accumulate in loose mutual contact, and an intercumulus liquid (red) fills the interstices.

Development of cumulate textures typical of layered mafic intrusions.

b. Orthocumulate: intercumulus liquid crystallizes to form additional plagioclase rims plus other phases in the interstitial volume (colored). There is little or no exchange between the intercumulus liquid and the main chamber..

After Wager and Brown (1967), Layered Igneous Rocks. © Freeman. San Francisco

Example of orthocumulate texture. In this case, the cumulus mineral is orthopyroxene and it is surrounded by later-forming intercumulus plagioclase (showing albite twinning). Stillwater complex, Montana. Field width 5 mm.

Development of cumulate textures (cont.)

Adcumulates: open-system exchange between the intercumulus liquid and the main chamber (plus compaction of the cumulate pile) allows components that would otherwise create additional intercumulus minerals to escape, and plagioclase fills most of the available space. Some liquid is trapped and forms the small intercumulus pyroxenes, etc.

Heteradcumulates: The texture illustrated above is common in the larger layered intrusions. In the example illustrated, pyroxene nucleates from the intercumulus liquid and grows to form giant poikilitic crystals (grapefruit size) enclosing numerous cumulus plagioclase crystals. During growth of the oikocrysts, communication between the intercumulus liquid and main magma is maintained.

Example of adcumulus texture. Anorthosite from Bushveld Complex, SA

LayeringLayer: any sheet-like cumulate unit distinguished by its compositional and/or texture

Phase layering: the appearance or disappearance of cumulus minerals in the crystallization sequence, e.g., appearance of plagioclase at contact between Ultramafic Series and Banded Series in the St6illwater Complex

Cryptic Layering (not obvious to the eye): Systematic variation in the chemical composition of cumulus minerals (pyroxenes, olivines, plagioclase feldspars) with stratigraphic height in a layered sequence

Rhythmic layering: Layering pattern is repeated. Macrorhythmic layering: individual layers are several meters thick. Microrhythmic layering, layers are a few cm thick

Uniform layering. Layer is mineralogically and texturally homogeneous.

Intermittent layering: Irregular patterns: A common type consists of rhythmically graded layers interlayered with uniform layers.

Modal layering (aka. mineral graded layering): Smooth and gradual variations in the modal abundances of the cumulus minerals which are commonly plagioclase and pyroxene (norites and gabbronorites) or plagioclase and olivine (troctolites)

Size graded layering: Smooth and gradual variations in the grain sizes of cumulus minerals. This type of layering is not common, except in the Duke Island Intrusion, AK.

Uniform Layering Uniform chromite layers alternating with plagioclase-rich layers, Bushveld Complex, S. Africa.

Graded LayeringModal layering: This common type is defined by smooth and gradual variations in the modal abundances, commonly plagioclase and pyroxene or plagioclase and olivine

Size graded layering: This type defined by smooth size variations in cumulus minerals (not common except in the Duke Island Intrusion)

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

Inch-scale layering in norite in the Stillwater Complex, Montana. This type of layering is not formed by crystal settling processes. Note the doublet units (hammer for scale).

Modally graded layers in the Skaergaard intrusion, East Greenland. Commonly referred to as rhythmic layering. Graded layers are separated by uniform layers (hammer for scale).

Other layering types

Cross-bedding in modally-graded gabbroic cumulate layers. Skaergård Intrusion, E. Greenland.

Cross-bedding in size-graded and modally-graded cumulate layers in the Duke Island Intrusion, Alaska. The main cumulus minerals are olivine and orthopyroxene

Muskox Intrusion, NWT, Canada (funnel shape with a well-defined feeder dike)

The Bushveld Complex, South Africa (2.1 Ga)

The biggest LMI: 300-400 km x 9 kmThe Red Graniteintruded 50-100 Maafterwards

Simplified geologic map and cross section of the Bushveld Complex. After Willemse(1964), Wager and Brown (1968), and Irvine et al. (1983).

The Bushveld is the source of most of the world’s Pt, Pd, Cr & V

The Skaergård Intrusion in E. Greenland

Map from Stewart and DePaolo (1990) Contrib. Mineral. Petrol., 104, 125

This intensively studied body of mafic igneous rock is a classic example of extreme closed-system fractional crystallization, e.g., olivine ranges from Fo70 at bottom of LZ to Fo0 at the top of UZ

After Stewart and DePaolo (1990) Contrib. Min. Pet., 104, 125

95

68

67

53

102

8983

27

4 75

18

34

We will continue discussion of the Skaergaard Intrusion in the next class

Numbers refer to thin sections studied in lab

Skaergaard Intrusion, East Greenland

Pegmatite patch in layered gabbro

Skaergaard Intrusion, East GreenlandWager peak

Triple group

BearskinGlacier

The Stillwater Complex, Montana (2.7 Ga)

East Boulder Pd-Pt mine

Mountain View area

Stillwater Pd-Pt mine

View of Beartooth plateau looking South from Chrome Mountain

Macrorhythmic layers in Banded series, Stillwater Complex

Irregular layering in gabbronorite (GN III), Banded series, Stillwater Complex, MT. In addition to uniform layers, the darker units are rich in pyroxene and the lighter units are rich in plagioclase. This wispy type of layering is most likely due to high temperature deformation of pre-existing more regular layers.

Pyroxene snowball

UW Field camp at Stillwater Complex, 2004

Stratigraphy of Stillwater Complex•Basal Series: a 50-150 m thick complex unit consisting of a sill/dike complex, norites, gabbros, and o-pyroxenite. The series also contains xenoliths of country rock (pyroxene cordierite hornfels) and is the site of massive sulfides (Cu-Ni ores)

•Banded series: Complex group of cumulates all of which contain cumulus plagioclase (norites, gabbros, gabbronorites, anorthosites, troctolites). The main Pt-Pd reef occurs ~ 400 m above the contact between the Ultramafic series and the Banded series

•Ultramafic Series: The base of this series is marked by first appearance of cumulus olivine (phase layering). The UM series consists of two zones:

:Lower Peridotite Zone: >20 cycles (each ranging from 20-150 m thick) of macrorhythmic layering with a distinctive sequence of lithologies. The lowermost unit is an olivine plus chromite cumulate with intercumulus opx, plag and minor cpx, overlain by harzburgite (olivine + opx cumulate with intercumulus cpx and plag) in turn overlain by orthopyroxenite (opx cumulate with intercumulus cpx and plag)

:Upper Orthopyroxenite Zone: single, thick (up to 1070 m), fairly uniform sequence of orthopyroxenite (cumulus opx with intercumulus cpx and plag) and rare thin layers of olivine-rich harzburgite.

Modal stratigraphy of Stillwater complex after McCallum et al. (1980) and Raedeke and McCallum (1984)

Compaction and crystallization of intercumulus melt

Country rock

Country rock withthermal aureole

Multiple influxes of primary magma from deeper source (recharge)

Chilled margin

Venting of fractionated magma to surface volcano

Schematic representation of processes associated with the formation of layered mafic complexes

Assimilation of country rock

Density plume

Layered sequence accumulated along floor

Convection

Density currents

“Fountain”

Heat loss