Chapter 12 Layered Mafic Intrusions Table 12 1

Chapter 12: Layered Mafic Intrusions Table 12 -1. Some Principal Layered Mafic Intrusions Name Bushveld Dufek Duluth Stillwater Muskox Great Dike Kiglapait Skaergård Age Precambrian Jurassic Precambrian Precambrian Eocene Location S. Africa Antarctica Minnesota, USA Montana, USA NW Terr. Canada Zimbabwe Labrador East Greenland 2 Area (km ) 66, 000 50, 000 4, 700 4, 400 3, 500 3, 300 560 100 Large or particularly well-studied LMIs exposed in continents (many in flood basalt provinces)

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.

Layering layer: any sheet-like cumulate unit distinguished by its compositional and/or textural features F uniform mineralogically and texturally homogeneous

Uniform Layering Figure 12 -3 b. Uniform chromite layers alternate with plagioclase-rich layers, Bushveld Complex, S. Africa. From Mc. Birney and Noyes (1979) J. Petrol. , 20, 487 -554.

Layering layer: any sheet-like cumulate unit distinguished by its compositional and/or textural features F uniform mineralogically and texturally homogeneous F non-uniform vary either along or across the layering s graded = gradual variation in either i mineralogy i grain size - quite rare in gabbroic LMIs

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

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 F may contain uniform layers, graded layers, or a combination of both

Layering (or stratification) 2) Phase layering: the appearance or disappearance of minerals in the crystallization sequence developed in modal layers F Phase layering transgresses modal layering

3) Cryptic Layering (not obvious to the eye) F Systematic variation in the chemical composition of certain minerals with stratigraphic height in a layered sequence

The regularity of layering l Rhythmic: layers systematically repeat Macrorhythmic: several meters thick F Microrhythmic: only a few cm thick F l Intermittent: less regular patterns F A common type consists of rhythmic graded layers punctuated by occasional uniform layers

Rythmic and Intermittent Layering Figure 12 -3 a. 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 nongraded gabbroic layers. Skaergård Intrusion, E. Greenland. From Mc. Birney (1993) Igneous Petrology (2 nd ed. ), Jones and Bartlett. Boston.

The Bushveld Complex, South Africa The biggest: 300 -400 km x 9 km The Red Granite intruded 50 -100 Ma afterwards 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).

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

Stratigraphy Basal 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. Si. O 2 portion of the Fe. O -Mg. O-Si. O 2 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, Montana Figure 12 -8. After Wager and Brown (1968) Layered Igneous Rocks. Freeman. San Francisco.

Stratigraphy l Basal Series F l a thin (50 -150 m) layer of norites and gabbros Ultramafic Series base = first appearance of copious olivine cumulates (phase layering) F Lower Peridotite Zone s s F 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 s 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: s olivine + chromite s olivine + orthopyroxene s orthopyroxene + plagioclase + augite

Stratigraphy The Banded Series F Sudden cumulus plagioclase significant change from ultramafic rock types (phase layering again) F The most common lithologies are anorthosite, norite, gabbro, and troctolite (olivine-rich and pyroxene-poor gabbro)

The Skaergård Intrusion E. Greenland Figure 12 -10. After Stewart and De. Paolo (1990) Contrib. Mineral. Petrol. , 104, 125141.

Magma intruded in a single surge (premier natural example of the crystallization of a mafic pluton in a single-stage process) F Fine-grained chill margin F

Stratigraphy Skaergård subdivided into three major units: F Layered Series F Upper Border Series F Marginal 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 Hoover (1978) Carnegie Inst. Wash. , Yearb. , 77, 732 -739.

Upper Border Series: thinner, but mirrors the 2500 m Layered Series in many respects F Cooled from the top down, so the top of the Upper Border Series crystallized first s The most Mg-rich olivines and Carich plagioclases occur at the top, and grade to more Fe-rich and Narich compositions downward s Major element trends also reverse in the Upper Border Series as compared to the LBS

Sandwich Horizon, where the latest, most differentiated liquids crystallized F Ferrogabbros with sodic plagioclase (An 30), plus Fe-rich olivine and Opx F Granophyric segregations of quartz and feldspar F F & G = immiscible liquids that evolve in the late stages of differentiation?

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 Skaergård Figure 12 -13. After Mc. Birney (1973) Igneous Petrology. Jones and Bartlett.

The Processes of Crystallization, Differentiation, and Layering in LMIs l l l LMIs are the simplest possible case More complex than anticipated Still incompletely understood after a half century of intensive study

l l 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

l l 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

l l l 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 l Nucleation and growth of minerals in a thin stagnant boundary layer along the margins of the chamber F F F 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 s Boundary layer interface inhibits material motion

l l 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 case r temp) 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 F A series of convecting layers Figure 12 -14. After Turner and Campbell (1986) Earth-Sci. Rev. , 23, 255 -352.

Double-diffusive convection situation F A series of convecting layers Figure 12 -14. After Turner and Campbell (1986) Earth-Sci. Rev. , 23, 255 -352.

Density currents l Cooler, heavy-element-enriched, and/or crystal-laden liquid descends and moves across the floor of a magma chamber F Dense crystals held in suspension by agitation F Light crystals like plagioclase also trapped and carried downward

Figure 12 -15 a. Cross-bedding in cumulate layers. Duke Island, Alaska. Note also the layering caused by different size and proportion of olivine and pyroxene. From Mc. Birney (1993) Igneous Petrology. Jones and Bartlett Figure 12 -15 b. Cross-bedding in cumulate layers. Skaergård Intrusion, E. Greenland. Layering caused by different proportions of mafics and plagioclase. From Mc. Birney and Noyes (1979) J. Petrol. , 20, 487 -554.

l 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 motion relative to the cell

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 calcalkaline magma series (after Sparks et al. , 1984) Phil. Trans. R. Soc. Lond. , A 310, 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 Mc. Callum (1984) J. Petrol. , 25, 395 -420.