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

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.

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

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  1. 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)

  2. 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

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

  4. 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

  5. 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

  6. Figure 12.2. Modal and size graded layers. From McBirney and Noyes (1979) J. Petrol., 20, 487-554. Graded Layers

  7. 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

  8. Layering (or stratification) 2) Phase layering:the appearance or disappearance of minerals in the crystallization sequence developed in modal layers • Phase layeringtransgressesmodal layering

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

  10. 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

  11. 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.

  12. 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

  13. 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

  14. 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.

  15. 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.

  16. 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.

  17. 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.

  18. Upper Zone Appearance of cumulusmagnetite (Fe-rich) Well layered: anorthosite, gabbro,andferrodiorite Numerous felsic rock types = late differentiates

  19. 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.

  20. 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.

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

  22. The Stillwater Complex, Montana Figure 12.8.After Wager and Brown (1968) Layered Igneous Rocks. Freeman. San Francisco.

  23. 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

  24. The crystallization sequence within each rhythmic unit (with rare exception) is: • olivine + chromite  • olivine + orthopyroxene  • orthopyroxene  • orthopyroxene + plagioclase  • orthopyroxene + plagioclase + augite

  25. 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)

  26. The Skaergård Intrusion E. Greenland Figure 12.10.After Stewart and DePaolo (1990) Contrib. Mineral. Petrol., 104, 125-141.

  27. 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

  28. 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)

  29. Figure 12.11. After After Hoover (1978)Carnegie Inst. Wash., Yearb., 77, 732-739. Cross section looking down dip.

  30. 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

  31. 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?

  32. 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.

  33. Chemistry of the Skaergård Figure 12-13. After McBirney (1973) Igneous Petrology. Jones and Bartlett.

  34. 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

  35. 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

  36. 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

  37. 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

  38. 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

  39. 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

  40. 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.

  41. Figure 12.14. After Turner and Campbell (1986) Earth-Sci. Rev., 23, 255-352. Double-diffusive convection situation • A series of convecting layers

  42. 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

  43. 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.

  44. 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

  45. Figure 12-17.After Irvine et al. (1998) Geol. Soc. Amer. Bull., 110, 1398-1447.

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

  47. 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.

  48. 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.

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