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Diversification of Magmas

Diversification of Magmas. Differentiation of magmas. Crystal-melt fractionation Seen best in layered mafic intrusions Large intrusions in short time period. Layered mafic intrusions. 2. (km. ). Area. Name. Age. Location. Bushveld. Precambrian. S. Africa. 66,000. Dufek. Jurassic.

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Diversification of Magmas

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  1. Diversification of Magmas

  2. Differentiation of magmas • Crystal-melt fractionation • Seen best in layered mafic intrusions • Large intrusions in short time period

  3. 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) Biggies are Precambrian

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

  5. 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

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

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

  8. 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.

  9. 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.

  10. What can happen to cumulate minerals • A) Cumulate olivine, px, chromite • B) Cementation of cumulate minerals • C) ignore • D) ignore

  11. 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).

  12. Stratigraphy Series defined on mineralogy Presence or absence of minerals Formed by gravity settling and cooling from top and bottom 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.

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