Chapter 20: Anorthosites

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Presentation transcript:

Chapter 20: Anorthosites Most slides are from your text, provided online by Dr. Winter and Prentice-Hall. Modifications by Prof. Smart

Anorthosites Plutonic rocks with over 90% plagioclase No known volcanic equivalents Highly felsic nature and their location in continental areas they share with granitoid rocks The felsic mineral, however, is a calcic plagioclase, which, along with associated high-temperature mafic minerals, suggests a stronger similarity to basaltic rocks The mafic similarity is supported by our previous encounter with anorthosites: as layers in layered mafic intrusions (Chapter 12)

Anorthosites Ashwal (1993) listed six major types or anorthosite occurrences: 1. Archean anorthosite plutons 2. Proterozoic “massif-type” anorthosite plutons 3. Centimeter-to-100m thick layers in layered mafic intrusions 4. Thin cumulate layers in ophiolites/oceanic crust 5. Small inclusions in other rock types (xenoliths and cognate inclusions) 6. Lunar highland anorthosites We will concentrate here on the first two types, because they are the classic examples Type 3 was discussed in Chapter 12 Type 4 is created by cumulate processes at mid-ocean ridge magma chambers similar to those discussed in Chapters 12 and 14 Type 5 is a very minor occurrence, and may represent fragments of material derived from the other types Although type 6 is extra-terrestrial, it presents an interesting opportunity to compare igneous processes on Earth with those on other bodies, so I will describe them briefly

Anorthosites Figure 20-1a. a. “Snowflake” clusters of plagioclase crystals from the Fiskenæsset complex, W. Greenland. Myers (1985) Stratigraphy and structure of the Fiskenæsset complex, West Greenland. Grønl. Geol. Unders. Bull 150. Photograph courtesy John Myers. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Anorthosites Figure 20-1a. a. Typical texture of Archean anorthosite. From the Fiskenæsset complex, W. Greenland. Myers (1985) Stratigraphy and structure of the Fiskenæsset complex, West Greenland. Grønl. Geol. Unders. Bull 150. Photograph courtesy John Myers. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Anorthosites Figure 20-2. Chondrite-normalized rare earth element diagram for some typical Archean anorthosites from the Bad Vermillion (Ontario) and Fiskenæsset (Greenland) bodies. Data from Seifert et al. (1997) Can. J. Earth Sci., 14, 1033-1045; Simmons and Hanson (1978) Contrib. Mineral. Petrol., 66, 19-135; and Ashwal et al. (1989) Earth Planet. Sci. Lett., 91, 261-270. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Anorthosites a. Mantle-derived magma underplates the crust as it becomes density equilibrated. Figure 20-2. Model for the generation of Massif-type anorthosites. From Ashwall (1993) Anorthosites. Springer-Verlag. Berlin. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Anorthosites b. Crystallization of mafic phases (which sink), and partial melting of the crust above the ponded magma. The melt becomes enriched in Al and Fe/Mg. Figure 20-2. Model for the generation of Massif-type anorthosites. From Ashwall (1993) Anorthosites. Springer-Verlag. Berlin. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Anorthosites c. Plagioclase forms when the melt is sufficiently enriched. Plagioclase rises to the top of the chamber whereas mafics sink. Figure 20-2. Model for the generation of Massif-type anorthosites. From Ashwall (1993) Anorthosites. Springer-Verlag. Berlin. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Anorthosites d. Plagioclase accumulations become less dense than the crust above and rise as crystal mush plutons. Figure 20-2. Model for the generation of Massif-type anorthosites. From Ashwall (1993) Anorthosites. Springer-Verlag. Berlin. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Anorthosites e. Plagioclase plutons coalesce to form massif anorthosite, whereas granitoid crustal melts rise to shallow levels as well. Mafic cumulates remain at depth or detach and sink into the mantle. Figure 20-2. Model for the generation of Massif-type anorthosites. From Ashwall (1993) Anorthosites. Springer-Verlag. Berlin. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.