Saturday, September 3, 2011

Migmatites:A Mixture of Igneous and Metamorphic Rocks

Metamorphism transforms pre-existing rocks into distinctly new rocks (metamorphic) as the result of high temperatures and high pressures. Metamorphic rocks, when raised to temperatures and pressures at or near their melting point, form a migmatite— a rock that has almost melted and characteristically produces a fantastic display of mixed igneous and metamorphic rocks. A migmatite, intermediate between metamorphic and igneous rocks, is a mixed rock in which at least one part is igneous.

This migmatite, a mixture of igneous and metamorphic rocks, is actually a glacial erratic deposited by a retreating glacier at the end of the Ice Age in the Rocky Mountain National Park.  This migmatite is along the shore of Sprague Lake.  Photo date Aug, 2004, by S. Veatch.
High temperatures cause partial melting with segregation of granitic melt bands that form swirled banding. This banding reveals that the light colored minerals (felsic) have undergone melting and flow while the dark colored minerals (mafic) have not yet reached their melting point and have been contorted by flow. If a rock undergoes extensive metamorphism and light and dark minerals have been segregated, it is gneiss. If a rock undergoes partial melting with segregation of granitic melt bands it is a migmatite. To find out if a rock is a migmatite, carefully look at the felsic layers, if they have completely melted and re- crystallized, the rock is a migmatite. If re-melting has not taken place, it’s a gneiss.


The swirled banding of light-colored felsic minerals and dark-colored mafic minerals seen in this boulder are characteristic of a migmatite. Photo date Aug, 2004, by S. Veatch.





7 comments:

  1. Oh my god..From far it does not look like a rock at all..Amazing..

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  2. i like to know the types of migmatites and P-T conditions of their origin!

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  3. "The Trouble with Physics" could be the trouble with geology as well.

    An alternative model of metamorphic rock suggests this could be authigenic gneiss with a leucosome layer that crystallized at the cold-junction ice ceiling of an Oort cloud comet ocean, then sank and crumpled forming ptygmatic folds onto the more-mafic mineral layer which crystallized at the warmer temperatures near the sedimentary comet core.

    Subsequently, during diagenesis and lithification of the authigenic gneiss core, hydrothermal fluids may have formed hydrothermal (authigenic) schist and dolomite over hot and cool smokers respectively. Thus the concentric layering of mantled gneiss domes could be merely growth rings of a sedimentary comet core.

    On our high gravity planet, authigenic mineral grains are the clay-sized particles, but in the micro-gravity near the center of an Oort cloud comet ocean, authigenic mineral grains may be the size of gneiss crystals, and thus, gneiss could be authigenic sedimentary rock formed in the micro-gravity of an Oort cloud comet core.

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  4. I enjoyed looking at the many wall paintings that you have done. Not being very handy with a paintbrush, even though I know what I like in the way of art, I took the easier option to order this canvas print from the site wahooart.com .
    It’s an unusual work called Forest music 1, by Remedios Varo Uranga, a Spanish-Mexican woman artist.

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    1. Hi Scott, I just started lessons in watercolor, so I am still learning. If I can do it, anyone can.
      ---Steve

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  5. Thanks for your kind comments. I have been taking art lessons for one year.

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