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Thread: Gabbro intrusion

  1. #1 Gabbro intrusion 
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    Hello all,

    I wonder if someone can confirm that my thinking is correct on a question:

    A gabbro intrusion cuts across a layer of schist and a layer of sandstone above. I have to comment on the geological processes that probably took place, in the correct order, based on my geology knowledge. So, here goes:

    The schist would have originally been deposited first as a layer of mudstone, which through regional metamorphism formed into the layer of schist. An unknown layer (or layers) of rock above the schist would have been present for the gabbro to then intrude into (since it could not have formed by rapid cooling at the surface). Then through erosion/ weathering etc, the layers of rock above the schist would have eventually exposed the upper part of the gabbro intrusion. Finally, the layer of sandstone would have been deposited and lithified (through compaction and cementation) to form the sandstone. I assume the sandstone wasn't present when the intrusion took place because it hasn't undergone contact metamorphism. I also assume that the upper surface of the schist is therefore an unconformity.

    Is this about right??

    I am wondering what happens to schist when it undergoes contact metamorphism? I assume that as it is baked, the mineral structure recrystalizes and becomes finer?

    Many thanks for your comments!

    Jules


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  3. #2  
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    The gabbro instrusion cuts through the sandstone, so what came first?

    Incidentally I'm not a geologist but I have done some field work before, I remember finding an igneous intrusion in some sandstone, it was remarkable that it was pretty much impossible to tell just from the sandstone alone that there was a gret big intrusion less than a metre away. For some reason (and don't take this as a fact) it seemed that the sandstone hadn't really metamorphosed and if it had then neither I nor my 2 geologist companions could discern any signs of it.


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  4. #3  
    Universal Mind John Galt's Avatar
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    As you've phrased the question [A gabbro intrusion cuts across a layer of schist and a layer of sandstone above.]it seems, as billiards pointed out, as if the gabbro has been intruded between the schist and the overlying sandstone. That being the case the sequence would be
    1: Formation of schist as you have described.
    2: Uplift and erosion
    3: Uncomnformable deposition, then lithification of the sandstone.
    4: Intrusion of the gabbro.

    However, you also say the sandstone has no signs of contact metamorphism. I would ask, is this a very pure sandstone? [In which case changes will be microscopic and textural, rather macroscopic and mineralogical.] What is the character of the gabbro/sandstone interface?
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  5. #4 Gabbro intrusion 
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    Hi,

    Sorry, maybe I wasn't clear! 'Cuts across' means that there is a horiztonal layer of schist and a layer of sandstone above and the gabbro cuts across both layers in a vertical (rather than horizontal orientation). Does this then make sense of my explanation?

    many thanks for your guidance!

    Jules
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  6. #5  
    Forum Junior Twaaannnggg's Avatar
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    The degreee of methamorphosis depends on the size of the plutone, the time the magma chamber feeding the gabbro intrusion was active or "hot" and also the type of rock this intrusin was cutting through. For the sandstone I do not expect a great deal of metamorphosis or at least not visible to the naked eye. You would have to x-ray the sandstone in order to identify high-temperature modifications of SiO2. Also the thermalconductivity of the sandstone itself is'n stellar so I'd also expect the area of metamorphosis to be pretty narrow.

    So I'd say the order was as ophiolite suggested.
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