Geopuzzle #6

This weeks mission, should you choose to accept it, is to work out what’s going on in this rather strange outcrop:



Categories: geology, geopuzzling

Comments (13)

  1. Silver Fox says:

    Possibly some kind of magmatic segregation process? The dark material, however, seems rather fine-grained. In places it looks like the lighter material is cutting the darker, in other places it looks as though the darker material is embaying outward into the lighter??
    On the other hand, maybe it’s a soft-sediment deformational swirling effect?

  2. Dave S. says:

    My first thought was that we’re looking at the wall of a magma chambre with a few xenoliths.
    That’s also my only thought.

  3. flounder says:

    Magma mixing.

  4. Divalent says:

    “what’s going on in this rather strange outcrop?”
    Looks like weathering to me. 🙂

  5. Bob O'H says:

    Err, yeah, sorry about that folks. You know they say you shouldn’t run with scissors in your hand? Well, you shouldn’t run with an open tin of grey paint, either.

  6. Not enough details! I’d be looking at that a lot closer, so I could see crystal size. But it does in fact look like a mafic injection into a cooler felsic melt. The mafic material intermingles with the felsic melt at the top of the outcrop (which may not be up in the chamber) and cools quickly to leave a fine-grained intrusive. You can see a second instrusive event occur especially on the right side of the first photo: there is a thin and large crystal intrusive that cuts through both the mafic and the felsic parts (but I’d want to get closer to confirm).
    Now I’m wondering if the felsic material was already slushy with crystals when the mafic magma intruded.

  7. Andrew says:

    It’s not stoping, is it?

  8. clay says:

    yep another vote for magma mingling, with the more melanocratic unit being the latter on the scene.

  9. fnord prefect says:

    I’m voting self-propagating nano-machines. At least we got prior warning. I, for one, and going to go ahead and live this weekend under the assumption its the last.

  10. Kim says:

    Why’s the felsic stuff blobbing into the mafic stuff? I’ve seen mingled magmas in which the mafic stuff looks almost pillow-like (and that makes sense, because it crystallizes at a higher temperature, so a granite melt could chill it). I’ve never seen lobes of granite blobbing into mafic rock, though.

  11. Silver Fox says:

    There is an odd “circular feature” in the upper photo, with a center of the lighter material encircled by a thin line of darker material. The outer part of this feature appears to be larger crystals of the lighter stuff, like a thin pegmatitic circular “dike.” Reminds me a bit of orbicular granite.

  12. Ron Schott says:

    Here’s where a Gigapan image would be really useful. I agree with Dean – I’d like to zoom in. The cuspate margin between the felsic and mafic units does call to mind magma mingling, but I have the sense (and I want to zoom in to confirm this) that the felsic unit is not a rhyolite, but in fact a granite (at least away from the contact), which would suggest that it is the older rock and that the cuspate margin is merely a remelted bit of the granite and therefore this is an example of stoping/assimilation. Since you’ve recently been to the Bushveld I could certainly see this as essentially a chill margin of that very large LMI, which would certainly have enough heat to partiallty melt some of the country rocks into which it was intruded (especially felsic ones with a much lower melting point). I’d also like to zoom in a bit more because I think I see xenocrystic feldspars in that mafic unit, as well. Of course, the other thing that would be very useful in interpreting the field relations is some sense of the third dimension, which I don’t think we really see well in just these two photos.

  13. PaulG says:

    Magma mingling. I have no desire to look any closer as I have already pushed the envelope of my knowledge(?) to the point of brittle failure.