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Arkose or Paragneiss or something else?
  
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Kara




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PostPosted: Dec 01, 2018 06:36    Post subject: Arkose or Paragneiss or something else?  

Please help me again. I am thinking this boulder could be either layered arkose or paragneiss. The surface is tanned and blotchy and at first I thought it was migmatite, but inside it is layered with lots of colours, mostly green and gray, like a gneiss. It has some calcite in it for it fizzes slowly in vinegar. I can see merged grains on the surface, which looks also "sugar" flaky like quartzite.

What is this rock type? What causes the green colour, can it be fuchsite or is it some amphibolething? From Finland.

Kara



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Bob Carnein




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PostPosted: Dec 01, 2018 10:53    Post subject: Re: Arkose or Paragneiss or something else?  

This definitely is not an arkose. However, we keep running into the same problem with your posts, which is that you've asked a question that can't be answered by looking at a photo. Definitive identification (and genetic interpretation) of a rock really depends on examination of thin sections and determining the proportions of the minerals present, as well as detailed field observations. This can't be done by looking at a photo, no matter how good it is (or how many there are). This isn't a criticism; you ask good questions and give us interesting things to look at, but any answers are likely to be guesses.
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Kevin Schofield




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PostPosted: Dec 01, 2018 11:46    Post subject: Re: Arkose or Paragneiss or something else?  

I agree with Bob, in as much as this definitely isn't an arkose, which is simply a descriptive term for a sandstone with a particular composition...over 25% feldspar. In order to establish that a rock is an arkose, one really (again as Bob notes) needs a petrographic analysis.

I will part from Bob a wee bit, in as much as some things can be determined from a photograph...although not to the point of coming up with a definitive answer, and certainly not to the level of determining the mineralogical cause of a colour.

In this instance, I think that I can say fairly definitively that your rock is not a gneiss. It lacks the strongly differentiated mineral banding and flow textures that most gneisses exhibit. I would guess, however, from your close-ups that the rock has been metamorphosed (although even with a very sketchy knowledge of Finnish geology, that's not a great intellectual stretch!). My (educated) guess, based largely on the macrotextures seen in the first and second photographs, is that this is a meta-sedimentary rock that was deposited as a very poorly sorted conglomerate with a sandy matrix and a preponderance of shale lithoclasts. If that reading is correct, the overall texture suggests that it was probably deposited from a mass-flow of some sort, either a debris flow or very dense turbidity current.

To further Bob's comment, it would also help with these sorts of questions if you gave a little more context...where are you in Finland, what is the regional geology, what do the surrounding rocks look like? I would also encourage you to keep posting...our answers may indeed be (educated) guesses, but it gives both us and you food for thought.

Finally, to help you get that context, you might consider looking out work of the Geological Survey of Finland. As a generality, Surveys are always a good place to start with background knowledge, because their work in all countries is apolitical, non-economic, and (by definition) country-wide. In this case, the Geological Survey of FInland 2017 Special Paper #60 "Bedrock of Finland at the scale 1:1 000 000 – Major stratigraphic units, metamorphism and tectonic evolution" edited by Mikko Nironen might be a valuable companion on your field excursions.

https://tupa.gtk.fi/julkaisu/specialpaper/sp_060.pdf
(Link normalized by FMF)

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Bob Carnein




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PostPosted: Dec 01, 2018 13:40    Post subject: Re: Arkose or Paragneiss or something else?  

Kevin's post illustrates the problem--it's a guess (and I don't mean that in a negative way). I see greenish layers that "could be" lime silicate rocks deposited as impure carbonate sediments, but then again...Also, I'd definitely call the rock a gneiss, but Kevin would not. That's a pretty basic difference of opinion.

Although I agree that Kevin's hypothesis is plausible, it's still not based on the kind of detailed study that one should do before saying "this is a paragneiss" , "this was originally an impure carbonate sediment", etc. I definitely agree that the starting point should be to find publications on the area and see what the "experts" say, followed by using that information to make your own observations. That might provide a satisfactory answer to your questions. But that would best be followed by thin-section analysis. I know this sounds picky, and the last thing I want to do is discourage your interest and enthusiasm, but it's how science is done. Even "experts" disagree about interpreting geology.

Sorry if I'm beating a dead horse!?
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fuss




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PostPosted: Dec 01, 2018 19:09    Post subject: Re: Arkose or Paragneiss or something else?  

Educated guesses such as yours Bob, Kevins and others here are what I and probably most other novices look for when posting here, as well as suggestions on tests to perform and what to look for in those tests to help narrow down the possibilities. I personally am not expecting much more since I understand the difficulties in doing so.
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