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Monday, 23 March 2015

Where did the Sleek Stone giant erratics come from?


Now I'm trying to work out where all these giant erratics found near the Sleek Stone, Broad Haven, have actually come from.......

I know that Rob says it's a mug's game, trying to work out provenances from photographs taken by amateurs like me, but I'm going to hazard a guess and suggest that the greenish and bluish rocks that dominate this erratic collection are quartz-feldspar porphyritic rhyolite with quartz and feldspar phenocrysts, of Ordovician age, from the south-western side of Ramsey Island.

Above I have juxtaposed the biggest close-up I could get from my photos of the erratics (on the left) with a photo of a porphyritic rhyolite from Mexico.  There seem to me to be considerable similarities.

Here are some more close-ups of the erratics:






The two lower photos are clearly different.  How would we describe these?  Quartz-diorite?  Porphyritic microtonalite?  These might come from outcrops on the northern part of the island -- expert advice welcomed!

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5 comments:

  1. Myris Of Alexandria23 March 2015 at 14:04

    Its a mugs game.
    1 porphyry certainly zoned altered feldspar megacrysts
    2 Arkosic cg sst
    3Porphyry see 1.
    4 Scottish granite/granodiorite
    5 Only Sublime Apollo Knows.
    M
    Send freshly broken samples.

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  2. BRIAN JOHN23 March 2015 at 16:45

    Thanks Myris -- very useful, even if everything is speculation until such time as it isn't. Can you please educate us as to why you reckon pics 1 and 3 are porphyry whereas 2 is a coarse sandstone? We are all on a steep learning curve here, and specialist education is always welcome!

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  3. Myris of Alexandria23 March 2015 at 17:53

    The feldspar megacrysts are euhedral
    And zoned. There is no other large crystal/clast.
    Two probably has large feldspars but also has irregular clasts plus at least two of your porphyries have turned out to be cg arkosic sandstones and I am guessing this is another BUT seeing the real rock is a first step.
    M

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  4. BRIAN JOHN23 March 2015 at 19:34

    Thanks again -- excellent. I'll try and take another trip down there to collect some samples.

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  5. BRIAN JOHN24 March 2015 at 09:45

    I see now that the crystals or megacrysts are "complete" and clearly defined in the porphyritic rocks, and are set in a darker matrix. In contrast, the crystal forms in the second photo -- the one you refer to as a coarse-grained sandstone -- are much more fuzzy or broken, and the matrix seems to consist of similar material -- presumably bits and pieces of quartz? The question is this -- could those sandstones have come from Ramsey as well? The island is very complex geologically, but there are sediments there, mixed in with all those volcanics. Richard knows the island as well as anybody else.....if not better!

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