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Friday, 18 May 2012

Microtonalite and Dolerite



I was googling about, as one does, trying to inform myself as to what a microtonalite looks like, following Rob's kind identifications of those samples I collected.  The top photo is of my sample from a boulder on Carningli.  The bottom one is a photo of a microtonalite from somewhere in Wales.

I don't think the two are the same rock type.  The top one has much finer crystals, packed more densely with less whitish "matrix" -- whereas the bottom one has a flaky look about it, as if the dark crystals are elongated rather than round, and more variable in size.

I think I'll go with Rob's initial identification -- namely that the Carningli rock is a fine-grained dolerite.

All further opinions gratefully received!

4 comments:

Phil M. said...

Hello Brian,

Interesting stuff igneous rock, but maybe of equal, or even greater interest is your sample number 7, about which Rob says --

"Fine-grained indurated micaceous sandstone with quartz veining along joint plane. Devonian/Lower Palaeozoic? Not Permo-Trias."

Perhaps Rob would give a view as to the possibility of sample #7 and it's parent, the bigger, rounded boulder, originating in the micaceous sandstones of the Cosheston Group to the south of Newport, or the Senni Beds to the east of Newport?

Phil

BRIAN JOHN said...

Not sure what you are on about here, Phil. This sample is red -- the Cosheston Sandstones and the Senni Beds are greenish of buff in colour, which is why they have been debated with respect to the Altar Stone.

Are you suggesting that this boulder found near Newport (Pembs) has been carried by ice or human beings either north-eastwards from Milford Haven or westwards from the upper Towy Valley? There is not, as you will be aware, any evidence for ice movement in this area which would fit the bill.

What point are you trying to make?

Phil M. said...

Hello Brian,

I confess to being unfamiliar with the appearance of the Cosheston Group sandstones, but I have had dealings with the micaceous sandstones of the eastern section of the Senni Beds, a portion of which has been described by an eminent petrologist as being purple coloured in its natural state, and pinkish grey when cut,(5YR 7/1 on the Geological Society of America rock-color chart); as opposed to the, perhaps, more normal green/buff colour you cite. I posses a very large lump of eastern Senni Beds sandstone that is distinctly red in colour, far too heavy to send to Rob for his opinion; perhaps I'll send him a photo as a substitute. :-)

I agree with you when you say "there is not, as you will be aware, any evidence for ice movement in this area which would fit the bill". It may therefore be the case that the rounded boulder which produced your sample may have been brought from the east to the Newport area, by human endeavour.
Have you an estimate of the mass of the rounded boulder, please, as it may promote discussion?

Phil

BRIAN JOHN said...

I'm not a geologist, and am certainly no expert on the Senni Beds, but my impression is that the Senni sandstones are generally greenish in colour, with a few brownish beds as well -- very different from the spectacular red and pink colours we get in Pembrokeshire.

There is nothing special about my reddish boulders in this area --- the other day I had a load of footpath gravel delivered from one of the gravel pits near Moylgrove. In every shovel-load of this gravel there are reddish pebbles rather like the boulders I found up the road. I have concluded that the glacial and fluvio-glacial deposits of this area have this reddish material as one of their standard components.... but it's still rather interesting to know where it all came from.