Something for Rob and Richard. I hadn't noticed this before, but that big detached block of quartz resting on the bedrock floor just upslope of the "proto-othostat" is very interesting indeed. It's very brittle and sharp-edged, so it hasn't been subjected to much erosion -- and that suggests strongly to me that it is more or less in situ. But if you click to enlarge these pics you'll see that there is a large bluish rhyolite inclusion within it. There are traces of fine fractures within the rhyolite, but it doesn't seem to be strongly foliated. I don't suppose there is any great significance in this, but it's a nice geological curiosity.
Precisely what might have been the mechanism of formation? I thought quartz was often created as a "secondary" mineral in pre-existing volcanic or sedimentary deposits? But here it looks more as if the rhyolite is the thing that is secondary.....
It is interesting, most probably it is a qtz vein that entrapped a bit of country rock.
ReplyDeleteIn thin section it is common at CRyf.
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