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Friday, 26 September 2014

Rhyolite inclusion in a quartz block

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.....

1 comment:

Myris of Alexandria said...

It is interesting, most probably it is a qtz vein that entrapped a bit of country rock.
In thin section it is common at CRyf.