10 October 2016
Oddball Icelandic rocks, part II: granite!?!?
Posted by Callan Bentley
Around the corner from the Hvalnes Lighthouse in eastern Iceland is the second non-basaltic igneous rock I saw in Iceland.
I couldn’t drive past something like that and not stop. Seriously – this was the only high-albedo rock I saw on the entire island!
(Note the lens cap for scale, as in all the photos in this post.)
Yes, that’s what it appears to be – here on the flanks of the Mid-Atlantic rift, partial melting of the mantle yields basalt and partial melting of the basalt has apparently generated a granite.
This was unexpected (to me), as I associate granites with more evolved crust – like that of the continents, where the material has been cycled through multiple rounds of “distillation” to pull out the most easily-melted components. (Felsic minerals melt at lower temperatures than mafic ones, so if you partially melt a given rock, it’ll segregate into a more-felsic liquid and a more-mafic solid residue.) Iceland’s apparently got a sort of “hot spot” underneath it (like Hawaii) as well as being situated on a divergent boundary (the Mid-Atlantic Ridge), so I would have expected nothing but basalt. (And indeed, that’s mostly what Iceland is made of!)
I wouldn’t have guessed any of the rocks here would have had the opportunity to get the granite sweated out of them. But: apparently it has happened, at least once.
The granite contains xenoliths of more mafic compositions:
And now for a few GigaPan views of this place:
Link GigaPan by Callan Bentley
Link GigaPan by Callan Bentley
Link GigaPan by Callan Bentley
Link GigaPan by Callan Bentley
Finally, a GIGAmacro view of a sample I collected at the site. You can explore its phaneritic (coarse-grained) texture and see its quartz content readily with this image:
Link GIGAmacro by Robin Rohrback
A granite in Iceland! Who’d have thunk it?
Well,that’s a thing – I was just watching new Magic Schoolbus series, and finding granite in Iceland came up. My reaction was “In Iceland??? Not bloody likely!” 🙂 Nice to see you’ve proved otherwise
Hi! My friend and I just drove past this today and had to stop. Your post is the first one that pops up when I google Iceland granite. Have you found out anything more about it? Source? We thought this was magma mixing with granite and a mafic magma due to some rounded edges and the intermediate mixed margin around some of the enclaves, but I see you’re calling them xenoliths as opposed to enclaves – I’m curious to know what you think of this, can both processes be there? Did you find out any more about this? 🙂
Nothing much. Sure, I don’t see any reason both processes couldn’t be present. If you’re more of an igneous petrologist than I am, I defer to you.
Robin Gill, in their book “Igneous Rocks and Processes,” mentions the presence of felsic rocks along the mid-Atlantic Ridge. This looks like a zoned pluton that has undergone fracturing during various stages of cooling – it is Iceland, after all. If you look at your top “Gigapan” photo, you can see (at least) four different igneous rock types within the overall exposure AND you can see that different rock types enclose other rock types. Talk about a picture worth a thousand words. Thanks for sharing this.