9 August 2019

Friday fold: crumpled green metavolcanics from St. Anthony

Posted by Callan Bentley

It’s Friday, so it’s time to turn our alliterative attention to folds.

Here is a sweet specimen I collected this summer in St. Anthony, northern Newfoundland:

Link 1.39 Gpx GIGAmacro by Callan Bentley

This was from the top of the Dare Devil Trail, a wooden staircase that ascends a cliff at the east end of town, on a small peninsula by the lighthouse.

I’m still trying to wrap my mind around just how this rock formed. Here’s what I’ve got so far, and I’d appreciate any insights you have to share. This appears to be a porphyritic volcanic rock that has been metamorphosed (green color), but its most distinctive feature is the very thin layers which record the deformation so well. The white “phenocrysts” are not individual crystals however, but lumpy aggregates that appear to include both opaque feldspar and transparent/translucent quartz. Having quartz phenocrysts would imply a felsic original composition to the volcanics, but I’m not sure how that gets us to what I would guess (again based solely on color) to be chlorite, which needs a healthy dose of iron and/or magnesium.

Note also the differential weathering that is apparent between some of these layers. Some bright green lichens have roosted in the interstices provided by the recessive layers.

At the same site, there were more massive outcrops that showed a well-developed visible alignment of mineral grains. I spotted this distinctive rock halfway up the cliffs, long before I found the thinly layered, folded lithology up top.

Two views of one sample of the more massive lithology here:

Link 1.24 Gpx GIGAmacro by Callan Bentley

Link 1.05 Gpx GIGAmacro by Callan Bentley

(The first one of these is untreated; the second is polished.)

My first instinct was to lock on to their green and white pattern as a mylonitic fabric. But upon closer inspection and pondering, I have shifted to interpreting the alignment as a primary volcanic flow banding, on the basis of lack of ribboning of quartz and the presence of small void spaces in some locations (empty spaces wouldn’t persist under the high pressures needed to make a mylonite).

What do you think of these rocks? I tried to find out more about them, but unfortunately the site was not written up in any substantial detail in either of the two field guides I used in Newfoundland this summer. (One said, in essence, “There are a bunch of volcanic rocks near St. Anthony.”) And I was not able to access a detailed geologic map of this part of the island. So I’m left with a vague sense that I’m missing something, and I suppose the next step would be to make some thin sections to get a better handle on the petrology and mineral assemblage.

Regardless, have a happy Friday!