Exploring Knockvologan

Final day of the short geologising trip to explore the Ross of Mull Granite. Classic locations, today's was one of the best - Knockvologan and the coast opposite Erraid Island. Bitingly cold wind but beach was deserted. After spending the morning there I went up to Bendoran Cottage to have a look at the other classic ROMG/ Moine contact on the hill behind the cottage. More of that later, but here are some pics of the Knockvologan area with some brief details

(I used the VMSG 2017 Guide. I was on the VMSG field trip last year and Knockvologan was one of the places we visited. However there were a few locations that werent included then so I had a look today)

First stop to look at the distinctive aplite vein running through the granite near the shore

 The aplite vein runs across the rock face

 In close up

The granite is very coarse with large K-feldspar phenocrysts. RM2 facies

The area is famous for the diorite enclaves in the granite. All sorts of interactions and textures to be seen:

Xenocrysts of the granite in the diorite

 Diorite enclaves can appear as large rounded and lobate

Here is a later intrusion cutting across the granite

Large xenoliths of Moine metasedimentary rocks are common in the granite:

 Large xenolith - rucksack for scale

 And a small pegmatite just level with the sand

Some huge xenoliths seen in this picture looking across a small bay

The VMSG referred to a small dyke with cordeirite in it

 I think this is it!

The dyke has a rough spotted look - presumably this is the cordeirite - wasnt too sure on this point. The dyke must be later than the granite - Permian? Tertiary?

It also has a vein of some dark material running across it

Moving down the coast a bit there are some great views:

It really is one of the nicest places on the west coast

I headed over to Eilean nan Griogag, which is tidal, but as the tide was well out at this stage it was no problem

A large xenolith with pegmatite right beside it

 The bare slabs on the island show the diorite enclaves in the granite really well

 More diorite enclaves

 A large xenlith in the middle has weathered out and forms a pool

Contact of the xenolith with the granite

Back to the other side before the tide came in! Some good examples of diorite enclaves that have become flattened - described in the VMSG field guide

Flattened and squeezed diorite enclaves

Heading back towards the road, I spotted a couple of distinctively psammitic xenoliths in the granite:

Also clearly visible on the way back was this rocky outcrop which looks a bit like the tors you see in the Cairngorms:

And then back to the car - an excellent morning spent in a classic location. Many thanks to John Faithfull for his advice on this area - its brilliant!


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