Day 4: Metasediments, Intrusions, and Fossils

Day 4: Metasediments, Intrusions, and Fossils
May 20, 2018, Island of Mull

Hello Geonerds!

Today was a very exciting day full of many interesting outcrops, exciting geology, and a bit of rain (but what can you do in Scotland?). We even had an appearance of the Banana Crew in their beautiful yellow suits!
Michael and Akuei displaying the latest and greatest in rain suit fashion.
 
Our first stop was at the Ross of Mull in Ardlanish Bay at a knob looking outcrop sticking out of the beach. The outcrop was a foliated metapelite rich in biotite and quartz. Also present in the rock were small crystals of kyanite and retrograde andalusite, as well as spectacular euhedral tourmaline crystals up to a centimetre in size. The rock was originally metamorphosed to the amphibolite facies, where kyanite is stable, however while temperature and pressure were decreasing it became less stable and converted to andalusite, kyanite's more stable polymorph. This retrograde mineralogy represents a disequillibrium in the rock that has been preserved for millions of years. 

Further south of the first outcrop the amphibolite facies was very obvious. Along with a very large amount of amphibole, garnets were also found within the rock, showing both the high pressure and abundance of aluminum within the rock.

Many dykes intruding a metasilstone host rock.


The second outcrop exhibited a beautiful intermingling of sedimentary and igneous rocks. In Carraig Mhor exists a metasedimentary host rock intruded by a multitude of dykes. The first intrusion was a felsic granite containing xenoliths of the host rock. After the felsic intrusion were mafic microdiorite dykes, which are the youngest rock type in the area. Folding and faulting also occur in the area.

The same metasiltstone but with Peter for scale. Which one is more beautiful?


An ammonite fossil with pencil for scale. Yes, its full of poop.
The last outcrop of the day was at Carsaig Bay after a walk through a very wet bog and over some very slippery rocks. The slippery rocks were mainly shales that are Jurassic in age and very fossiliferous, containing ammonite fossils up to 30cm in diameter. Above the shale is a unit of sandstone that displays crossbedding and above that, further up the hillside and beside the road, are units of flood basalt. Finally, the outcrop shows a great example of preferential weathering. Two mafic dykes intrude the Jurassic shale and protruding out of the outcrop, even jutting up out of the ocean. These dykes show that shale is a lot easier to weather than the dykes, which is why they are left behind while its surrounding host rock weathers away.
Just some scenery to enjoy on Mull.

That's all for now, keep checking back for more updates on the 2018 capstone trip to Scotland!

-Ivano Gennaro

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