The Belmullet Field Trip


Field Report. Field Trip to Belmullet Sat 21st / Sun 22nd June 2008

Leader : Julian Menuge (UCD)

As the forecast predicted, Saturday morning was overcast and uninviting. A large gang met at the Broadhaven Bay Hotel and Julian gave out a very well composed handout in glorious Technicolour. We were to visit five locations around the Belmullet peninsula and a further location near the Ceide Fields. We moved off in a reduced number of cars to Annagh Head to view the spectacular outcrops of Proterozoic rocks of the Grenvillian Orogeny. These metamorphic rocks, gneisses, amphibolites, migmatites, associated pegmatites and metadolerites range in age from 1,753 Ma to 460 Ma, through Grampian orogenic times, and have seen serious metamorphic changes at various times throughout this period.

At Annagh Head the Mullet gneisses are granitic in form with colourful banding and foliation, with large K feldspar crystals and are probably intrusive igneous rock in origin. The age of the original rock, before metamorphism, is 1,753 MaÉ give or take 3Ma. They were dated using the U-Pb isotope radiometric dating method.

The gneisses contain layers and pods of amphibolite; a very attractive and dark micaceous rock of fibrous biotite and hornblende. They are also cut through by several later dykes of metadolerite and pegmatite with large pink orthoclase feldspar crystals. We saw metadolerite dykes of probable Grampian age and also very large boulders of gneiss, which had been thrown up on the beach during storms.

Some of the rocks exposed on the Mullet peninsula are fragments of the ancient North American (Laurentian) continental crust that moved eastwards when the Atlantic Ocean started to open circa 200 Ma. Similar rocks are exposed in Greenland and in Canada near Quebec and Ontario.

Next, we went to Scotchport and saw the Scotchport Schist of semipelitic (muddy quartzitic) content, probably of Dalradian origin. Metadolerite sills intrude the schist. This metadolerite is probably the same age as those at Annagh Head.

On the northern side of the cove the rocks are strongly striped. These are shear zones exhibiting crystallized lenses of epidote, quartz and amphibole. These steeply dipping rocks made the terrain hard to negotiate on foot and, particularly so, as none of us are getting any younger (I think that puts it politely enough). Peter Lewis was in good rock collecting mode, as were others who offered to help increase his load and to whom he is greatly indebted.

Lunch was an interesting break at the supermarket after which the intrepid group set off to Cross Point. This was the site of the graveyard where the skeletons were being exposed by coastal erosion. The rain still just holding off we walked a short distance to the cove to see rocks, similar to those at Annagh head, with nice banded gneisses (less pegmatitic) and with finer layering. These gneisses are younger than those at Annagh Head – about 1,271Ma. Pegmatites cross these gneisses in bands of varying thickness with large feldspars. Some migmatites are also evident.

We moved along to our fourth destination at Belderg Harbour, nearly an hour away. By this time the rain won out and we arrived to be met by Seamus Caulfield and Killian Driscoll. We first inspected Belderg harbour where a folded metadolerite sill is overlain by psammite beds with almost horizontal undulating layers creating a whaleback-like structure. We saw a metadolerite sill that intruded the psammite beds.

According to Killian, the vein quartz in these rocks was used by a Neolithic (Stone Age) community that lived here around 6,000 B.C. They probably used quartz implements to hunt, fish, gather and prepare foods. Seamus and Killian gave us kindly of their time, and as archaeologists working on the site, they explained the work in the field, with reference to the new interpretive centre. Seamus is well known from his work on the Ceide Fields nearby.

Saturday evening was enjoyed very much at the Broadhaven Bay Hotel, where most of us availed of the great meal and service. Sunday was a wet start at the Hotel where we met and departed to Spinkadoon on the north Mullet peninsula again. Here we clambered once again over beds dipping at 45 degrees to try and find feldspar pebbles on the surfaces of the psammites. They were hard to find and interpret. Claire McAteer has spent some time working on these rocks and has shown that the pebbles may have been derived from the Annagh Gneisses.

Our last visit was to Doolough on the mainland to look at more gneisses that were made up of thick granitic layers and darker layers; both layers having a common foliation. These layers had been spherically foliated with pegmatite dykes cutting the foliations. A time frame for dyke intrusion and the formation age of the gneisses is about 1177Ma. Adjacent to these gneisses, pegmatites and amphibolites, we saw beds of peralkaline granite with a brown colouration. As Julian explained; the relative ages of all these ancient metamorphic rocks can only be worked out after serious field and lab work by many people. We have all these people to thank for making the story of these rocks more understandable and Julian, in particular, for unfolding this history for us that weekend.

Many thanks are due to Julian again, and also to Susan and Angela, for their combined efforts to make this such an interesting field trip. Despite the rain dampening our spirits a bit, a great trip was had by all.

Peter Lewis (Min Sec), (Field Rep).

The Cork Geological Association have some pictures from the weekend available here.

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