The Connemara Field Trip



Leader: Barry Long (formally Geological Survey of Ireland)

Saturday 13th

Why do I always start with the weather? Anyway, as predicted, it mainly remained cloudy and but for a few outbreaks of rain was typically Irish summer. 20 people turned up at Joyce’s of Recess to hear Barry in his inimitable style describe to all and sundry where we were positioned. The northern part of Ireland in late Precambrian times, approximately 750 million years ago, was situated on the equator at the northern margin of proto-Laurentia and deep within the supercontinent of Rodinia. It was joined to the pre-Andean margin of today’s south west Peru, long before the Iapetus Ocean opened in latest Precambrian times. A major ice age (Slush Ball Earth) event is recorded here in the Dalradian rocks.

I will not dare to compete with Barry’s erudition so please allow for my reinvention of the genealogy of the 12 Bens, we may lose some Bens or Ben otherwise allocated to other families. At least we can say Ben there done that.

Anyway, joking apart, we headed off up the road to Lissoughter village in pooled cars to arrive on the lower slopes of Lissoughter hill where after negotiating fences and bogs we climbed to our first outcrop. Barry, here as always, was keen to be informative and demonstrative by way of maps (large) and sketches (short). Sometimes Long winded, as he had to compete with the windy elements at this elevation. Fair play to Camille for her agility to climb along and ahead of most of us.

The amazing views from this altitude helped all to try to appreciate the complexity of the D1, D2, D3, and D4 folding as explained by Barry and his flip chart. Barry had tried effectively to reduce this complexity by simplifying the terminology in the handout.

In Connemara the oldest part of the Dalradian Succession is missing. Sediments were initially deposited in a shallow stretching basin (like pulling a Mars bar apart), a sagging trough. The later sediments were deposited in generally deepening water on the northern edge of where the Iapetus Ocean eventually was to open at about 590Ma. These Precambrian sediments underlie Connemara. The opposite margin of the new ocean was inland from the coast of what would become modern Peru, but before the Andes formed, showing us how plate tectonics has relocated the continents over the many centuries in between.

The entire Dalradian Succession (in Scotland) spans from about 800-480 Ma (mid Neoproterozoic to earliest Ordovician). The stretching of Rodinia eventually brought instability and rifting. Extensional faulting on the continental margins created a number of depositional basins. Volcanism was symptomatic of these tectonic forces that led to opening of the Iapetus Ocean.

The Dalradian Succession comprises four Groups, Grampian (oldest), Appin, Argyll and Southern Highland (youngest). We were mostly concerned with the Appin and Argyll Groups, and rocks of these were deposited as sediments mainly on a stable shelf that became unstable. Most of their sedimentary features have been destroyed, and in most areas they are strongly and repeatedly folded, often upside down and strongly metamorphosed during tectonic and magmatic events in Ordovician times. Barry explained that the sediments first became schists deep in the crust during their regional metamorphism and were modified soon after that by district scale contact metamorphism caused by great heat from a large volume of gabbroic magma intruded mainly in the south that was itself metamorphosed to become metagabbro.

On Lissoughter we first saw folded, metamorphosed pelitic schists of the Barnanoraun Schist Formation. In several adjacent locations 50 yards apart we saw the effect of pressure and heat where abundant sillimanite, Al2SiO5, (as conspicuous knots of matted fibres known as faserkiesel) stood out from the surface of aluminium-rich pelites. The sillimanite was formed during large scale thermal (= contact) metamorphism caused by intrusion of very hot gabbroic magma to the south. It had replaced earlier kyanite and staurolite, produced during the regional metamorphism, as the temperature had increased and the pressure fallen. The pelitic schists also had small needles of tourmaline (black schorl). Other rocks we saw in the Barnanoraun Schist Formation included calcareous schists sometimes with abundant hornblende.

Further up the hill we saw bedded glacial metasediments, a diamictite, from ‘Snowball’ (or ‘Slush Ball’) Earth times (Sturtian ~720 Ma), and interbeds that can include large dropstones, all representing the Cleggan Boulder Bed Formation (best seen at Cleggan Head). The Bennabeola Quartzite Formation was present as a narrow unit, but we were to see this elsewhere later.

Nearby we encountered the famous Connemara Marble Formation, the oldest Dalradian formation in Connemara. This was once a dolomitic marble, with forsteritic olivine, that was altered by contact metamorphism to become serpentine marble with yellow-green serpentine, sepia phlogopite, pale green diopside (a clinopyroxene) and white tremolite (an amphibole). Nearby, the rock varieties trondhjemite and skarn were explained and a whitish skarn largely composed of diopside was inspected. We were all very good, only attacking the Connemara Marble Ònaturally erodedÓ boulders for our collections, minimizing any other plunder, and using Dan’s sledgehammer. This is almost the most easterly occurrence of the Connemara Marble.

The Derryclare Anticline (D2) is a huge district scale isoclinal fold that is refolded by a stack of tight D3 folds and the whole lot then arched over the Connemara Antiform (D4). The D2 and D3 folding give rise to repetitions of the stratigraphy and often result in the rocks being upside down.

On our way down from Lissoughter village back to Recess we looked briefly at the pelitic, semi-pelitic and psammitic schists of the Streamstown Schist Formation overlying the Bennabeola Quartzite Formation. These are distal turbidites and have occasional small irregular patches of granite caused by partial melting and recrystallization of the melt. There are some thin amphibolite sills and a broad band of granite related to the Oughterard Granite.

After lunch by the lake shore at Recess we proceeded up the road from Recess to Kylemore, past Derryclare Lough and Lough Inagh. We made several stops on the way, firstly near Recess to see the Bennabeola Quartzite Formation mainly with quartzite and psammite, but with minor microgranite and pegmatitic veining, and amphibolite sills, as well as folds of different ages. The granite veining related to the Oughterard Granite, and amphibolite sills related to basic volcanism that we saw higher in the succession later in the afternoon.

The Lakes Marble Formation was examined at two separate localities, firstly to see a unit of amphibolite (north of Inagh Lodge Hotel), formed by regional metamorphism of the original basic volcanic basalt that erupted as lava flows and also formed from associated volcaniclastic sediments. No original features have been preserved. The volcanism was a prelude to further volcanism prior to continental break up. The second locality was in one of the calcitic marble units of the Lakes Marble Formation. Siliceous beds were interbedded between pale marble beds. Elsewhere, beds of pebbly grit are part of the formation and micaceous schists and quartzite also occur.

Between our two stops in the Lakes Marble Formation we stopped to walk eastwards across the flat bog west of Letterbreckaun to see the Ballynakill Schist Formation. Graded quartzite pebble beds here are ideal for determining which way up the beds are. They are a feature associated with major continental faulting and rifting. A conspicuous fold was related to one of the many large D3 folds. Nearby, richly aluminous beds in outcrops of generally micaceous schists contain small staurolite crystals, easily seen after you find the first one. These were produced during the regional metamorphism. Tiny fibres of sillimanite are present, but a slice of rock must be examined by microscope to see them. They formed during the later contact metamorphism despite the distance from the metagabbros in the south. Small garnets are present and we heard of andalusite in nearby pegmatite bodies.

As time was pushing on, and the lateness of the hour, we headed home and later met up for a meal at the very modern Station House Hotel where Dan had organized the evening meal and chat.

Sunday 14th

Sunday started with very early rain which dried off in time for us to head for the Errislannan peninsula adjacent to Clifden, where Barry regaled us behind a wall, with maps charts etc. (what did the passers by think?). Some gawked and earwagged! The Alcock and Brown Memorial was the venue, and the local paragneisses of the Cashel Schist Formation are the more extremely metamorphosed southern stratigraphical equivalent of the Ballynakill Schist Formation seen yesterday. The garnets we saw in the paragneisses were a product of thermal metamorphism caused by great heat from the adjacent Metagabbro and superimposed on the regional metamorphic effects. The garnets are well formed with undeformed crystal shape (i.e. euhedral) and thereby contrast with the earlier garnets with ragged outlines produced during the regional metamorphism. Narrow veins represent recrystallized partial melt derived from the rock and sillimanite was present.

We moved on to the beautiful Mannin Bay just short of Ballyconneely to view the metagabbroic rocks in the hanging wall of the Mannin Thrust that underlies all of Connemara, except in the centre of the eroded Delaney Dome where one can see the rocks of the footwall.

Walking northwards along the beach of Mannin Bay, on the western margin of the Delaney Dome, we noticed that the mineral grain size diminished and all fabrics became aligned into almost a single plane. Pyroxene in the metagabbro had been entirely replaced by amphibole. The metagabbro has been transformed into mylonite as a result of southerly-directed shearing of the hanging wall over the footwall metarhyolite. The Mannin Thrust is marked by this thick zone of mylonite affecting rocks of both hanging wall and footwall. The hanging wall comprises the whole of Connemara and not just the metagabbro. The Ordovician footwall rocks of the Delaney Dome Metarhyolite Formation are mid Ordovician pre-462.5Ma and may relate to acid volcanic rocks of south Co. Mayo. On reaching these pale coloured mylonitic footwall rocks we returned to our vehicles for lunch in the sun beside the sea and its seals.

We learned from Barry of the real lineage of the white Coral beach, which is of algal origin. The Lithothamnium algae secrete calcium carbonate within their fronds that after their death can form beach deposits.

After lunch we moved to the east of the Delaney Dome to see more metagabbros, well beyond the mylonites, and there we met a local inhabitant rock lizard who posed for our cameras. The rocks here present us with lenses of Metagabbro surrounded by Quartz Diorite Gneiss that invaded the Metagabbro accompanied by shearing, thus breaking up the gabbroic rocks into smaller pieces on all scales, including district scale. These rocks are of Mid Ordovician age (late Llanvirn), and this is the minimum age of formation of the Mannin Antiform (D4) of the Delaney Dome. The Mannin Thrust only marginally predates the Mannin Antiform (formed between 467Ma and 462.5Ma) because it was folded by the antiform, and the later Dolan Antiform, and yet post-dates gabbro and quartz diorite intrusion. These two folds intersect to form the Delaney Dome structure.

The next stop was at Toombeola where a quarry exhibited nice white Roundstone Granite emplaced at about 400Ma showing no signs of stress or strain in the fabric. There were also some interesting patches of bright green epidote crystals in joints, and some pink K-feldspars giving two types of granite colouration, white and pink.

The final stop was just a bit further down the road where the thermal aureole of the Roundstone Granite yielded some conspicuous and abundant andalusite in the paragneisses of the Cashel Schist Formation.

As is customary with the IGA, the big speech has to be made and due to the needs of various participants to make tracks early el Presidente made an utterance and token gesture on behalf of the group to thank Barry, and to help him go to sleep at night.

On behalf of the IGA, I would like to thank Barry for his time and effort in compiling the notes and researching the venues to facilitate our very enjoyable trip.

Peter Lewis

el Presidente

Ed. C.B.Long

This entry was posted in Field Reports. Bookmark the permalink.

Leave a Reply

Your email address will not be published. Required fields are marked *