The Rosslare Complex Sat/Sun 21/22 October 2006


Field Report. The Rosslare Complex Sat/Sun 21/22 October 2006

Leader : Barry Long

Weather forecasting was again proven hit and miss and very wrong yet again,when 13 dedicated and prepared to be weather-beaten attentive Holocene Homo-sapien students and real geologists + canis minoris ( Charlie Brown ) faced the elements at Forlorn Point, Kilmore Quay, Co. Wexford.

This aptly named spit of outcrop disappears under the waves and at times and different places displays itself as differing lithologies.We know that the problem of deciphering these rock facies is due to their extreme old age ( pre-cambrian ) and being seriously metamorphic with numerous intrusions of dykes and sills makes dating and sequencing very difficult. Barry explained even his abilities to understand the Complex needed to be augmented by several specialists namely: Daniel Tietzsch-Tyler, Stephen Daly UCD Geology and M.D.Max to name but a few.

We made the first attempt to see the rock outcrops at the west of Forlorn Point beach. Here we saw banded gneiss of the Kilmore Quay group, known as grey gneisses and dark gneiss. They were interspersed with metamorphosed granites to a form known as Trondheimite. There were several minor intrusions of dyke. (granodiorites and quartzites ) We moved up the beach eastwards and out along an outfall pipe to view more dyke intrusions, some very clear and dramatic.

Barry explained a rough genealogy as far as we now know of the Wexford complex. Back about 620Ma near the South pole of today’s world, Ireland south and most of England and Wales was part of Avalonia – a smaller offshore breakaway land from the old supercontinent of Gondwana and Rhodinia. There are, it must be said, differences of opinion here due to age of formation i.e. Timescale versus position. These lithologies at Kilmore Quay represent a time when these were continental rocks as distinct from marine rocks. These are also well metamorphosed with pressure and re-melting so we see thinly banded gneiss and schistose type rocks with micaceous structures.

Due to the ongoing threat of yet another drenching we moved back to the car-park and had a break for lunch. The weather then improved somewhat and we headed refreshed up the eastern coast of Kilmore Quay to try to view the contact of the Saltees granite and the Carnsore granite which meet at this location. Unfortunately the tide was not on our side due to strong southerly winds and we only got close to the contact with the tide swirling around. Some loose pieces of Saltees granite were in evidence and the main contact remained out of reach. These granite formations occurred much more recently- in Cambrian times for the Carnsore granite intrusion about 550Ma, then the Saltees granite about 437Ma. These would have evidenced the Monian Orogeny from Cambrian to Ordovician and then the Caledonian Orogeny from Ordovician to Silurian.

Much of these altering the granites to gneiss and amphibolites.

We decided to move rather promptly back to the harbour at Kilmore Quay as the tide was advancing on us and the weather getting worse overall.As we had some distance to travel to Kilrane we decided to call off the day early and met at Culletons of Kilrane later for a meal and banter.

Sunday 22 October a much better day started well weather –wise and we all met up at St.Helens harbour where we moved off at the early hour of 09:45 towards the south of the bay to find the dark gneisses of the similar Kilmore Quay group. This comprises the oldest part of the complex, M.Max (1975) termed the Initial Complex, the dark gneiss , a fine grained amphibolite and has basic to intermediate volcanic elements. Also grey gneiss rich in phyllosilicates representing a semi pelitic meta-sedimentary sequence of greywacke origin making this gneiss a para-gneiss.All of these were highly cooked metamorphosed and migmatised.There is evidence of two main events of migmatism and intrusion of younger Granodioritic gneiss.These may represent the Caledonian orogeny. The Rubidium-Strontium dating methods of half-life decay suggest ages for these rocks of 1600 Ma and not more than 2400 Ma . Old eh !

On the other side of the small beach 60 metres to the north of the gneiss were the outcrops of the meta-gabbro. Various features can be seen, injectioned dykes of quartzitic fabrics and more basic dykes.

We proceeded further north by the rocks ,to see more amphibolitec type very dark rocks of questionable content needing laboratory equipment to analyse. We chipped our way through many outcrops and many of us came back to the car with prized examples.

With the dark metagabbies we see the pyroxene content being changed to amphibole and epidote on the surfaces of some gneiss. The beach contacts between the Gabbro and the Greenore group are hard to see.. The ongoing discussions were very interesting and Barry kept everyone onside despite the Hecklers.

Lunch was called back to Kilrane and most arrived at the same time except Phelim whose car decided enough was enough. After 1 green bottle had already fallen of the wall (PJ and Charlie Brown) it was Phelims turn to bow out. We hope he fared ok in the end as he needed a recovery lorry. We then made off towards Tomhaggard at the south contact of the Ballycogly group with the Kilmore Quay north contact. The rock is seen in excellent form in a small disused quarry east of the village of Tomhaggard. This is a phyllitic mylonite sometimes abbreviated to Phyllonite and is very like fine mica schist. This is a good example of a much metamorphosed contact type rock. These mylonites have been formed by the intense deformation of the gneisses of the Rosslare Complex. They form past of a NE/SW belt which separates the Rosslare Complex from the Cullenstown Formation and the lower Palaeozoics to the north.

We took this opportunity to thank Barry Long very much for all the preparation and effort he put into the weekend as well as the very informative notes which somehow survived Saturday’s drenching due to Barry’s plastic note coats. The customary bottle of wine was handed over and Barry waxed poetic. Our thanks in absence to Angela for her input regarding accommodation.

On a sad note we all would like to express our heartfelt sympathy to the Poole family and Gordon on the loss of Owen whose friendship and humour will be deeply missed. He passed on the duties of treasurer to Ed McKenna for anyone who needs to know and we also would appeal to any new members and any old members to continue to support our efforts as we have great plans for the immediate future.

Peter Lewis & Ed McKenna

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