OUGS FIELD EXCURSION TO DONEGAL 2 - 4 May, 1998

A group of fourteen enthusiastic OUGS members assembled in Ardara in south-west Donegal on a fine Saturday morning ready to set out into the unknown with our leader John Reavy and his colleague Adrian Finch. John gave a short summary of the local geology which consists of many small, basic, intrusions and several different, larger and later, Devonian granite bodies all intruded into Dalradian meta-sediments.

We drove through rugged country along the Main Donegal Granite, which is some 40km long by 10km wide, to Doochary. Here we looked at strikingly banded granite, the bands in widths of less than a metre, the granite very variable in colour and texture. It was suggested that the Main Donegal Granite had been intruded in numerous small increments along a ductile NE-SW shear zone. Shearing continued after emplacement further deforming the granite.

Onward to scenic Lackagh Bridge where we ate lunch all the while trying to stop it blowing away in the strong breeze! Here, very close to the contact with the Main Donegal Granite, we saw granite sheeting in the Lackagh Quartzite (= Ards Quartzite).

Next stop was Rosguill in northern Donegal to examine granite stoping in the Fanad pluton. Stoping occurs particularly in the roof of an intrusion where intruding magma prises away blocks of country rock which sink into the magma and, depending on their size, may be absorbed or greatly altered. We saw plenty of such xenoliths in the granite. Cordierite was noted in the xenoliths.

Our final stop for the day was on the coast at Downings to see a basic amphibolite dyke intruded into the sediments. On the long return drive to Ardara we noted the widespread effects of recent glaciation on the landscape.

Sunday dawned fair and we were to concentrate on the almost circular Ardara pluton and its surrounding features. On the coast at Rosbeg we had our first good look at the Dalradian which had been intruded by relatively fine-grained, dark-coloured, basic dykes and sills and one fine-grained, light-coloured felsite dyke. All had later been thermally metamorphosed, the country rocks, originally muds and silts, now forming pelitic schists. There were abundant garnets (~0.5cm) throughout, mostly replaced by chlorite, though a few relatively unaltered red ones were also seen.

Around the Ardara Granite are several appinites. These mafic plutonic rocks, often with abundant volatiles, form a petrological series. They may be closely associated with a particular granite. Some are forcefully intruded as an extremely fluid mush which splits off pieces of country rock on its way up. At Birroge we saw a breccia formed in this manner with clasts of quite variable size, shape and composition in a dark matrix.

At Portnoo harbour the appinites we saw were coarse-grained, dark-coloured gabbros with large hornblendes which had been intruded into Dalradian limestones.

Having skirted the granite all morning, during the afternoon we began moving in closer. At Clooney an outcrop of strongly foliated, pelitic country rock possessed spectacular andalusite crystals several centimetres long lying in a plane parallel to the edge of the intrusion.

A short drive away, near Maas, we crossed into the granite proper. A distinct break in slope marked the probable edge of the pluton and about 100 metres into the granite we saw a strong foliation parallel to the edge of the intrusion and to the pelite at Clooney.

Our last stop was near the centre of the pluton where the granite was undeformed. The model proposed for this intrusion was of a conduit formed at the intersection of the NE-SW shear zone with another EW lineament. The granite magma ascended through this conduit until it reached a level where it spread out sideways in a flat mushroom shape. Fresh pulses of magma entered at the centre and pushed against the pre-existing, cooling magma which became increasingly deformed toward edges where it was forced against the country rock.

Back to Charlie's for dinner and then to a nearby hostelry for liquid refreshment and Irish music.

Monday's main event was a visit to see the dramatic quartzite sea-cliffs of Slieve League some 600 metres (2,000feet) high, reputedly the highest in Europe. A short discussion tied the local geology into the larger picture. The two lineaments mentioned above mimic the NE-SW trend of the Leinster Granite and the EW trend of the Munster Basin. Was Donegal once closer to, or further from, Leinster / Munster? Was this evidence for an ancient ocean opening and closing? Perhaps, perhaps, perhaps. Our sincere thanks to John Reavy who led an extremely interesting field trip investigating many types of igneous intrusion in superb surroundings. Susan Pyne

The photo of the cliffs ar Sleive League was taken from the car park at

the end of a rather hairy road along the cliff top.