Joint Ireland Wales Branches Field trip 2008 : The Granites of the Wicklow Mountains

Five members of GCOUGS joined many members of the Ireland Branch in Wicklow at Easter to explore the granite facies of these stunning mountains.  Torrential rain on the first morning did nothing to quench our enthusiasm for our first international fieldtrip.  We would like to thank the Ireland Branch, in particular Phyllis Turkington, for welcoming us, for arranging an excellent leader in Michael John O’Mahoney and for organising our accommodation at the Glendalough Hotel which has a spectacular setting.  The trip was thoroughly enjoyed by all.

 

The Leinster Granite in South East Ireland is the largest outcropping granitoid complex in the British Isles with an area of 1500km².  Geophysical surveys indicate a continuation to the South west under the Upper Palaeozoic.  The granite complex intrudes the Lower Palaeozoic which comprises, according to Michael John’s article, “a complex marginal sequence of metasedimentary and metaigneous domains of Avalonian/Iapetus affinities which have undergone pre-granite green schist metamorphism”.  Wow!  The Iapetus suture zone lies to the north west but the boundary is unclear due to the extensive Lower Carboniferous cover and complex Iapetus closure history.  It is thought that closure caused the base of the crust to melt to create granites and the geochemical and isotope studies indicate derivation from partial melt.  Around the granite there is a contact aureole of garnet, biotite, andalusite, staurolite, cordierite, muscovite, tourmaline and other minerals.

 

At the hotel on the Friday evening Michael gave us flavours of what was to come; the granite complex is divided into five plutons (see map below) which are arranged in en eschelon fashion obliquely to the regional Caledonion trend in the country rocks.  These have undergone a complex tectonic history through the Lower Palaeozoic and the granites have been emplaced post orogenically.  The units are often separated from one another by schist zones.  Many granite facies can be observed due this probable island arc tectonic setting where mixing of different melt creates interesting and varied textures as a result of thermal and compositional differences.

Five granite units separated by schist shear zones, after O’Mahoney, 2001.

Out in the field along the Glendalough Valley we observed the small folds of the siltstone/mudstone country rocks close to Glendalough and as we progressed along the valley into the hills the rocks became more granitic.  Michael pointed out appinite blocks up on Camaderry Mountain identified by a change of vegetation.  The geological divide between granite and schist is clearly visible in places like Glendalough where coarse granite boulder scree suddenly gives way to smoother shiny schist.  At the west end of the Upper Lake lie the ruins of an abandoned miners' village where the mining of lead took place from 1850 until about 1957.  Lots of minerals were spotted in the spoil heaps.

Mining was also in evidence at our second stop at Glendasan in a parallel valley towards the Wicklow Gap.  Here we observed the contact between the Lugnaquilla and the Ribband units and highly strained granites were in evidence. At the Wicklow Gap we saw greseining or elongated quartz crystals in the granite which indicated deformation at emplacement.  Here the schist septum was visible as more rugged landscape in contrast to the smooth granite outcrops on either side.  Passing further west to Hollywood and along a forestry track, we experienced the western contact of the granite of unit 1, metasediments and along the road, high above, vertical bedding and other country rock structures were clearly visible.

On Sunday morning we drove to the stunning Glenmacnass waterfall where we closely observed ‘megacrystic granites cross cutting high strain gneissose granite’ outcropping along the road cutting.  The views from the top of the waterfall along the glen were superb and we were able to see the contact of the northern unit 1 with the Ribband Group quite clearly delineated by outcrop and vegetation zoning.  We drove along the panoramic Sally Gap  and at Brockagh mountain we were delighted to find good examples of coticule…..neatly folded quartz veins associated with this kind of granite setting but still remaining somewhat of a mystery to most of us!  Excellent photos are included here from Tony James.

Coticule                                                      Glenmacnass in schist with waterfall in granite.

 

References:

Grogan, S.E. and Reavy, R.J., Disequilibrium textures of the Leinster Granite Complex, SE Ireland: evidence of acid-acid magma mixing. Mineralogical magazine, December, 2002, Vol 6, p929-939.

O’Mahoney, M.J., (2002) The Structure and Metamorphic features of the Central and Southern Units of the Leinster Granite Complex, SE Ireland, Unpub. PhD thesis, NUI, Cork.