Participants listening to the leader Dr. Menuge as they gather to make their way out to the rocks at Annagh Head.The Mullet Gneisses at Annagh Head on the Mullet Peninsula, the oldest rocks on the mainland of Ireland.The Mullet Gneisses, originally probably intrusive igneous rock like granite or granodorite have been dated to 1753+or- 3 million years using the U-Pb isotope analysis of zircon crystals. The Mullet Gneisses contain layers and pods of amphibolite ( dark minerals, hornblende, biotite, etc.) probably originally emplaced as dykes. Similar rocks to these gneisses are found in Canada and Greenland.IGA members examining the rocks at Annagh Head. Cutting through the gneisses are coarsegrained pegmatite sheets ( milky quartz and orthoclase feldspar) that have been dated to 990 Ma. Cutting across the pegmatites are some metadolerite dykes which probably date from about 460Ma.The gneisses and related rocks on the Mullet Peninsula, on the offshore of Inishglora and Eagle Island and on the mainland nearby around Doolough have been known as the Annagh Gneiss Complex. The rocks in this area were changed and metamorphosed during the Grenvillian and Caladonian orogenies.Looking south from Annagh Head to Inishglora, Inishkea Islands and Achill Island.Looking north from Annagh Head to Eagle Island.Large boulder of gneiss thrown up during a storm. Great care should be taken in this area during stormy weather and high tides.A storm beach near Annagh Head on the Mullet Peninsula.Scotchport Bay: On the left hand side of the bay we see the Scotchport Schists. These biotite schists have foliation in one direction. They were originally sedimentary rocks with some mud and quartz and probably part of the Dalradian Supergroup.Group of IGA members clambering over the schists to get a closer look at a metadolerite sill intruding the schists. The foliation in both probably took place during the Caledonian orogeny.A very different rock on the right hand side of the bay - a mylonitised rock containing pegmatite parallel to the bands in the rock. It started off as gneisses like those at Annagh Head.Another view of the strongly striped rock. This is a Grampian shear zone.Cross Point: Cross Poimt Gneiss showing a tightly folded banded gneiss and pegmatite. These are granitic gneisses similar to those at Annagh Head but the igneous rocks, from which they were later formed, were intruded as magmas only 1271 Ma ago.At Belderg Harbour psammite beds ( metamorphosed sandstone) form undulating layers over a folded metadolerite sill. Far away from the AGC the rocks here are mainly Dalradian metasediments and Stone Age communities used the quartz to manufacture tools.The Doolough gneisses on the mainland shore opposite the Mullet Peninsula have thick granitic and dark layers and share a common foliation. The banded foliated gneiss has been folded in many places. Here the Doolough Gneiss shows intrusion by pegmatite bands. The gneiss represents the metamorphism of intrusive igneous rocks which originally crystallized at 1177Ma. The pegmatite bands are about 990 Ma. old.Another view of the lighter-coloured pegmatite bands intruding the folded Doolough gneiss.Further along the shore we came to Doolough Granite which was intruded at 1015Ma. Here we see coarse-grained darker patches surrounding a finer-grained pink/orange patch of the granite. This granite is rich in many minerals including potassium and aluminium.Another view of the group as they gather at Annagh Head.
The Irish Geological Association organised a field trip to the Mullet Peninsula on June 20th - 21st last. The trip was led by Dr. Julian Menuge, U.C.D.. Even though the weather was not too kind to us it was a very interesting trip. Below is a photo gallery of some of the pictures taken over the weekend. A full  account of the trip, written by Peter Lewis, is to be found  on the IGA website at www.geology.ie under "Field Reports" .