GEOCHRONOLOGY AND ND-SR SYSTEMATICS OF LATE CALEDONIAN GRANITES IN WESTERN IRELAND: NEW IMPLICATION FOR THE CALEDONIAN OROGENY

U-Pb and Pb-Pb zircon age determinations by single grain evaporation, conventional multigrain fraction analyses have been used to portray the magmatic and tectonic history along the Iapetus suture in W-Ireland. We report crystallization ages for the different granite phases of the Galway Granite and the Omey Granite which define three age groups, the first at ~420 Ma, the second at ~ 400 Ma and the third at ~ 380 Ma. eNd(t) values for the various phases of the Galway Granite show values of -1.5 to -2.6 and Nd mean crustal residence ages of 1.3 Ga to 1.4 Ga. A binary isotopic modelling exercise shows that the ~380 Ma granite phase can be generated entirely by large-scale melting of the older ~400 Ma phase, triggered by voluminous intrusion of mafic mantle material into the lower crust. One of the youngest granitoid phases of the Galway Granite, the Murvey granite suite, displays a completely different picture to other intrusion phases. These rocks are highly evolved, restitic leucogranites and carry magmatic garnet. The eNd(t) values for the Murvey granites, assuming an emplacement age of ~380 Ma, range from +2.3 to +3.1, and the Nd mean crustal resident ages are 0.9 Ga to 1.0 Ga. The Nd data suggest involvement of isotopically primitive material, therefore the Murvey suite seems to have originated from different sources compared to the rest of the batholith. Based on these data it is postulated that magmatism along the Iapetus suture is characterized of at least four separate granitic phases. The first and second phase at~420 and ~400 Ma intruded after closure of the Iapetus Ocean. The second phase at ~380 Ma was emplaced due to input of mantle material into the crust that caused remelting of the older material. The last phase which is so far undated is indicated by the Murvey Granite suite and might be associated with late extentional features during the Caledonian Orogeny.