Emplacement timing of the Galway Granite: a window in the thermo-tectonic processes along the Caledonian subduction.

Conemara W-Ireland

The inter-relationship between magmatism and tectonic processes, especially in collision zones, provides a window into the thermo-tectonic evolution of orogenic belts. The closure of Iapetus between 450 and 400 Myr, the climax of the Caledonian Orogeny, raised great mountains in Scotland and Ireland. The suture line between the two continents on either side of Iapetus lies beneath the Scottish Borders and extends along the Northern-Ireland and Ireland border up to Connemara at the west coast of Ireland. Along this suture a number of syntectonic granites intruded. The Galway Granite occupies a key location in the west of the caledonian mountain belt. The late Caledonian batholith is 80 km long, WNW-ESE trending axis lies astride and stitches the EW trending Skird Rocks Fault which is probably a westerly extension of the Southern Uplands Fault. The Skird Rocks Fault separates high grade metamorphic rocks of the Connemara Massif from Lower Ordovician greenschist facies metavolcanic and metasedimentary rocks (S.Connemara Group).

U-Pb and Pb-Pb zircon age determinations by single grain evaporation, conventional single grain analyses have been used to portray the magmatic and tectonic history along the Iapetus suture in W-Ireland. We received crystallization ages for the different granite phases of the Galway Granite and the Omey Granite which define three age groups, the first at approximately 420 Ma, the second at approximately 400 Ma and the third at approximately 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 approximately 380 Ma granite phase can be generated entirely by large-scale melting of the older approximately 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 approximately 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 approximately 420 and approximately 400 Ma intruded after closure of the Iapetus Ocean. The second phase at approximately 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 extensional features during the Caledonian Orogeny. At the moment the relation between Scandinavian, Scottish Irish and American Granites are under investigation to understand the relationship of large scale fault zones and the melting processes in crustal levels.

Buchwaldt, R., Kröner, A., Feely, M., Todt, W., Toulkeridis, T.: Geochronology and Nd-Sr systematics of Caledonian Granites: New implication on the Caledonian Orogeny.- in prep.

Oliver, G. J. H., Chen, F., Buchwaldt, R., Hegner, E. (2000): Fast tectonometamorphism and exhumation in the type area of the Barrovian and Buchan zones Geology, 28, 459-462.

Buchwaldt, R., Kröner, A., Feely, M., Todt, W., Toulkeridis, T. (2001): Geochronology and Nd-Sr systematics of late Caledonian Granites in western Ireland: New implications for the Caledonian Orogeny Abstract at the GSA Northeastern Section Meeting, Berlington, March.[Invited].