DECONSTRUCTING THE LATE-CALEDONIAN GALWAY BATHOLITH: P-T-T PERSPECTIVES OF GRANITE MAGMATISM IN CONNEMARA, WESTERN IRELAND

Well established intergranite field and temporal relationships are combined with hornblende geothermobarometry to present P-T-t perspectives of the Galway Batholith's magmatic history during the late–Caledonian. The batholith's country rocks are the 474.5-462.5 Ma Metagabbro-Gneiss Suite to the north, and the Lower Ordovician greenschist facies rocks to the south. It occupies a key location in the Caledonides of Ireland and the British Isles. Its 80 km long, WNW - trending axis lies astride and stitches the EW-trending Skerd Rocks Fault, which is part of the orogen-parallel Southern Uplands Fault system.

Major syn-emplacement NNE and NW -trending faults define the boundaries between a western, central and eastern block in the batholith. The western and eastern blocks expose granodiorite through granite to alkali granite varieties. In contrast, coeval diorite and granite magmas with pure flattening fabrics intruded by a later suite of unfoliated granites characterize the upthrown deeper level central block.

U-Pb zircon geochronology indicates that emplacement of an early suite occurred over at least 8 Ma (~394-402Ma) in the central block. Zircon from one non-foliated granite yielded a U-Pb age of 380 ± 6 Ma and a single concordant monazite yielded 383.6 Ma. Furthermore, Re/Os ages for granite related molybdenite mineralisation in several granites range in age from ~410 Ma to ~380Ma. Age determinations using both chronometers indicate a magmatic history punctuated in time by at least three significant emplacement (and mineralisation) events i.e. at approximately 410Ma, 400Ma and 380Ma.

Hornblende geothermobarometry, from eight new locations in the Galway Batholith, give spatially controlled crystallization pressures that range from 3.16±0.6kb to 4.13±0.8kb. When combined with existing data they show significant differences in calculated crystallization pressures (and temperatures) across the batholith. Finally, geothermobarometry is used in conjunction with the geochronologic data to generate an intergranite P-T-t model for the late-Caledonian Galway Batholith.