LATE CALEDONIAN MAGMATIC EVENTS FROM U-PB ZIRCON GEOCHRONOLOGY OF THE GALWAY GRANITE, CONNEMARA, IRELAND -IMPLICATIONS FOR CROSS-ATLANTIC CORRELATIONS

The Galway Granite occupies a key location in the west of Ireland Caledonides. The late Caledonian batholith's 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). We present new isotopic U-Pb zircon age determinations (using TIMS) for five granites from the central block of the Galway Granite. Field relations in the central block exhibit unequivocal evidence for several phases of granite emplacement. Interactions between coeval diorite and granite magmas are concomitant with the development of pure flattening fabrics and are succeeded by emplacement of a suite of generally unfoliated granodiorites and granites intruded in a brittle fracture regime. We present new data for four samples (G1, 2,3 & 4) from the early suite of foliated diorites and granites and one sample (G5) of the later suite of unfoliated granites. The results of the U-Pb zircon geochronology indicate that the emplacement of the early suite occurred over at least 8Ma and belongs to the late Emsian stage( G1=394.4 ± 2.2 Ma; G2=397.7± 1.1 Ma; G3=399.5 ± 0.8 Ma; G4 ~ 402Ma). Samples of commingled granite (G2) and diorite (G3) yield ages that are indistinguishable within error. Sample G5 is from a silica rich (SiO2 ~ 76wt.%) alkali feldspar leucogranite which cuts the early suite. Zircon from this granite yields a U-Pb age of 380 ± 6 Ma and single concordant monazite yields 383.6 Ma indicating emplacement at the Givetian -Frasnian boundary and a gap of ~20Ma between intrusion of the two granite suites. Emplacement ages and character of magmatism of the Galway Batholith are broadly correlative with those from New England. However, unlike the New England suite, field relations in the Galway Batholith indicate that it is entirely post-tectonic: deformation within being limited to structures directly associated with emplacement of the plutons themselves.