U-PB AND RB-SR GEOCHRONOLOGY OF MAGMATISM AND METAMORPHISM IN THE DALRADIAN-OF-CONNEMARA, WESTERN IRELAND

New geochronological data which clarify the timing of syn-orogenic mag matism and regional metamorphism in the Connemara Dalradian are presen ted. U-Pb zircon data on four intermediate to acid foliated magmatic r ocks show important inherited components but the most concordant fract ions demonstrate that major magmatism continued until 465 Ma whereas t he earliest, basic magmatism has been dated previously at 490 Ma; a fi ne-grained, fabric-cutting granite contains discordant zircons which a lso appear to be 465 Ma old. Are magmatism in Connemara therefore span ned a period of at least 25 Ma. Recent U-Pb data on titanite from cent ral Connemara which gave a peak metamorphic age of 478 Ma are suppleme nted by U-Pb data on titanite and monazite from metamorphic veins in t he east of Connemara which indicate that low-P, high-T regional metamo rphism ism continued there to 465 Ma, i.e. at least 10 Ma later than i n the central region dated previously. New Rb-Sr data on muscovites fr om coarse-grained segregations in different structural settings range from 475 to 435 Ma; in part this range probably also reflects differen ces in age from west to east, with three ages close to 455 Ma from the eastern area, which is also the site of the lowest pressure metamorph ism. Thermal modelling indicates that at any one locality the duration of metamorphism was probably as little as 1-2 Ma. The new dates empha size the complexity in the spatial and temporal distribution of high-l evel regional metamorphism caused by magmatic activity. The relatively simple overall distribution of mineral-appearance isograds revealed b y regional mapping masks the complexity of a prolonged but punctuated metamorphic history related to multiple intrusions, primarily in the s outhern part of Connemara. The later stages of magmatic activity follo wed progressive uplift and erosion after the onset of magmatism, and w ere localized in the eastern part of the region.