New geochronological data which clarify the timing of syn-orogenic
magmatism and regional metamorphism in the Connemara Dalradian are
presented. U-Pb zircon data on four intermediate to acid foliated
magmatic rocks show important inherited components but the most
concordant fractions demonstrate that major magmatism continued until
465 Ma whereas the earliest, basic magmatism has been dated previously
at 490 Ma; a fine-grained, fabric-cutting granite contains discordant
zircons which also appear to be 465 Ma old. Are magmatism in Connemara
therefore spanned a period of at least 25 Ma. Recent U-Pb data on
titanite from central Connemara which gave a peak metamorphic age of 478
Ma are supplemented by U-Pb data on titanite and monazite from
metamorphic veins in the east of Connemara which indicate that low-P,
high-T regional metamorphism ism continued there to 465 Ma, i.e. at
least 10 Ma later than in the central region dated previously. New Rb-Sr
data on muscovites from coarse-grained segregations in different
structural settings range from 475 to 435 Ma; in part this range
probably also reflects differences 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 metamorphism. Thermal modelling indicates that at
any one locality the duration of metamorphism was probably as little as
1-2 Ma. The new dates emphasize the complexity in the spatial and
temporal distribution of high-level regional metamorphism caused by
magmatic activity. The relatively simple overall distribution of
mineral-appearance isograds revealed by regional mapping masks the
complexity of a prolonged but punctuated metamorphic history related to
multiple intrusions, primarily in the southern part of Connemara. The
later stages of magmatic activity followed progressive uplift and
erosion after the onset of magmatism, and were localized in the eastern
part of the region.