Abstract

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. Arc 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 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.

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