Geochronology of Late Caledonian magmatism in northern Britain

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Abstract
Rb-Sr, K-Ar, Ar-Ar and Sm-Nd isotopic data are presented for mineral separates and whole rock samples from Old Red Sandstone (ORS) lavas, associated minor intrusions and 'Newer Granite' intrusive complexes from the Cheviot Hills north to Shetland. These data yield 16 internally consistent Rb-Sr mineral ages and one each of Ar-Ar, Sm-Nd and K-Ar ages. When considered with precise and reliable published data, it is clear that there are systematic differences in the age of Late Caledonian magmatism through the geological regions of Scotland. In the Grampian Highlands and the Scottish Midland Valley, magmatism was concentrated during the period 423 to 410 Ma. ORS volcanism in the Highlands occurred around 420 Ma, with plutonism continuing to about 408 Ma. Volcanism and plutonism in the Midland Valley took place between 415 and 410 Ma. In the Southern Uplands, magmatism occurred earlier in the north, with all magmatism close to the Southern Uplands Fault dated around 408 Ma and that close to the Iapetus suture dated at around 394 Ma. As no evidence exists for ocean closure prior to c . 400 Ma, the age of igneous activity north of the Southern Uplands Fault is consistent with chemical evidence for a relationship to active NW-directed subduction. The younger magmatism of the S part of the Southern Uplands cannot be subduction-related, and the c. 408 Ma plutons nearer the Southern Uplands Fault can only be subduction-related if tectonic construction of the Southern Uplands was a two-stage process.