Chemostratigraphy, K-Ar ages and illitization of Silurian K–bentonites from the Central Belt of the Southern Uplands–Down–Longford terrane, British Isles

Abstract

The Central Belt of the Southern Uplands Terrane, in both Scotland and Ireland, is a faulted and imbricated sequence of Caradoc to Llandovery shales and Llandovery turbidites with numerous interbedded K-bentonites. The bentonites are composed dominantly of R3-ordered mixedlayer illite/smectite (I/S) containing 90–95% illite, and represent the product of illitization during low-grade metamorphism. K-Ar age determination on the <0.5 μ m size fraction gave a range from 379 ± 10 to 406 ± 10 Ma. The fixed K is thought to have originated in the precursor ash, and was remobilized during the transformation of smectite to I/S. K-Ar ages record a retrograde thermal event that post-dates Wenlock prehnite—pumpellyite facies metamorphism and is contemporaneous with cooling and uplift during the end-Silurian—early Devonian collision of Laurentia with the East Avalonian terrane. Differences in Rb and other trace elements between the K–bentonite beds are due, in part, to differences in original ash composition, and can be used to group the beds within biostratigraphically-defined boundaries. The chemical identification of groups of K-bentonite beds offers additional criteria for their stratigraphical correlation on a regional scale.