The age of Acadian cleavage in northern England, UK: K–Ar and TEM analysis of a Silurian metabentonite

Abstract

SUMMARY K–Ar and 40 Ar– 39 Ar isotope analysis and transmission electron microscopy were used to study the cleavage developed in a metabentonite during the Acadian tectonometamorphic event in northern England. The Foredale metabentonite, from the Ribblesdale inlier in the Pennines, is one of numerous K-bentonites preserved in the hemipelagic Horton Formation of Lower Ludlow (Gorstian) age. It consists of over 90% phengitic white mica crystallized during deformation and cleavage development under mid-anchizonal metamorphic conditions. Temperatures approaching 250°C combined with strain energy to completely eliminate original smectite interlayers, producing stable crystallites of 1M and 2M mica polytypes. Mica concentrates analysed by the K–Ar method have a weighted mean apparent age of 397 ± 7 Ma; 40 Ar– 39 Ar total fusion analyses have a weighted mean apparent age of 418 ± 3 Ma. The true cleavage-forming event lies within this age range. An inferred geothermal gradient of <25°Ckm −1 , based on the white mica b lattice dimension, suggests that the Foredale metabentonite was buried under at least 10km of overburden, much of it Lower Old Red Sandstone, prior to late Lower Devonian (Emsian) uplift in northern England.