Palaeomagnetic and geochemical evaluation of a terrane-linking ignimbrite: evidence for the relative position of the Grampian and Midland Valley terranes in late Silurian time

Abstract

AbstractThe proposed correlation of Lower Old Red Sandstone dacitic ignimbrites between the Northern Midland Valley and Grampian terranes of central Scotland has been investigated in a combined palaeomagnetic and geochemical study. Statistically indistinguishable late Silurian palaeolatitudes (Grampian terrane 27.0° S±3.5°; Northern Midland Valley terrane 22.9° S± 1.8°), comparable remagnetization histories, a shared ‘normal’ polarity for the primary magnetization and equivalent gyroremanence acquisition, favour correlation across the Highland Boundary Fault. Similar chemistry for biotite phenocrysts further corroborates correlation which was initially made on the basis of a similar field appearance and petrography.This correlation, together with evidence from provenance studies, suggests that the separation of the Northern Midland Valley and Grampian terranes inferred for the Ordovician period was largely removed by late Silurian times, with scope for relative displacements of the order of only tens of kilometres after Lower Old Red Sandstone deposition.