An improved chronology for the Arthur’s Seat volcano and Carboniferous magmatism of the Midland Valley of Scotland

Abstract

Synopsis The Early Carboniferous Arthur’s Seat volcano in central Edinburgh is of geological significance and landscape value but lacks a precise chronology. New 40 Ar/ 39 Ar step-heating plateau analyses date the lowest Lava 1 of the eruptive sequence to 341.2 ± 0.8 Ma (Visean, Holkerian), with Lava 3a at 336.7 ± 4.7 Ma. Intrusive or lava-lake basalts within the Lion’s Haunch Vent are precisely dated at 338 ± 0.6 Ma at Dunsapie and 335.1 ± 0.6 Ma at Crow Hill (Visean, Asbian). Taken together with existing 40 Ar/ 39 Ar dates recalculated to recent decay constant and standard values, and U/Pb dates, a revised magmatic chronology consistent with biostratigraphy is presented for the Midland Valley of Scotland. The Garleton Hills Volcanic Formation is older than the Arthur’s Seat Volcanic Formation, whereas the Clyde Plateau and Arthur’s Seat volcanic formations are partly coeval, requiring minor re-adjustments to existing stratigraphy. Supplementary material: Excel spreadsheets with raw analytical data and recalculation of existing 40 Ar/ 39 Ar ages are available at www.geolsoc.org.uk/SUP18773 .