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

The Early Carboniferous Arthur’s Seat volcano in central Edinburgh is of geological significance and landscape value but lacks a precise chronology. New ⁴⁰Ar/³⁹Ar 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 ⁴⁰Ar/³⁹Ar 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.

位于爱丁堡中部的早石炭世亚瑟王座火山兼具地质研究意义与景观价值，但目前尚未建立精确的年代学序列。本研究通过全新的⁴⁰Ar/³⁹Ar步进加热坪年龄分析，确定该火山喷发序列底部的熔岩1定年结果为341.2 ± 0.8 Ma（维宪阶，霍克利安期），熔岩3a的定年结果为336.7 ± 4.7 Ma。位于狮臀火山口的侵入岩或熔岩湖玄武岩，经精确定年显示：邓萨皮测点的样品定年结果为338 ± 0.6 Ma，克劳希尔测点的样品定年结果为335.1 ± 0.6 Ma（对应维宪阶阿斯比安期）。结合经重新校准至最新衰变常数与标准值的已有⁴⁰Ar/³⁹Ar定年数据，以及铀-铅（U/Pb）定年结果，本研究为苏格兰中部谷地构建了一套与生物地层学相契合的修正版岩浆年代序列。研究表明，加尔顿丘陵火山岩组的形成时代早于亚瑟王座火山岩组；而克莱德高原火山岩组与亚瑟王座火山岩组存在部分同期的地层分布，这要求对现有地层格架进行小幅修正。