Geochronology and geochemical evolution of Carboniferous volcanism in Scotland’s Midland Valley: Insights from Arthur’s Seat

This project set out to temporally constrain Carboniferous (ca. 359-299 Ma) volcanic, magmatic, and interbedded sedimentary basins which host the important Midland Valley coal seams. We are interrogating rates of eruption and temporal variations in the magma source geochemistry to link these to palaeoclimate variability as recorded by these classic sedimentary successions.

Rifting of the Midland Valley, Scotland and synchronous short-lived volcanism (ca. 30 Myr) [7] is associated with the eruption of mildly alkaline olivine basalts and associated intrusive rocks (e.g. micro-gabbro, phonolite). Remnants of this volcanism are exposed across the Midland Valley, including the world-famous geological site, Arthur’s Seat, Edinburgh. Arthur’s Seat is a classic example of a volcanic cross section with approximately nineteen lava layers and numerous vents exposed and characterised [1, 6]. Previous mapping efforts established a stratigraphy for this Site of Special Scientific Interest (SSSI) landmark, constrained by interbedded palynological zones. This has since been succeeded by direct radio-isotopic dating including K-Ar and ⁴⁰Ar/³⁹Ar geochronological techniques [2, 3, 4]. The most recent published ages constrained the eruption and emplacement of the Arthur’s Seat volcano with a precision ranging between 0.18-1.40% [5, 6].

New high precision (<0.1%) ⁴⁰Ar/³⁹Ar geochronology on lavas and CA-ID-TIMS U-Pb zircon geochronology on intrusive rocks and explosive eruptions, has been completed in addition to a suite of geochemical and isotopic data (e.g. Nd, Sr and Pb isotopes; major and trace elements). New developments in sample preparation, mass spectrometry, standard mineral characterisation, and data reduction software have enabled significant refinement of the ⁴⁰Ar/³⁹Ar technique. For example, our re-examination of the Arthur’s Seat Volcanic Formation has obtained an improved precision, achieving eruption and emplacement ages ranging 0.018-0.052%.  

Our integrated geochronologic, geochemical and isotopic data underpin a robust temporal framework that tracks the evolution of the magmatic system, places the volcano-sedimentary sequences within the global Geologic Time Scale, and constrains the ages of key fossil-bearing horizons. By directly linking the magmatism to sedimentary basin development through dating of intercalated tephras, we aim to deliver a high precision chronology for Carboniferous climate change and the evolution of flora and fauna within the Midland Valley. Our preliminary results represent an order of magnitude improvement in precision and, in some cases, are sufficient to allow Carboniferous climate variability to be examined at orbital timescales (<100 kyr).

 

References

[1] Clarkson, E. and Upton, B. (2007) Geological Magazine, 144(3), pp. 603–603.

[2] De Souza, H.A,. (1979) Geochronology of Scottish Carboniferous volcanism. Ph.D. University of Edinburgh.

[3] De Souza, H.A,. (1982) ‘Age data from Scotland and the Carboniferous Time Scale’.

[4] Fitch et al, (1970) Isotopic ages of British Carboniferous rock. Sheffield, 1967, (2), pp. 771–790.

[5] Monaghan, A.A. and Browne, M.A.E. (2010) British Geological Survey, pp. 41.

[6] Monaghan, A.A., Browne, M.A.E. and Barfod, D.N. (2014) Scottish Journal of Geology, 50(2), pp. 165–172.

[7] Upton et al, (2020) Scottish Journal of Geology, 56(1), pp. 63–79.