The Wash Igneous Complex of eastern England: A candidate Avalonian source for Sandbian tephra in Baltoscandia and implications for closure of the Tornquist Sea

The BGS have revealed the "Secrets of an ancient supervolcano uncovered that change our understanding of England's geological past"

The origin of the Kinnekulle tephra, a huge volcanic ash layer spread across Norway, Sweden, the Baltic region, Belarus and Poland, has long been a mystery but new evidence may have uncovered the source of the ash for the first time.

Scientists at the British Geological Survey (BGS) and the University of Oslo have studied tiny crystals, smaller than the diameter of a human hair, extracted from boreholes 65 km apart in Lincolnshire and Norfolk. The analysis uncovered evidence of a 454 million year old supervolcano, now concealed beneath The Wash on the east coast of England.

Volcanic rocks of comparable age comprise the rugged topography of Eryri (Snowdonia) and the Lake District. A belt of arc volcanism extended from the Lake District towards Belgium in late Ordovician times, when England was separated from Scandinavia by an oceanic basin. The geography was probably comparable to the West Pacific region (Korea–Japan–Indonesia) today.

From the referenced paper:

The Wash Igneous Complex of eastern England: A candidate Avalonian source for Sandbian tephra in Baltoscandia and implications for closure of the Tornquist Sea

Tim C. Pharaoh; Jeremy Rushton; Dan Condon; Simon Chenery; Matt Horstwood; Diana Sahy; Chris Thomas; Elliott Hamilton; Charles J.B. Gowing; Jonathan Busby; Richard Haslam; Trond H. Torsvik

Author and Article Information

GSA Bulletin (2026)

https://doi.org/10.1130/B38831.1

The Wash Igneous Complex (WIC) is a component of a Sandbian (Late Ordovician) calc-alkaline magmatic arc that extends from the Lake District in northern England to the Brabant Massif of Belgium, associated with fast subduction of Tornquist ocean crust separating Avalonia (upper plate) and Baltica (lower plate). Buried beneath the East Midlands and The Wash of eastern England, the WIC is likely to comprise an extensive nested caldera system, underlain by a granite batholith, inferred from geophysical data. New whole-rock geochemical data confirm the calc-alkaline character of the WIC and elucidate relationships with other calc-alkaline magmatic complexes in eastern England. Detailed petrographic and geophysical data indicate that the WIC had the capacity to generate super-eruptions. Evidence for ultraplinian volcanism comes from contemporary, regionally distributed and thick metabentonite tephra in Baltoscandia. New, high-precision U-Pb isotopic (chemical abrasion−isotope dilution−thermal ionization mass spectrometry; CA-ID-TIMS) ages of 454.45 ± 0.13/0.23/0.54 Ma and 454.40 ± 0.11/0.22/0.53 Ma (2σ, Sandbian, Late Ordovician) are reported for zircons from a heterolithic dacitic tuff-breccia and a subvolcanic microgranite respectively, recovered from two deep boreholes that intersect the WIC. These dates are identical, within their analytical uncertainties, with equivalently high-precision U-Pb zircon dates for the Kinnekulle Metabentonite (KKMB; Baltoscandia). A population of younger grains from the breccia yielding a CA-ID-TIMS age of 453.70 ± 0.17/0.26/0.55 Ma (2σ) may correlate with the Lower Grimstorp Metabentonite. The matching U-Pb dates and closely comparable apatite microphenocryst chemistry from WIC rocks and the Baltoscandian tephra suggest the WIC as a primary candidate for much of the composite KKMB.