Columnar-jointed bentonite below a Doleritic Sill, Tideswell Dale, Derbyshire, UK: formation during prograde contact metamorphism

Abstract

<jats:title>Abstract</jats:title><jats:p>Columnar-jointed tuffs (bentonites) are located below the Lower Carboniferous Tideswell Dale Sill, Derbyshire, in an abandoned quarry. There are three zones of prismatic joints, columnar joints (∼60 cm thick and mean column width of 4.1 cm) and massive, conchoidally fractured rock sequentially from the dolerite contact downwards. The rocks are very fine-grained (1–10 µm) and consist mainly of sanidine, interlayered illite/smectite, and hematite, with minor quartz, apatite, montmorillonite, anatase and detrital minerals. High K<jats:sub>2</jats:sub>O and Fe<jats:sub>2</jats:sub>O<jats:sub>3</jats:sub>, low Na<jats:sub>2</jats:sub>O and low MgO are interpreted as due to alteration of felsic volcanic ash. The bentonite was contact-metamorphosed by the sill to temperatures of <jats:italic>c</jats:italic>. 300 °C. The progression of fracture geometries, thermal considerations and application of theories of column formation indicate that columns formed downwards away from the sill, due to prograde contact metamorphism of the originally clay-rich bentonite. The formation of columnar fracture networks by shrinkage due to igneous heating may have implications for the effectiveness of bentonite as a barrier for disposal of high-level radioactive waste. Regional dimensions include documenting widespread felsic explosive volcanism and inferring a Triassic oxidation event from palaeomagnetic data.</jats:p>

Dolomitisation of carbonate platform margins by fault‐controlled geothermal convection: Insights from coupling stratigraphic and reactive transport models

“…The continuation of igneous activity into the Pennsylvanian is manifest by the presence of basaltic and doleritic sills (Mullineux et al, 2020; Stevenson et al, 1970). Most igneous products on the Derbyshire Platform are basaltic and were sourced from tholeiitic magmas held in the lower crust prior to ascent (Macdonald & Walker, 1984).…”