U–Pb chronology and duration of late Ordovician magmatism in the English Lake District

Abstract

U–Pb zircon ages are presented from igneous rocks in the English Lake District for which a wide range of ages had been obtained previously by other methods. The Little Stand and Crinkle tuffs, from the Borrowdale Volcanic Group, have ages of 451.6 ± 1.4 and 452.8 ± 0.7 Ma (2σ), respectively, the Threlkeld Microgranite 451.0 ± 1.1 Ma (2σ) and the Carrock Microgranite 452.4 ± 4.1 Ma (2σ). These dates are identical to the previously published dates for the Ennerdale and Eskdale granites and together are considered to be a good estimate for both the age and length of Caradoc magmatism in the Lake District. This is the first of three magmatic events recognized in the Lake District, the other two occurring during Ashgill and Early Devonian times. Regional comparisons with the Leinster Terrane in SE Ireland and the largely concealed English Midlands basement emphasize the magnitude and significance of the Caradoc event along the Eastern Avalonia margin.