Siegenian generation of the Lizard ophiolite: U-Pb zircon age data for plagiogranite, Porthkerris, Cornwall

The long-controversial age of the Lizard ophiolite is resolved by U-Pb dating of three single magmatic zircons from a plagiogranite dyke near Porthkerris Point. This demonstrates that the axial oceanic crust was undergoing extensional ductile shearing, mylonitization, amphibolite facies metamorphism and, probably, anatexis at 397 ± 2 (2σ) Ma. In conjunction with published K–Ar, 40 Ar– 39 Ar and Rb–Sr data for the complex, this Early Devonian age predicates a protracted interval between intra-oceanic magmatism and deformation, on the one hand, and obduction during Late Devonian, northwestward displacement of the Normannian High, on the other.