SIEGENIAN GENERATION OF THE LIZARD OPHIOLITE - U-PB ZIRCON AGE DATA FOR PLAGIOGRANITE, PORTHKERRIS, CORNWALL

TTe long-controversial age of the Lizard ophiolite is resolved by U-Pb dating of three single magmatic zircons from a plagiogranite dyke nea r Porthkerris Point. This demonstrates that the axial oceanic crust wa s undergoing extensional ductile shearing, mylonitization, amphibolite -facies metamorphism and, probably, anatexis at 397 +/- 2 (2 sigma) Ma . In conjunction with published K-Ar, Ar-40-Ar-39 and Rb-Sr data for t he complex, this Early Devonian age predicates a protracted interval b etween intra-oceanic magmatism and deformation, on the one hand, and o bduction during Late Devonian, northwestward displacement of the Norma nnian High, on the other.