The Age of the Hoy Lavas, Orkney

Abstract
Synopsis: The Hoy Lavas are alkaline olivine basalts, occurring at the base of presumed Upper Old Red Sandstone, and resting unconformably on Middle Old Red Sandstone strata. Mean conventional K-Ar ages of the lavas are discordant and range from 300 to 333 Ma. A cross-cutting monchiquite dyke yields a conventional K-Ar age of 235 Ma. 40 Ar– 39 Ar stepwise degassing analyses of two samples indicate an age of approximately 370 Ma for the lavas. A revision of the age of the Givetian/Frasnian boundary is suggested.

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