Rb-Sr Whole-Rock chronology of Caledonian events in Northeast Scotland

Rb-Sr whole-rock isochrons are presented for four late Caledonian granites from northeast Scotland. These isochrons are believed to define a single 460-m.y. B.P. magmatic event, which caused isotopic homogenization of strontium on a whole-rock scale in slate of the Macduff Group, which is part of the Upper Dalradian Series intruded by the granites. The geochronology of the region supports the suggestion that all Caledonian deformation and metamorphism took place during a relatively short episode between the Early Ordovician sedimentation of the Upper Dalradian Series and the Middle to Late Ordovician granitic magmatism. A premetamorphic granite at Portsoy, which intrudes strata of the Middle Dalradian Series, yields a Precambrian isochron age of 669 ± 17 m.y. This is probably a late or postorogenic granite related to a pre-Caledonian metamorphism already recognized within the Moine Series. This Precambrian age cannot be used to support a recent hypothesis in which the Older basic igneous rocks of northeast Scotland are related to a Cambrian(?) subduction zone. All the granitic rocks of northeast Scotland have initial Sr87/Sr86 ratios of 0.714 to 0.717, irrespective of age. This indicates that the granites are derived from a deep, underlying crustal layer, possibly Lewisian (Precambrian) gneiss, and not from the mantle or subducted oceanic lithosphere. © 1974 Geological Society of America.