Many studies have reported U-Pb dates of zircons that are older than the rocks that contain them, and they are therefore thought to be inherited from older rock complexes. Their presence has profound geodynamic implications and has been used to hypothesize about concealed micro-continents, continental crust beneath ocean islands, and recycling of continental material in the mantle beneath mid-ocean ridges. However, there is skepticism in the scientific community whether these zircons are truly inherited from crustal fragments or represent contaminants from sample preparation and processing. In this study, we combine single zircon U-Pb dates with structural radiation damage determined by Raman spectroscopy from a Pliocene mid-ocean ridge gabbro and show that the thermal history of Precambrian zircons found in the sample contradicts an inherited origin. To confirm that our approach allows distinction between contamination and true inherited zircons, we investigated igneous rocks that formed during the opening of the Atlantic Ocean and find that the calculated radiation damage age of old inherited grains corresponds with the crystallization dates of the young magmatic zircons.

The Danish island Bornholm on the southwestern margin of the Baltic Shield was subject to dyke injection during the Proterozoic. The dykes probably result from several magmatic events. We present U-Pb geochronological data for the largest of the dykes, the tholeiitic Kelseaa dyke. The resulting age, 1326 ±10 (2σ) Ma, places the dyke significantly earlier in the Proterozoic than previously assumed. No other dykes of this age have been reported from the western part of the Baltic Shield. The NE–SW strike of the Kelseaa dyke is evidence for extension oblique to the border of the Baltic Shield.The Kelseaa dyke is the first evidence for this event that was subsequent to the emplacement of theBornholm and Karlshamn (SE Sweden) granites and prior to the intrusion of the Central Scandinavian Dolerite Group, and possibly also the majority of mafic dykes on Bornholm.