The ongoing search for the oldest rock on the Danish island of Bornholm: new U-Pb zircon ages for a quartzrich xenolith and country rock from the Svaneke Granite

Abstract

Previous geochronological studies on the Danish island of Bornholm have not identified any rocks older than c. 1.46 Ga. New LA-ICP-MS U-Pb zircon ages are presented for a xenolith within, and the country rock gneiss adjacent to, the Svaneke Granite on Bornholm. The xenolith is fine-grained and quartz-rich and was likely derived from either a quartz-rich sedimentary protolith or a hydrothermally altered felsic volcanic rock. The relatively fine-grained felsic nature of the country rock gneiss and the presence of large zoned feldspars that may represent phenocrysts suggest its protolith may have been a felsic volcanic or shallow intrusive rock. A skarn-like inclusion from a nearby locality likely represents an originally carbonate sediment and is consistent with supracrustal rocks being present at least locally. Zircon data from the xenolith define an upper intercept age of 1483 ± 12 Ma (2σ, MSWD = 2.5) with a poorly defined lower intercept age of 474 ± 250 Ma, and a weighted average 207Pb/206Pb age of 1477.9 ± 4.6 Ma; both these ages are older than the host Svaneke Granite (weighted average 207Pb/206Pb age of 1465.0 ± 4.8 Ma). Zircons from the gneiss define an upper intercept age of 1477.7 ± 6.8 Ma when anchored at 0 Ma, and a weighted average 207Pb/206Pb age of 1475.4 ± 6.6 Ma which overlaps statistically with the Svaneke Granite age. These ages are currently the oldest ages determined for in situ rocks on Bornholm. Evidence for substantially older basement lithologies (e.g. 1.8 Ga as observed in southern Sweden) remains absent. The zircons display clear oscillatory zoning, have Th/U typical of magmatic zircons and in some cases preserve inherited cores, all of which suggest that the ages are robust and do not represent resetting due to incorporation within or intrusion by the Svaneke Granite. Inherited zircons are not common; they have ages (c. 1.6–1.8 Ga) that are similar to those observed in other felsic basement lithologies on Bornholm. These new results suggest that prior to intrusion of the Svaneke Granite, the upper crust on Bornholm was dominated, at least locally, by lithologies similar in composition to the currently exposed felsic basement. The protoliths to the two samples investigated here must have been buried to mid-crustal depths over a relatively short time period (c. 10 Ma) prior to intrusion of the Svaneke Granite. This suggests a dynamic tectonic environment and is consistent with evidence for broadly simultaneous magmatism and deformation in basement rocks at 1.46 Ga in southern Scandinavia and burial and metamorphism of sediments in southern Skåne.