U-Pb analys på zirkon från COSC-2 borrhålet i centrala Jämtland

In Central Sweden, several generations of dolerite dykes and sills can be found intruding into the crystalline Precambrian basement east of the Caledonian front. The extent and continuation of these mafic intrusions beneath the Caledonian nappes remain largely unknown, and direct information about them is limited to a few irregularly distributed basement windows. A notable exception is the second drill core of the Collisional Orogeny in the Scandinavian Caledonides (COSC) scientific drilling project, COSC-2, which discovered a dense network of dolerite sheets in the underlying basement. U-Pb dating of minerals from the dolerite sheets has revealed ages of ca. 1.47 Ga and 1.27-1.26 Ga, but the possible presence of younger intrusions and the proportions of the different dolerite generations remain a subject of investigation. Given this, this project aimed to perform U-Pb zircon dating using Secondary Ion Mass Spectrometry (SIMS) to determine zircon ages from samples of the COSC-2 drill core. Analysis revealed zircon ages of 1268.4±7.0 Ma, with BSE and CL imaging showing only slight oscillatory zoning, typical for zircons of igneous origin. The result of this study is aligned with the initial interpretation of the dolerite ages, as they fall within the well-established Central Scandinavian Dolerite Group (CSDG) age range. The absence of metamorphic indicators further supports the idea that the ages revealed in this study represent the primary magmatic crystallisation age. These findings contribute to the knowledge about different dolerite generations´ location in Central Sweden, although further research is necessary to refine the age estimates of dolerites at other depths in the Precambrian basement.