Browsing by Author "Nikkola, Paavo"

In situ U–Pb zircon dating was done for the host rocks of the Juomasuo gold deposit, located in the Paleoproterozoic Kuusamo supracrustal belt, eastern Finland. For dating purposes, five samples were chosen to represent the typical volcano-sedimentary rock types of the deposit in their mineralized and unmineralized form. The samples were studied petrologically and dated using the LA–MC–ICP/MS technique at the research laboratory of the Geological Survey of Finland and the Finland Geoscience Laboratory (SGL). The U–Pb data revealed heterogenic detrital zircon populations even for the felsic volcanic rocks, and thus, all the samples were delineated as sedimentary/volcano-sedimentary sequences. The maximum deposition ages fall in the 2.75–2.65 Ga window, and not a single Paleoproterozoic age was measured in the bulk of 159 measurements. The pervasively altered samples from the mineralized sequence of the Juomasuo deposit hosted a younger population of homogenized, BSE-pale/CL-dark zircons and domains within zircon grains, with 2.62–2.60 Ga ages. This age can be linked to the Neoarchean crustal growth event at 2.7–2.6 Ga, described from the Archean terrains all over the world, including the Archean of the Karelia Province in Finland. Unfortunately, in the limits of this thesis, the homogenization age cannot be interpreted as the minimum depositional age of the studied metasedimentary rocks, as it remains unclear, whether the homogenization of the zircons took place before or after the deposition. Although the majority of the precursors of the studied supracrustal rocks from Kuusamo were presumably deposited between 2.43 and 2.21 Ga, there are no corresponding ages of volcanism in the area. The lack of U–Pb ages of that time frame is a global phenomenon, and the topic is thus pertinent to modelling of Paleoproterozoic plate tectonics. It may be that the Kuusamo supracrustal belt represents a deposition during a global slowdown in plate tectonics, thus recording an anomalous period in the history of planet Earth.