Why Baltic Shield zircons yield late Paleozoic, lower-intercept ages on U-Pb concordia

摘要

In the Baltic Shield, the Paleozoic is known as a period of regional Pb mobilization that resulted in new lead deposits and the addition of radiogenic Pb to older deposits. In addition, U-Pb data from zircons in the Baltic Shield commonly yield late Paleozoic lower intercepts on the U-Pb concordia regarded as "geologically insignificant." Results from fission-track studies reveal that the rocks that form the present bedrock surface in Sweden and Finland were heated at the same time. This fact is explained by upper Paleozoic sedimentary deposits, much thicker than hitherto known, that covered the basement for at least 200 m.y. These erosion products of the Caledonides formed terrestrial or shallow-water ("Old Red") deposits in a foreland basin that covered Sweden and, to a lesser extent, Finland. Hydrothermal solutions leached metamict zircons in the basement rocks for a long time. resulting in Pb loss and Paleozoic lower-intercept ages on U-Pb concordia. The radiogenic Pb was redistributed and preferentially concentrated into fractures and veins or was added to older sulfide deposits.