The 87Rb-86Sr isotope geochemistry of the blueschist and greenschist metavolcanics of the Rýchory Mts crystalline complex, West Sudetes, Bohemian Massif.

Abstract

The blueschists and greenschists of the Radčice Group (the Rychory Mts complex, West Sudetes) were analysed for Rb-Sr isotopes. The 87Sr/86Sr values of the metabasites, recalculated to Early Cambrian-Early Ordovician ages presumed for their magmatic origin, are mostly scattered between values of mantle-derived igneous rocks and Early Paleozoic oceanic waters. Either an introduction of seawater Sr into protolith or secondary alteration of Rb-Sr system during regional metamorphism can be a reason of this feature. Two extremely high Sr isotope values possibly indicate an input of radiogenic Sr leached from rocks of continental crust by post-magmatic hydrothermal alterations and/or metamorphic fluids. Due to the scatter of the measured values in the Rb-Sr diagram it is impossible to derive an isochron and initial Sr isotope ratio valid for the whole sample set of the metabasites. However, two blueschists, only weakly altered, are giving value 0.7039 as initial 87Sr/86Sr ratio. Thus a mantle-derived origin of the primary rocks can be presumed. The „isochron” denned by these samples in the Rb-Sr plot may be an indication of the Late Proterozoic to Early Ordovician magmatic age of the metabasite protolith. The Early Ordovician age may be accepted provided that the Radčice Group metabasite protolith took origin during hypothetic early-stage seafloor spreading, following on the intracontinental rifting dated to the same period by bimodal volcanism of the East Krkonoše Mts. This event would be set into the concept of Early Paleozoic rifting and opening of oceanic basins along the northern periphery of Gondwana.