The Central-Sudetic ophiolites (SW Poland): Petrogenetic issues, geochronology and palaeotectonic implications

The Central-Sudetic ophiolites (CSO), including the largest and best-studied Ślęża ophiolite, are relatively well-preserved, complete Palaeozoic ophiolitic sequences at the NE margin of the Bohemian Massif, near the eastern edge of the Variscan Belt in Central Europe. Despite a detailed study and a well-documented MORB-type affinity, the geochemical complexity of these ophiolites has only recently been appreciated, allowing construction of petrogenetic models. We review here petrological and geochemical data indicating a contrast between the plutonic and volcanic members of the Ślęża ophiolite. This contrast was previously interpreted in terms of a model of two-stage incremental melting of a depleted asthenospheric source producing basaltic magmas and subsequent melts crystallizing as gabbros. However, we argue that the geochemical differences between the plutonic and volcanic members can be explained by differentiation and cumulation processes of broadly similar parental magmas. Preliminary interpretation of new SHRIMP zircon age data from plutonic and volcanic members are consistent with earlier U–Pb zircon ages and provide evidence for magmatic crystallization of the Ślęża ophiolite at c. 400 Ma. The CSO, interpreted as traces of the Rheic Ocean, are key for reconstructing the early- to mid-Palaeozoic rifting processes. Distinct similarities exist with time-equivalent ophiolitic units in the Western Variscan segment of the NW-Iberian Massif. The CSO represent c. 400 Ma oceanic-crust fragments, and their structural position delineates likely major Variscan tectonic sutures and tectonic-mosaic domains within the Variscan accretionary prism.