Geochemical constraints on the origin of the mid-Palaeozoic diabases from the Holy Cross Mts. and Upper Silesia, southeastern Poland

A geochemical study of Palaeozoic diabase dykes and sills from the Holy Cross Mts. and Upper Silesian Block (southeastern Poland) has revealed that these diabases are most probably derived from fractional crystallization of three distinct primarymelts. All diabases are relatively evolved subalkaline tholeiites with low mg-numbers that vary between 50.7 and 60.5. Their mantle normalised incompatible element patterns are intermediate between that of enriched mid-ocean ridge basalt (E-MORB) and ocean island basalt (OIB). The compositional differences within the diabases are interpreted in terms of slightly variable degrees of partial melting of their mantle sources combined with variable degrees of subsequent gabbro fractionation in high-level magma chambers. Some incompatible element ratios (e.g. Zr/Nb, Y/Nb) in the diabases and Nd isotope data from the cogenetic diorites seem to be consistent with mixing of partial melts from both enriched lithospheric and depleted asthenospheric mantle sources. Both Upper Silesian and Holy Cross diabases show strong chemical similarity to the continental flood basalts (CFB), which are associated with extensional tectonics. The Bardo diabase, located in the northern part of the Małopolska Block, can be hypothetically linked to the detachment of this unit from the Baltica margin and subsequent displacement to its final position between the late Ludlovian and Emsian, whereas the Milejowice-Janowice diabase (Łysogóry Unit) possibly relates to the extension of the Baltica passive margin during the Late Silurian, in the final stage of its collision with East Avalonia.