The petrology and geochemistry of mantle-derived basic and ultrabasic rocks from the Szklary massif in the Fore-Sudetic Block (SW Poland)

Abstract

This paper presents the results of petrological investigations of rocks from the Szklary serpentinite massif in the eastern margin of the Góry Sowie Block. Among the weakly serpentinised ultrabasites present, harzburgites, lherzolites and pyroxenites have been distinguished. Small enclaves of metabasites within the serpentinites represent low-Ti varieties and - like rodingites - they developed from initial gabbroides or picrites. They reveal a characteristic depletion in incompatible elements. This feature is reminiscent of high-Mg (boninite) basic magmas formed in island arc or back-arc basin environments. Plagiogranites, present within the serpentinites, most probably represent trondhjemites - more acid derivates formed during the fractional crystallisation of the same initial basic magma.
On the basis of the petrological investigation results, the hypothesis was put forward that - with regard to their initial composition - the ultrabasites of the Szklary massif may represent heterogeneous spinel residual peridotites impregnated with pyroxenite veins. Enclaves of metabasites and plagiogranites, having the properties of mafic cumulates and/or fractional differentiates, may reflect a stage of further magmatic processes which took place in a zone of lithospheric plate convergence.