Petrogenesis of metabasite olistoliths from the Magura Nappe in the Polish Western Outer Carpathians

Abstract

We provide new geochemical and geochronological data on metabasite blocks exposed in the Osielczyk Stream within the Polish Western Outer Carpathians. These blocks, the only known occurrence of this rock type in the Carpathian flysch, form part of an olistostrome within the flysch deposits of the Pasierbiec Sandstone Formation of the Magura Nappe. The metabasites are dark green, medium-grained, deformed rocks containing porphyroclasts of albite and Mg-hornblende, with secondary minerals (phengite and chlorite) defining the foliation. Bulk geochemical analyses reveal moderate contents of Fe₂O₃, MgO, TiO₂, P₂O₅ and transition elements (Cr, Ni, Co and V), alongside low concentrations of high field strength elements (HFSEs: Zr, Hf, Th, U), large ion lithophile elements (LILEs: Sr, Ba, K, Rb), and total rare earth elements (ΣREE: 95.5–92.3 ppm). Chondrite-normalized ratios include (La/Sm)ₙ = 2.4–2.3 and (Tb/Yb)ₙ = 1.3–1.2, with a slightly negative Eu anomaly (Eu/Eu* = 0.83–0.91). The chemical composition corresponds to basaltic andesite or andesite magma, exhibiting intermediate characteristics between arc tholeiite and calc-alkaline series. These metabasites resemble high-alumina basalts (maximum Al₂O₃ content 18.6 wt.%) and display geochemical signatures akin to volcanic arc basalts, suggesting an origin within a supra-subduction zone ophiolite sequence. Laser ablation ICP-MS U-Pb zircon dating yields a Neoproterozoic protolith age of 611.7 ± 4.7 Ma, while K-Ar dating of phengite indicates a late Variscan episode (348.1–311.4 Ma), likely reflecting post-metamorphic cooling. The olistostrome material is interpreted as derived from the Fore-Magura Ridge.