In the Late Jurassic to Early Cretaceous ophiolite mélange from the Mt. Medvednica (Vardar Ocean) blocks of boninite rocks have been documented. They emerge as massive lavas made of augite, spinel, albite and secondary hydrous silicates (e.g., chlorite, epidote, prehnite, and pumpellyite). An established crystallization sequence (spinel→clinopyroxene→plagioclase±Fe-Ti oxides) was found to be typical for the boninite series from the suprasubduction zones (SSZ). Augite crystallization temperatures and low pressures of ~1048 to 1260 °C and ~0.24 to 0.77 GPa, respectively, delineated the SSZ mantle wedge as a plausible source of boninite parental lavas. Their whole-rock geochemistry is characterised by low Ti, P2O5, Zr, Y, high-silica, and high Mg# and Cr# values. Low and U-shaped REE profiles are consistent with the negative Nb-Ta, P and Ti anomalies indicative for SSZ. Thorium and LILE enrichment, and very low initial Nd-isotopic values (εNd(T = 150 Ma) + 0.49 to +1.27) act as vestiges of mantle-wedge metasomatism. The mantle source was likely depleted by the MORB and IAT melt extraction and was contemporaneously affected by subduction fluids, prior to the large-scale adiabatic melting of the mantle hanging wall. This eventually gave rise to boninite lavas and an ultra-refractory harzburgite residiuum. The genesis of boninites is related to the Tithonian mature forearc setting that evolved from an intra-oceanic, Callovian to Oxfordian, island-arc environment. The Mt. Medvednica boninite rocks stand for the youngest SSZ-related Jurassic oceanic crust from the NW segment of the Dinaric-Vardar Tethys that are nowadays obducted onto the passive margins of Adria. Taking into account the existence of similar rocks in the ophiolite zones of Serbia, Albania and Greece, the boninites of Mt. Medvednica strongly favours the single Tethyan oceanic basin that existed in this part of Europe during the Late Jurassic.
- Dinaric-Vardar ophiolite zone
- Mount Medvednica
- Ophiolite mélange
- Suprasubduction zone