Ophiolitic detritus in Cretaceous clastic formations of the Dinarides (NW Croatia): evidence from Cr-spinel chemistry

Cr-spinel is a common heavy mineral in the sandstones of Cretaceous synorogenic sedimentary formations of the NW Dinarides, Croatia. The rocks occur in isolated exposures in the uplifted basement units of Medvednica, Ivanica, A1/2umberak and Samobor Mountains near Zagreb. In this area, evidence of the early Alpine evolution of the Dinarides is obscured due to strong dismemberment of pre-Tertiary tectonostratigraphic units resulting from an intense tectonic history, as well as due to the widespread sedimentary cover of the Pannonian Basin. Electron microprobe analyses of detrital Cr-spinels from the Otrc Formation reveal that in the Early Cretaceous the ophiolitic source area was predominantly composed of harzburgite peridotites and associated cumulate rocks, which developed in a supra-subduction zone setting. The supply of Cr-spinels with the same chemical signature remained dominant until the end of the Cretaceous, suggesting that exposed remnants of the same ophiolite belt persisted through the Cretaceous and/or that recycling was significant. Similarities with data reported from the Northern Calcareous Alps and the Transdanubian Central Range imply that a rather extensive harzburgitic ophiolite belt probably extended along the Adriatic margin during the Early Cretaceous. A slight trend of increasing variation in the Cr# is observed from the Early to the latest Cretaceous, suggesting that the source areas became more heterogeneous with the ongoing Cretaceous tectonic evolution. Differences in Cr-spinel compositions in two contemporaneous latest Cretaceous formations are well in line with existing data on heavy mineral proportions, which together identify contrasting hinterland geology for these formations and strongly suggest the coeval existence of two separate basins.