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Ophiolitic detritus in three Cretaceous clastic formations of the Croatian Dinarides: evidence from Cr-spinel chemistry.

The Dinarides represent a major part of the Alpine orogenic belt in SE Europe. Among the more prominent features of the Inner Dinarides are ophiolites, pointing to the existence of ancient oceanic realms, the evolution of which is still a matter of debate. The study area in NW Croatia is a very complex region where the junction of the Dinarides, Tisia and the Alps is obscured by intense tectonic movements and the sedimentary cover of the Pannonian Basin. Thus, evidence of the early Alpine evolution of the Dinarides is hidden in patchy and isolated outcrops of Cretaceous syntectonic clastic units. The chemistry of detrital Cr-spinels from three Cretaceous units was studied in order to better describe the character and geotectonic setting of the ophiolites being obducted and eroded in that time. The Oštrc Fm. is a Lower Cretaceous turbiditic succession whose heavy mineral spectra show a very high proportion of Cr-spinel (>90%) proposed to have derived from propagating ophiolitic nappes of the Inner Dinarides (Zupanič et al., 1981, Babić et al., 2002). The Lower Cretaceous Bistra Fm. includes shallow-marine sediments (Crnjaković, 1989) thought to be deposited on top of a previously exhumed Jurassic ophiolitic melange, while the Albian-Cenomanian Kravljak Fm. is part of the NW, inner margin of the Outer Dinarides. The sandstones of these two units contain dominantly ZTR, along with chrome spinel (2-34% and 6-24%, respectively) and minor proportions of other minerals (Crnjaković, 1989, Crnjaković et al., 2000). Our new Cr-spinel chemistry data shows, in all three formations, a dominance of peridotitic grains (<0.2% TiO2) whose Cr# and Mg# values predominantly fall within the range of ophiolites with an island arc affinity (Dick & Bullen, 1984). Only the shallow marine Bistra Fm. contains a somewhat higher percentage of grains indicating a mid-ocean ridge setting, which possibly reflects a more localized input. The proportion of volcanic-derived Cr-spinel grains (>0.2% TiO2) is significant in all studied formations, their chemical composition pointing mainly to volcanic sources initially formed in an island arc setting (Arai, 1992). These results imply that peridotites and volcanic rocks of mainly island arc origin were exposed adjacent to the basins receiving clastic input, which reflects the shortening and accretion processes occurring between the Adriatic plate and the Dinarides between the Barremian and Cenomanian.

Cretaceous; Cr-spinel; Dinarides; Croatia

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