Origin of the Jurassic Tethyan Ophiolites in Bosnia: A Geochemical Approach to Tectonic Setting

The ophiolites from Bosnia are products of the Jurassic spreading of the Neo-Tethys, and are fragments of a long chain of ophiolites of the same origin. Their geological characteristics are the absence of a sheeted dyke complex and the tectonic disruption of the cumulate sequence. Upper crust volcanics as well as lower crust cumulates show low metamorphic changes of the prehnite/laumontite type, except for some marginal localities of large ophiolite massifs (Visegrad, Konjuh, Krivaja, Ozren) in which the metamorphic grade reaches the amphibolite facies. The characteristic features of these ophiolites are as follows: The mantle rocks are lherzolites and harzburgites, both characterized by a negative Eu-anomaly and a few of them have a negative Ce anomaly, which is explained as alteration due to contact with sea water. The cumulate rocks are gabbroic to ultramafic. Numerous amphibolites reveal a chemical character of cumulus rocks. Early crystallization of plagioclase in the magma chamber caused positive Eu anomalies in these rocks. The upper crust rocks are basalts, diabases and gabbros. Most of them reveal REE patterns which are typical for basalts from constructive plate margins. A few of them which have a slight enrichment of LREE are explained as the partial melt products of a mantle less depleted than the one which produced the majority of basalts. Chemical discrimination diagrams show that the magmatic rocks are of the mid-oceanic ridge type, i.e. that they formed at the spreading of oceanic plates without any influence from a subducted slab. A comparison with recent spreading zones shows that ophiolites of the described type (absence of a sheeted dyke complex, areas of tectonic disruption in the upper and lower crust, local amphibolitisation and eruptions of “enriched” MORB) probably have formed in areas of the intersection of spreading and a transform fault. In most of their characteristics the ophiolites from Bosnia show a close similarity to the Ligurian ophiolites which obducted in about the same period of time and which are also products of the spreading of Neo-Tethys