engleski

Does the Sava-Zone represent a remnant of the Vardar ocean and when did it close? - structure, geochemistry and age of the Kozara ophiolites (northern Bosnia-Hercegovina)

The internal Dinarides of Former Yugoslavia are characterised by the widespread occurrence of ophiolites. Having formed part of the Vardar ocean, they were obducted onto the distal passive margin of the Adriatic platform in the Late Jurassic. Later, they were incorporated into the Late Cretaceous to Palaeogene SW-verging nappe stack of the Dinarides, where they occupy a structurally high position today. While there is general consensus about the (Late Jurassic) age of obduction of these ophiolites, recent investigations in the northernmost Dinarides of Bosnia-Hercegovina have revealed the existence of a mafic rock suite of Upper Cretaceous age in a yet more internal (i.e. more northerly) position of the Dinarides’ nappe stack. This mafic rock suite, exposed in the northern part of the Kozara Mountains (termed N Kozara from here on), very likely belongs to the “ Sava Zone” , a Mid-Cretaceous to Early Palaeogene accretionary prism, volcanic arc and back-arc basin that underwent Early Tertiary metamorphism and that remained open until collision between the Dinarides and the northerly adjacent Tisza block in the Mid-Eocene. Whether or not the mafic rocks of N Kozara truly represent ophiolites that could be regarded as a Cretaceous remnant of the Vardar ocean, is yet very improperly understood. Solving these questions, however, will improve timing constraints on the final closure of this domain and the subsequent collision with the Tisza block. It is therefore the aim of this study (a) to comprehend the structural position of the Kozara ophiolites with respect to the “ classic” Dinaric ophiolites, obducted in the Late Jurassic, and the northerly adjacent units within the Sava Zone, (b) to characterise their geodynamic setting using their geochemical signatures and (c) to obtain radiometric ages of the mafic rocks and biostratigraphic ages of associated sediments. Going from S to N, the massif of Kozara reveals the following succession of WNW-ESE-trending units. In the S, serpentinised lherzolithes and gabbros sit on top of a presumably Jurassic mélange formation, containing limestone olistolithes of Triassic age. Dolerites are found to be intersected by albite-rich felsic rocks. A Sm-Nd-age, obtained from a well-preserved olivine cumulate gabbro, yielded an age of 158 ± ; ; 8 Ma. This ophiolitic succession itself is unconformably covered by Palaeocene/Eocene Discocyclinae-bearing shallow-water limestones and subordinately shales that post-date the age of obduction. The mafic rocks of N Kozara are separated from the ones in the S by a 5 to 7 km wide belt of siliciclastic flysch of presumably Late Cretaceous to Palaeogene age. The mafic rocks of N Kozara, overlying the flysch, consist of basalts, dolerites and gabbros. Ultramafics are absent. Occasionally, basalts exhibit pillow-lava textures and are intercalated with red, pelagic limestones containing Globotruncanae that indicate an Early Campanian age (or, alternatively, Late Campanian to Early Maastrichtian. Gabbros and dolerites frequently reveal the growth of secondary, green amphibole on the expense of clinopyroxene, rendering these lithologies unsuitable for Sm-Nd-dating. Moreover, the N Kozara mafics are often intersected by Na- and K-rich felsic dikes. Geochemical analyses of mafic rocks from S and N Kozara show differences in both their major and trace element concentrations. Gabbros from the S show Mg-contents typical of primitive MORB. Basalts, dolerites and gabbros from N Kozara, on the other hand, exhibit significantly lower Mg-contents and enrichment of incompatible elements like Ti and P, suggesting a more differentiated or enriched magma source. Chondrite-normalised REE concentrations of samples from S Kozara (particularly gabbros) reveal a strong depletion of LREE and lesser depletion of HREE. Samples from N Kozara, in contrast, show enrichment of LREE over HREE. These features suggest an N-type MORB origin of the S Kozara samples, whereas N Kozara samples are more characteristic of an island-arc or transitional MORB setting with a low degree of partial melting. There is evidence that the mafic rock suites of S and N Kozara have been juxtaposed tectonically. This is indicated by basaltic pebbles from the separating flysch belt, which clearly bear the geochemical signature of the overlying N Kozara mafics. In agreement with published data we therefore invoke the possibility that the N Kozara mafics could represent a Late Cretaceous to Early Palaeogene (?) remnant of the Vardar ocean, which was juxtaposed against the “ classical” Dinaric ophiolites (obducted in the Late Jurassic) during the final closure of an oceanic domain within the Sava Zone. Future work will include the U-Pb-dating of zircons from highly differentiated gabbros and dolerites of N Kozara, as well as Nd- and Sr-isotope measurements on samples from both S and N Kozara. While the former should yield the first radiometric ages of the N Kozara mafic suite, the latter will allow characterising its mantle reservoir. Moreover, fission track analyses will be carried out in order to constrain the cooling history.

Tisza; Sava Zone; Dinarides; Vardar; ophiolites; geochemistry

nije evidentirano