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Petrologic and geochemical characteristics of the Krivaja-Konjuh ophiolite complex (NE Bosnia and Herzegovina) - petrogenesis and regional

Based on its petrological and geochemical characteristics, the Krivaja-Konjuh Ophiolite Complex (KKOC) and the surrounding ophiolitic mélange make an integral part of the Central Dinaridic Ophiolite Belt (CDOB) of the Internal Dinarides. The Jurassic ultramafic and mafic sequences form about 80 % of the KKOC, whereas the rest belongs to the metamorphic sole, which is found concentrating in the northwestern and southern margins. The KKOC ultramafic rocks, comprising lherzolites, pyroxenites, dunites, along with rocks from the metamorphic sole and chromitites were subjected to an extensive analytical investigation, which included EMPA and SEM studies, and XRF and ICP-MS analyses, in order to reveal their mineralogical, geochemical and petrological characteristics. The lherzolitic modal mineralogy consists of olivine, orthopyroxene, clinopyroxene, spinel and occasionally plagioclase. With respect to clear differences in modal mineralogy, as well as in phase and bulk-rock chemistry, one is able to distinguish two main varieties among the KKOC lherzolites. The first one renders spinel lherzolites, whilst the second variety is known as plagioclase lherzolites. Geothermometric estimations yielded the main equilibration range, for both spinel and plagioclase lherzolites, to range from 809 to 1012 °C (T1). The Fe-Mg exchange between olivine and spinel provided a temperature range of 550-682°C (T2), which is found to be indicative for subsolidus reequilibration processes. Using the oceanic and rift-ridge geotherms, the equilibration pressures extending from 1.2 to 2.0 GPa (ca. 40-65 km depth) were inferred for T1 temperatures. Numerous geochemical parametres are pointing to the KKOC lherzolites as fertile solid residues, which, before metamorphism, underwent low to moderate degrees of MOR melting. The mineral chemistry of spinel and the REE normalisation levels (relative to chondrite) yielded an average batch melting degree of ~7.7 % for spinel lherzolites. Part of the eastern segment of the KKOC is reported to deviate from the MOR fertile geochemistry. The occurrence of pargasite, coupled with a Cr-enriched spinel, call for enhanced mantle melting (~ 15 %) in a SSZ-type of setting. Contrary to the amphibole formation in a SSZ mantle wedge, geochemical evidences clarify the growth of plagioclase through sub-solidus equilibration (T2) and melt metasomatism of the lithospheric mantle at a MOR-type setting. It can be concluded that the main part of the KKOC lherzolites presents a segment of mantle column, which experienced relatively fast and continuous adiabatic ascent under the spreading MOR environment. Within the KKOC metamorphic unit, a variety of paragenesis, textures and structures was recognised. Based on the general classification criteria set for metamorphic rocks, the following varieties among KKOC metamorphic rocks were distinguished: (1) granoblastic amphibolite, (2) garnet-diopside amphibolite, (3)garnet-diopside-hypersthene amphibolite, (4) diopside gneiss and (5) plagioclase garnet-diopside gneiss. Major and trace element geochemistry, as well as petrographic data show that 2/3 of metamorphic rocks bear geochemical signatures of mafic to ultramafic gabbroic rocks, whereas the rest corresponds to more evolved basalts of tholeiitic affinity. Geothermobarometric calculations, based on the phase chemistry of different coexisting minerals, yielded ranges of temperature and pressure for the different KKOC petrographic varieties, as follows: (a) ~ 880 °C and 0.90-1.30 GPa for granoblastic amphibolites, (b) 802-1028 °C and 0.94-1.40 GPa for garnet-diopside amphibolites, (c) for garnet-diopside-hypersthene amphibolites the metamorphic conditions are analogue to the previous variety, recording slightly reduced pressure values, (d) 0.84-0.89 GPa for diopside amphibolites, with no temperature data, and (e) 729-890 °C and 1.10-1.60 GPa for splagioclase-garnet-diopside gneisses. Due to the striking similarities of CDOB with ophiolitic complexes in Albania and Greece, the geodynamic evolution of KKOC is bound to the Mesozoic evolution of the Pindos Ocean, a Tethyan back-arc type of ocean. In order to reconcile the features of two contrasting geotectonic settings (MOR and SSZ) displayed by almost all KKOC lithological units, one suggests that a subducted slab must have been placed under the Krivaja-Konjuh mantle domain. The metamorphic sole is also related to such a defined intra-oceanic subduction and underplating of the cold oceanic crust under the hot mantle wedge. During the late Jurassic to early Cretaceous transition period, the subduction trench collided with the Adriatic microplate, and the KKOC among other CDOB ultramafic complexes was thrust southwestwards on the Adriatic carbonate platform. Throughout the emplacement, the eastern KKOC mantle portion was most probably tectonically exhumed along the ancient subduction channels, causing an erosion of the above placed plagioclase-containing lithospheric mantle.

Dinarides; Krivaja-Konjuh; ophiolite; lherzolite; metamorphites; geochemistry; intraoceanic subduction; MORB; petrogenesis

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