Two types of peridotites in the area of Banovina, Croatia and their petrogenesis

Numerous outcrops of ultramafic rocks consisting mostly of peridotites occur in the area of Banovina, in Croatia. These rocks were formed as parts of the former Earth's mantle and belong to the Central Dinaride Ophiolite Belt (CDOB), which is direct proof of the existence and closure of the Neothetys ocean in the northern part of the Balkan area. Previous studies have considered these peridotites as fertile, subcontinental parts of the mantle with complex chemistry. This research presents a more detailed petrographic and chemical analysis, focused solely on peridotites as dominant ophiolitic member, with the intention to sort between different types of Banovina peridotites and offer the model for their petrogenesis.   

Detailed field work, mapping and petrographic analyses have revealed that Banovina peridotites occur as two texturally, lithologicaly and mineralogicaly different types, that crop out in two geographically different belts, the northern (N-belt) and the southern (S-belt). The N-belt contains mostly serpentinite breccias and serpentinized, depleted and mostly porphyroclastic spinel lherzolites that occur in the form of mélange, while S-belt comprises larger masses of peridotites which consist predominantly of fertile spinel lherzolites with equigranular to porphyroclastic textures. Bulk rock analyses have shown that spinel lherzolites from the S-belt have lower Cr# and Mg# and higher content of Al₂O₃, CaO, Na₂O, TiO₂ and REE than spinel lherzolites from N-belt, and same relations, excluding the REE, can be seen in the chemistry of clinopyroxenes and orthopyroxenes. Spinels from the N-belt spinel lherzolites have a significantly higher Cr# (12,7 – 50,7) then those from the S-belt spinel lherzolites (7,7 – 10,8). Two types of dunites, which were found only within S-belt peridotites, have very different petrographic and chemical characteristics. Pyroxene rich dunite is characterized by a coarse-grained protogranular to porphyroclastic texture, high modal pyroxenes (up to 10 vol. %) and spinels enriched in MgO, Al₂O₃ and NiO. The second type of dunite has small-grained equigranular texture, contains amphibole (up to 1 vol. %), pyroxene (< 1 vol. %) and spinels enriched in Cr₂O₃ and FeO_T. Geochemical analysis of all peridotites indicate that the S-belt peridotites represent a subcontinental mantle which have been formed through the initial rifting phase during which they ascended to the upper crust. Peridotites from the S-belt are classified as orogenic peridotites. The geochemical characteristics of N-belt peridotites indicate their origin from a suboceanic mantle formed within mid ocean ridge environment and are classified as ophiolitic peridotites. Dunites show different geochemical characteristics and may have been formed by different geological processes. The diverse lithology of ultramafics in the limited space of the S-belt indicates very heterogeneous nature of the subcontinental mantle. As a part of the CDOBs, peridotites from Banovina indicate that the CDOB record three different phases of ocean evolution, the early phase of the initial rift and opening of the ocean (S-belt peridotites), later phase of the already developed ocean (N-belt peridotites) and also the phase of ocean closure which is evident from the mélange occurrences.