Petrography and geochemistry of heterogeneous mantle peridotites from the Banovina Ophiolite Complex, Croatia

Abstract

This study investigated the petrographic and geochemical characteristics of highly serpentinised and heterogenic mantle peridotites exposed in Banovina, central Croatia, which represent the basal ophiolitic unit of the Central Dinaridic Ophiolite Belt (CDOB). Results indicate the existence of two structurally and chemically different sets of peridotites that outcrop along two geographically distinct belts. The northern belt contains mostly serpentinite breccias and serpentinised, depleted and predominantly porphyroclastic lherzolites (N spinel lherzolites and N plagioclase lherzolites) that form part of an ophiolitic mélange. In contrast, the southern belt contains larger masses of peridotites that comprise predominantly fertile lherzolites (S spinel lherzolites) with equigranular to porphyroclastic textures. Bulk rock chemistry reveals that the S spinel lherzolites have lower Cr# and Mg# and higher Al₂O₃, CaO, Na₂O, TiO₂ and REE contents than the N lherzolites. The same relations are also observed when comparing pyroxenes compositions from S and N lherzolites. Trace element signatures of clinopyroxene and orthopyroxene from the two belts are completely different and indicate a different petrogenetic history. Spinels from all N lherzolites have higher Cr# than those from the S lherzolites. Differences between N plagioclase and N spinel lherzolites are most evident from the significantly higher Cr# and TiO₂ in the spinel from plagioclase lherzolites, reflecting subsolidus plagioclase formation. Geochemical and petrographic analysis indicates a subcontinental origin of the S spinel lherzolites and a suboceanic (MOR) origin of the N lherzolites. Both types come with their own petrographic and geochemical heterogeneities that suggest complex and multi-phase petrogenesis. Overall, the results correlate well with studies from other neighboring CDOB localities.