MORB-type vs. BARB-type ophiolites of the Dinarides: geologic and geochemical data

Abstract

The Dinaridic Alpine ophiolites can be divided into two groups: 1) those of the Jurassic Dinaride Ophiolite Zone (DOZ) and 2) those of the Cretaceous-Early Paleogene (?) Sava-Vardar Zone (SVZ). However, geologically and petrologically they have so far been treated as a whole, and their geotectonic setting has not been reliably established. We have investigated ophiolite terranes from both regions in order to determine their tectonic setting. Our comparison is based on trace element data and characteristic elemental ratios correlated with ophiolite settings in recent oceans. We show that the DOZ tectonite peridotites are enriched in LREE and have high Ti/V ratios (up to 23), whereas the SVZ mantle rocks are LREE-depleted and have low Ti/V ratios (1-4). The DOZ cumulate gabbros and peridotites are characterized by increased Ni content and high Ba/Sr ratio, positive Eu anomalies and lower Cr# of spinels, comparable with MORB cumulates. The SVZ cumulates do not show a positive Eu anomaly and are characterized by higher concentrations of V, Zr and Y, and higher K/Nb, Ti/Zr, Zr/Sm ratios, which correlate with those from some recent back-arc settings. Essential differences between DOZ and SVZ basaltic rocks are as follows: high-Ti vs. low-Ti ophiolites: Ti/V = 20-50 vs. <20; higher Mg#, Ni, Cr, Nb and Ta vs. higher Rb and Ba; elemental ratios of Sr/Zr, Ti/Zr, Y/Nb, Zr/Nb, Th/Nb, Th/Ta, La/Nb, La/Ta and Sm/Hf are similar to those from recent back-arc settings. These geochemical data confirm the earlier geologic model of Pamić et al. (2002). These contrasting geochemical signatures are the result of different geochemical processes taking place in two different settings: a) MORB along an accretionary plate margin underlain by oceanic upper mantle, and b) BARB along a supra-subduction zone characterized by an obducting upper mantle wedge underlain by subducted oceanic crust, which was a source of fluids.