Geochemistry and tectonic significance of meta-ophiolitic mafic rocks in the high-grade metamorphic basement of the eastern Rhodope Zone, Bulgaria-Greece

In the Alpine orogen of the north Aegean region, the eastern Rhodope Zone consists of widespread high-grade metamorphic basement exposed in Bulgaria and Greece. In this high-grade basement, the lithologically variegated upper unit contains meta-ultramafic bodies, which are considered as dismembered Precambrian meta-ophiolite association (Kozhoukharova 1984). In the same unit, the voluminously predominant amphibolites, having mafic igneous precursors of boninitic-tholeiitic affinity, are in turn considered of Precambrian-Paleozoic island arc origin (Haydoutov et al. 2004), or part of the amphibolites of Ordovician age have back-arc origin (Bonev et al. 2013). The upper unit, together with the overlying Circum-Rhodope belt Jurassic ophiolite, constitutes the hanging wall of the Eocene extensional system consisting of meta-granitoids with Carboniferous protoliths in the footwall. Here, we report on the geochemistry of the amphibolites from the upper unit in Bulgaria and Greece, and discuss their composition and tectonic setting, which might shed a light on the mid-late Paleozoic-early Mesozoic tectonic architecture of the region.

The amphibolites occur intercalated with para- and ortho-metamorphic lithologies within the upper unit. Texturally, they are represented mainly by massive or banded amphibolite and garnet-bearing amphibolite. The bulk mineral assemblage contains amphibole and plagioclase ± quartz ± garnet ± epidote-clinozoisite ± chlorite ± sphene ± rutile, which resulted from the main metamorphic overprint in amphibolite-facies and variable retrogression to greenschist-facies. The meta-mafic rocks cover the range of basalt to andesite composition, with elevated MgO, variable alkali and low-K contents, having mainly tholeiitic to weak calc-alkaline affinity. The range of TiO₂ defines two groups of high-Ti (>1%) and low-Ti (<1%) meta-mafic rocks. Mostly flat to slightly LREE-depleted chondrite-normalized patterns characterize the high-Ti group, which overlaps N-MORB and E-MORB compositions. The low-Ti group exhibits pronounced LREE-depleted and fractionated REE patterns, rarely U-shaped boninitic-like pattern. N-MORB-normalized trace element profiles define high LILE/HFSE ratios, moderate to strong HFSE and HREE depletion of the low-Ti group, and close to N-MORB to slightly enriched HFSE-HREE trend of the high-Ti group. A negative Nb anomaly characterizes part of the low-Ti group, whereas other samples from both groups show no Nb anomalies and have contents higher than N-MORB. On various trace element discrimination diagrams the majority of high-Ti group meta-mafic rocks display clear MORB affinity and few samples plot in the WPB field of oceanic island tholeiites, whereas low-Ti meta-mafic rocks show island arc tholeiite (IAT) affinity or have transitional MORB/IAT signature. 

The compositional diversity of the meta-mafic rocks from the upper unit with MORB, transitional MORB/IAT and IAT affinity, in turn call for the origin of the protoliths in a paired ocean ridge-island arc environment, and thus could hints their supra-subduction zone origin in an island arc/back-arc setting.

 

References

Bonev, N., Ovtcharova-Schaltegger, M., Moritz, R., Marchev, P., Ulianov, A. 2013. Geod Acta 26, 3-4, 207-229.

Haydoutov, I., Kolcheva, K., Daieva, L., Savov, I., Carrigan, Ch.  2004. Ofioliti, 29, 2, 145-157.

Kozhoukharova, E. 1984. Geologica Balc., 14, 4, 9-36.

 

Acknowledgements: The study was supported by the NSF Bulgaria KP-06-N54/5 contract.