Cadomian and post-cadomian tectonics west of the Rhodope Massif – The Frolosh greenstone belt and the Ograzhdenian metamorphic supercomplex

The Frolosh Greenstone Belt (FGB) is traced at a distance of more than 200 km in the territories of Bulgaria, Macedonia and Serbia. It consists of various greenschist-facies rocks (actinolite schists, phyllites, calcareous schists, impure marbles, metasandstones, metadiabases, massive green rocks, etc.) of the Frolosh metamorphic complex with bodies of metabasites (including lherzolites), and inliers (retrogressed mica gneisses and migmatites) from the Ograzhdenian supercomplex. The complex is in­truded by bodies of gabbro (occasionally with ultramafic cumulates), diorites to granites (Struma diorite formation). U-Pb studies on zircons yielded Cadomian ages within the time span between c. 574 and 517 Ma. The Frolosh complex covers the ultrametamorphic (migmatized gneisses and amphibolites; tourma­line-biotite schists; quartzo-feldspathic gneisses; lensoid bodies of metaperidotites to norites) of the Ograzhdenian supercomplex. The Ograzhdenian rocks are intersected by diatectic metagranites over­printed by amphibolite-facies metamorphism. Dominant U-Pb ages vary between 470 and 430 Ma. The contact between the Frolosh complex and the Ograzhdenian supercomplex has been subject of long dis­cussion and controversial interpretations. Now we emphasize on the multistage developments of both complexes as demonstrated both by field evidence and isotopic dating. The Ograzhdenian supercomplex has been subject of Precambrian tectonometamorphism witnessed by Rb-Sr whole-rock isochron data and relict U-Pb zircon data. Ordovician to Silurian anatectites (metatectic migmatization, diatexis) are in­truded by Permo-Triassic granites. The contact between the Ograzhdenian supercomplex and the covering Frolosh complex is regarded as a thick complex zone of multistage tectonometamorphic development rather than a “razor-blade” surface of one-stage origin. As a boundary between suprastructure and infra­structure, it played an important role throughout the Phanerozoic, and acted as a screen with a steep ther­mal gradient during the Ordovician-Silurian anatexis and metamorphism in the Ograzhdenian supercom­plex. For to verify this hypothesis, new detailed structural and isotopic studies are needed

Influence of the surface deformability and variable viscosity on buoyant -thermocapillary instability in a liquid layer

The primary stationary and oscillatory Bénard-Marangoni instability is investigated in a fluid layer of infinite horizontal extent, bounded below by a rigid plane and above by a deformable upper surface, subjected to a vertical temperature gradient. Since the viscosity is temperature-dependent the consequences of relaxing Oberbeck-Boussinesq approximation and free surface deformability are theoretically examined by means of small disturbance analysis. The problem has been solved numerically by the Taylor series expansion method. The results obtained confirm that when the free surface is undeformable, stationary convection develops in the form of polygonal cells, and oscillatory motion cannot be detected. When the surface deformability is considered, stationary convection sets in, either as a short-wavelength hexagonal instability or as a long-wavelengh mode or as both, and oscillatory convection is also possible. The stability threshold for the short-wavelength mode depends mainly on the viscosity variation while the long-wavelength mode is determined by the surface deformation. Numerically, it is found that the neutral oscillatory Marangoni numbers are only negative. When a variable-viscosity model is used the theoretical and experimental results are in better agreement

Genesis and tectonic implications of podiform chromitites in the metamorphosed ultramafic massif of Dobromirtsi (Bulgaria)

Podiform chromitites occur in the meta-dunite horizon of the Dobromirtsi ultramafic massif, in the Central Rhodope metamorphic core complex (southern Bulgaria). Although these were deformed and metamorphosed together with their host rocks several chromite cores from massive chromitite samples still preserve compositions unaffected by metamorphism - at least in terms of major elements. In situ laser ablation ICP-MS analyses of these chromite cores show a bimodality in the composition of chromite between high-Cr and high-Al {Cr# = [Cr/(Cr + Al)], atomic ratio = 0.74–0.55} with Mg# between 0.69 and 0.60 [Mg# = Mg/(Mg + Fe²⁺), atomic ratio]. These compositions of chromite and the whole rock enrichment of Os−Ir−Ru relative to Pt−Pd in the chromitites resemble that reported for podiform chromitites in ophiolites elsewhere. Magmatic platinum-group minerals (PGM), including laurite (RuS₂), Os−Ir alloys and sulfarsenides (irarsite, IrAsS), exhibit a wide range of ¹⁸⁷Os/¹⁸⁸Os (from 0.1097 to 0.1272) whereas ¹⁸⁷Re/¹⁸⁸Os is nearly zero. The distribution of the Os model ages calculated for these PGM ranges between 0.13 and 2.6 Ga, with age peaks at 0.25, 0.4, 0.7, 2.1 and 2.6 Ga. Two xenocrystic zircons were identified in the chromitites and yield concordant U−Pb age of 2257 ± 80 Ma (2σ) and 1952 ± 82 Ma (2σ). The oldest zircon exhibits a depleted-mantle Hf model age (TDM) of 2.77 Ga and a "crustal" Hf model age (TCDM) of 3.1 Ga, whereas the youngest has a TDM of 2.21 Ga and a TCDM of 2.35 Ga. We interpret the coexistence of both types of chromitites in a horizon of meta-dunites as the result of their precipitation from fractionating island arc tholeiite melts. The first reactions produced dunite; later the batches of magma were isolated from reaction with the peridotite wall-rocks and progressively fractionated in a network of dunite channels developed in the upper mantle beneath the back-arc spreading centre. The Os and Hf model ages suggest that the ultramafic protolith of the Dobromirtsi ophiolite was derived from an original Archean mantle at least 3 Ga old, which underlay a continental crust and was reworked in the Paleo-proterozoic and possibly even younger times.20 page(s

Metamorphic evolution of sulphide-rich chromitites from the Chernichevo ultramafic massif, SE Bulgaria

The upper mantle rocks of the metamorphosed ophiolite of Chernichevo, Rhodope Metamorphic Complex in southern Bulgaria, host small chromite ores with unusual mineralization of base-metal sulphides rich in platinum-group elements. Mineralogical and chemical data indicate that after their formation in the mantle the Chernichevo''s chromite ores were modified by the intrusion of an alkaline mafic melt, which resulted in the precipitation of a suite of metasomatic minerals (sulphides, calcite, apatite and ilmenite), accompanied by an increase in FeO, TiO2, Ga, Zn, Vn, Mn, and especially Ti and Fe2O3 contents in the chromite. The degree of chemical modification and abundance of metasomatic minerals are positively correlated and mark the extent of reaction of the chromitite with the intruding melt. Sulphide segregation promoted the concentration of high amounts of PGEs (up to 3661 ppb), particularly Pt and Pd, yielding chromite ores with a typical flat to positive-sloped chondrite-normalized pattern. Subsequently, the chromite ores were deformed and metamorphosed together with their host rocks at ultra-high pressure (UHP) (>2.5 GPa, >1200 °C) to be later retrograded under eclogite and finally hydrous amphibolite-facies conditions, giving rise to three microstructural types. Metamorphism of the most metasomatized (i.e., sulphide-rich) chromitites at temperatures >700 °C within the conditions of UHP and eclogite-facies resulted in the formation of (1) non-porous recrystallized chromite, consisting of a granoblastic microstructure made-up of coarse-grained blasts and finer-grained chromite neoblasts. In contrast, hydrous metamorphism on the less metasomatized (i.e., sulphide-poor) chromitite under the conditions of amphibolite-facies (ca. 482–483 °C) resulted in the formation of (2) partly altered chromite, characterized by unaltered cores surrounded by Fe2+-rich and Al-depleted porous chromite containing abundant clinochlore, and (3) porous chromite corresponding to a chromite that was entirely transformed by the metamorphic alteration to Fe2+-rich and Al-depleted porous chromite. During metamorphism magmatic Ni-Fe-Cu sulphides originally formed during the metasomatic event in the mantle were altered, resulting in a major leaching of Cu-rich sulphides, leading to significant remobilization of Pt and Pd