Summary
In the upper stratigraphic levels of the Thracian Circum-Rhodope Belt, pillowed or massive metavolcanics and metapyroclastic rocks occur. In the deeper part of the stratigraphic column a composite suite of greenschists, cumulate and noncumulate gabbros, metagabbros, serpentinites, chlorite and talc schists are found. Detailed petrographical study revealed that the metavolcanics consist of four lava types. From the basic to the more evolved types, these lavas are: pyroxeno-phyric lavas, aphyric oligophyric lavas, albite-rich lavas and porphyric felsites. Based on geochemical criteria the metavolcanics are classified as tholeiitic basalts and andesites, to dacites-rhyodacites. The content of MgO, Cr, Ni, TiO2, Zr and REE, and some petrographic features of the pyroxeno-phyric lavas suggest boninitic affinities.
The projection of chemical data on several discrimination diagrams, the REE patterns, the occurrence of lavas with boninitic affinity, the chemistry of clinopyroxenes, the crystallization sequence of the primary minerals, the presence of both basic and more evolved volcanic rocks, as well as the high ratio LIL/HFS indicate that the protoliths of the metavolcanics were formed in an immature island arc setting. The greenschists present both weak MORB and strong VAB characters suggesting that their protoliths developed in a short-lived back-arc basin. As the whole sequence of the metabasic and meta-ultrabasic rocks of the Thracian Circum-Rhodope Belt would be considered as an incomplete and dismembered ophiolite, the geodynamical environment of its formation is assumed to be a system of volcanic arc-marginal basin. Both subducted and obducted slabs were parts of the Palaeotethys oceanic realm, while the system was situated along the continental margin of the Rhodope Massif.
Zusammenfassung
In den oberen stratigraphischen Horizonten des thrakischen Zirkum-Rhodope-Gürtels kommen metavulkanische und metapyroklastische Gesteine mit Pillow-oder massigem Gefüge vor. In den tieferen Teilen der stratigraphischen Säule wurde eine komplexe Abfolge aus Grünschiefern, Gabbros, Metagabbros, Serpentiniten, Chlorit-und Talkschiefern festgestellt. Gründliche petrographische Untersuchungen zeigten, daß die Metavulkanite aus pyroxeno-phyrischen, aphyrisch-oligophyrischen und albitreichen Laven sowie aus porphyrischen Felsiten bestehen. Aufgrund geochemischer Kriterien werden die Metavulkanite als tholeiitische Andesite oder Dacite bis Basalte eingestuft. Der Gehalt an Mg0, Cr, Ni, TiO2, Zr und REE sowie petrographische Merkmale der pyroxeno-phyrischen Laven weisen auf einen boninitischen Charakter hin.
Die Protolithe der Metavulkanite wurden im Bereich eines unreifen Inselbogens gebildet. Hierfür sprechen: (a) die Projektionslage der chemischen Daten in mehreren Diskriminierungsdiagrammen; (b) die REE-Häufigkeitskurven; (c) das Vorkommen von boninitischen Laven; (d) der Chemismus der Klinopyroxene; (e) die Kristallisationsabfolge der primären Mineralphasen; (f) die Vergesellschaftung basischer mit mehr sauren vulkanischen Gesteinen; (g) das hohe LIL/HFS-Verhältnis. Die Grünschiefer zeigen Übergangsmerkmale zwischen MORB und VAB.
Die gesamte Folge der metabasischen und -ultrabasischen Gesteine des thrakischen Zirkum-Rhodope-Gürtels könnte als ein unvollständiger und zergliederter Ophiolithkomplex angesehen werden, der in einem Vulkanbogen-Randbecken war. Sowohl sub duzierte als auch obduzierte Anteile waren am Kontinentalrand des Rodope-Massifs angeordnet und stellen Teile des ozeanischen Bereiches der Paläotethys dar.
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Magganas, A., Sideris, C. & Kokkinakis, A. Marginal basin—volcanic arc origin of metabasic rocks of the Circum-Rhodope Belt, Thrace, Greece. Mineralogy and Petrology 44, 235–252 (1991). https://doi.org/10.1007/BF01166965
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DOI: https://doi.org/10.1007/BF01166965