Crystal chemistry, structure analyses and phase transition experiment on an omphacite from eclogitic metagabbro from Syros island, Greece

Summary Pyroxene samples, from the Greek island of Syros, taken from a blueschist-eclogite facies Mg-rich metagabbro, were investigated by chemical and XRD analyses and M?ssbauer spectroscopy. Single-crystal XRD and microprobe analysis showed that the natural sample is a typical omphacite of intermediate composition in the Ca–Na pyroxene solid solutions. The space group P2/n was confirmed and the cations Mg, Al and Ca, Na were found to be ordered in the M1 and M2 positions, respectively. M?ssbauer spectroscopy showed that there is both ferrous and ferric iron in the structure, with the ratio 1.38:1. The M2 sites are fully occupied by Ca and Na, thus the iron (Fe²⁺ and Fe³⁺) can substitute only for Mg and Al in the M1 sites. Partial disorder was attained by annealing the sample at 850 °C, 20 kbar for 7 days, as confirmed by decrease of intensity of reflections affected by the C-type extinction.     

Marginal basin—volcanic arc origin of metabasic rocks of the Circum-Rhodope Belt,Thrace, Greece

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.

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Randbecken- vulkanbogenherkunft metabasischer gesteine des Zirkum-Rhodope-Gürtels, Thrakien, Griechenland

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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With 8 Figures     

Geochemical and Sr-Nd isotopic evidence for petrogenesis and geodynamic setting of Lower-Middle Triassic volcanogenic rocks from central Greece: Implications for the Neotethyan Pindos ocean

Mineralogy and Petrology - Geochemistry and Sr-Nd isotopes were determined in Triassic calcalkaline metalava and metapyroclastic rocks from Attica and Argolida, central Greece, to ascertain their...     

New Data on Fiedlerite-1A from Ancient Slags of Lavrion,Greece: Crystal Structure and Hydrogen Bonding

Doklady Earth Sciences - The crystal structure (R = 0.0750) of fiedlerite-1A Pb3Cl4F (OH) · H2O from ancient slags of Lavrion (Greece) was studied on a single crystal. The mineral is...