Andreas Magganas

Constraints on the petrogenesis of Evros ophiolite extrusives, NE Greece

Abstract The incomplete and dismembered Evros ophiolite is situated in Thrace, NE Greece, and belongs to the Circum-Rhodope Belt (CRB). Its age is considered to be Jurassic–Lower Cretaceous. The geotectonic setting of the ophiolite was a volcanic arc-marginal basin system in the Vardar oceanic realm. In the uppermost level of the ophiolitic sequence, volcanic and pyroclastic rocks of tholeiitic composition are underlain massive and pillow lavas with a few tuffaceous rocks and lava breccia. All these rocks were involved in intense deformation and low-grade metamorphic events during and after their formation. The underlying tholeiites were regionally metamorphosed to greenschist facies ( lower metavolcanics (LMV)), whereas the upper lavas were extensively recrystallized mainly by ocean floor metamorphism of prehnite–pumpellyite facies ( upper metavolcanics ). REE data and wide ranges in Zr/Y, Ti/Zr and Al/Ti ratios suggest that the parental magma of the upper metavolcanics underwent extensive fractional crystallization. Tholeiitic basalts and mainly basaltic andesites to andesites were formed at the initial stages of the differentiation by crystal fractionation, which involved olivine, Cr-spinel, clinopyroxene and occasionally plagioclase. Subordinate dacites to rhyodacites were created when plagioclase became the main fractionating phase. The oxygen fugacity remained nearly stable during the formation of the main volume of the rock association, but it changed when rhyodacitic rocks started to crystallize. The protoliths of the upper metavolcanics was boninitic, produced in the forearc area by about 30% partial melting of an already depleted mantle source. This partial melting was induced by aqueous fluids and/or melts from the subducting oceanic crust and its sedimentary cover. The depleted mantle source of the upper metavolcanics was the residue of an earlier partial melting event that had generated the magma of the lower metavolcanic protoliths in an extensional regime.

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

SummaryIn 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.ZusammenfassungIn 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.

Geochemistry and diagenesis of Miocene lacustrine siliceous sedimentary and pyroclastic rocks, Mytilinii basin, Samos Island, Greece

Abstract A Late Miocene non-marine stratigraphic sequence composed of limestone, opal-CT-bearing limestone, porcelanite, marlstone, diatomaceous marlstone, dolomite, and tuffite crops out on eastern Samos Island. This lacustrine sequence is subdivided into the Hora Beds and the underlying Pythagorion Formation. The Hora Beds is overlain by the clastic Mytilinii series which contains Turolian (Late Miocene) mammalian fossils. The lacustrine sequence contains volcanic glass and the silica polymorphs opal-A, opal-CT, and quartz. Volcanic glass predominantly occurs in tuffaceous rocks from the lower and upper parts of the lacustrine sequence. Opal-A (diatom frustules) is confined to layers in the upper part of the Hora Beds. Beds rich in opal-CT underlie those containing opal-A. The occurrence of opal-CT is extensive, encompassing the lower Hora Beds and the sedimentary rocks and tuffs of the Pythagorion Formation. A transition zone between the opal-A and opal-CT zones is identified by X-ray diffraction patterns that are intermediate between those of opal-CT and opal-A, perhaps due to a mixture of the two polymorphs. Diagenesis was not advanced enough for opal-CT to transform to quartz or for volcanic glass to transform to opal-C. Based on geochemical and mineralogical data, we suggest that the rate of diagenetic transformation of opal-A to opal-CT was mainly controlled by the chemistry of pore fluids. Pore fluids were characterized by high salinity, moderately high alkalinity, and high magnesium ion activity. These pore fluid characteristics are indicated by the presence of evaporitic salts (halite, sylvite, niter), high boron content in biogenic silica, and by dolomite in both the opal-A and opal-CT-bearing beds. The absence of authigenic K-feldspar, borosilicates, and zeolites also support these pore fluid characteristics. Additional factors that influenced the rate of silica diagenesis were host rock lithology and the relatively high heat flow in the Aegean region from Miocene to Holocene.

Clastic Huntite in Upper Neogene Formations of the Kozani Basin, Macedonia, Northern Greece

ABSTRACT An unusual carbonate mineral, huntite (Mg3Ca(CO3)4), is present in Upper Neogene lacustrine formations of the Kozani Basin, Northern Greece. This carbonate, either pure or mixed with variable proportions of hydromagnesite and magnesite, forms extensive deposits that accumulated in ponds on a lake-margin carbonate flat. Although labile and metastable, huntite also is the main detrital component of fluvial-dominated delta sequences in some parts of the basin. Clastic huntite is present mainly in foresets, trough cross-stratified units, and channel forms in terrigenous lithofacies towards the basal part of deltaic assemblages. Detachment of fragments from cohesive huntite pond deposits may have been favored by expansion through wetting rather than by desi cation and further contraction of the chemical sediment. This first documented occurrence of clastic huntite places some uncertainties on the presumed chemical and mechanical instability of huntite under natural conditions.

Petrology and Diagenetic Changes in Miocene Marine Diatomaceous Deposits from Zakynthos Island, Greece

Silica diagenesis in a diatomaceous section of marly limestone and chert from central Zakynthos Island, Ionian sea, was investigated. This section, underlain by Cretaceous-Eocene reefal limestone and overlain by a middle-upper Miocene terrigenous—evaporitic succession, is defined as the Romiri Formation. Diatom and planktonic foraminiferal assemblages occur, the latter determining the biostratigraphic interval N4-N7 of the Blow zonation (Aquitanian-Burdigalian).

The Acheulian Site at Rodafnidia, Lisvori, on Lesbos, Greece: 2010–2012

Rodafnidia is an Acheulian site on Lesbos Island, in the north-east Aegean Sea. This chapter presents the model that guided Paleolithic investigations on the island, the history of research, and the results of the 2012 expedition of systematic work in the field, which consisted of surface survey and excavation. The typology and technology of lithic artifacts from the surface and the uppermost Unit 1, as well as the first cluster of luminescence dates, firmly place the early component of the site in the Middle Pleistocene. The Acheulian industry derives from fluvio-lacustrine deposits at a locale with abundant fresh-water and lithic resources. Situated in the north-east Mediterranean Basin, an area where research on early hominin prehistory is intensifying, Rodafnidia holds the potential to contribute to Eurasian Lower Paleolithic archaeology and fill the gap in our understanding of early hominin presence and activity where Asia meets Europe.

Boninitic and tholeiitic basaltic lavas and dikes from dispersed Jurassic East Othris ophiolitic units, Greece: petrogenesis and geodynamic implications

ABSTRACT Pillow lavas, massive lava flows, and sub-volcanic dikes of tholeiitic basaltic composition are found to be members of the Vrinena, Aerino, Eretria, and Velestino dispersed Middle–Upper Jurassic ophiolitic units in East Othris. The Vrinena and Eretria ophiolitic units appear to have been emplaced onto the Pelagonian continental margin during the Upper Jurassic–Lower Cretaceous, whereas the Aerino and Velestino units seem to have been finally emplaced during post-Palaeocene times. Geochemically these are divided into two groups: Group I includes subduction-related boninites and low-Ti basalts from the Vrinena and Aerino units, and Group II high-Ti basalts show spreading-type characteristics occurring in the Eretria and Velestino units. Primary magma of the Group I volcanics appears to have been formed after high partial melting degrees (~18%) of a highly depleted harzburgitic mantle source, under relatively high temperatures (mantle potential temperature ~1372°C). Petrogenetic modelling also suggests that the primary magma of the Group II volcanics were formed after lower partial melting degrees (~7%) of a moderately depleted mantle source. The petrological and geochemical data from the East Othris dispersed and diversely emplaced ophiolitic units provide evidence of a common intra-oceanic supra-subduction zone (SSZ) origin within the Pindos oceanic strand of the Western Tethys. Specifically, Group I lavas and dikes from Vrinena seem to represent the extrusive part of an almost complete fore- to island-arc ophiolitic sequence. Dikes of Aerino most likely correspond to fore-arc magmatic material that intruded within exhumed serpentinized ultramafic rocks through a subduction channel that developed close to the slab and towards the fore-arc and the accretionary prism. The Group II volcanics either corresponded to a fore-arc magmatic expression, which extruded earlier than Group I volcanics and prior to the establishment of a mature subduction zone, or represent back-arc to island-arc magmatism that was contemporaneous to the fore-arc magmatic activity during rollback subduction.

Pb2(AsO2OH)Cl2, a new phase from the Lavrion ancient slags, Greece: occurrence and characterization

Abstract Transparent prismatic crystals of Pb2(AsO2OH)Cl2 were collected in black metallurgical slag on the coast in the Punta Zeza area, 3 km south of the town of Lavrion, Greece. Analyses by energy-dispersive X-ray spectrometry (EDS) provided the following chemical composition: PbO 73.04, As2O3 15.97, Cl 11.42, O=Cl -2.58, total 97.85 wt.%. The empirical formula calculated on the basis of As = 1 is HxPb2.03(AsO3)1.00Cl1.995 (x = 0.96). The infrared spectrum of Pb2(AsO2OH)Cl2 has characteristic AsO33- bands at 707 and 594 cm-1, O-H stretching vibrations at 3310 and 2900 cm-1 and a band at 1107 cm-1 which is assigned to As-O-H bending vibrations. The structure, which is monoclinic P21/m, a = 6.4235(8), b = 5.5399(7), c = 9.321(1) Å, β = 90.767(2)°, V = 331.67(7) Å3, R1 = 0.035, is identical to that of synthetic Pb2(AsO2OH)Cl2 and contains two symmetrically unique Pb sites and one As site. The crystal structure is based on [Pb2AsO2OH]2+ double chains interconnected via weak Pb-Cl bonds to produce a three-dimensional framework which is closely related to that of Pb oxysalt minerals containing [O2Pb3]2+ chains of oxocentred [OPb4]6+ tetrahedra including mendipite, damaraite, rickturnerite and plumboselite.

Hilarionite, Fe23+(SO4)(AsO4)(OH) · 6H2O, a new supergene mineral from Lavrion, Greece

A new mineral, hilarionite, ideally Fe23+ (SO4)(AsO4)(OH) · 6H2O, has been found in the Hilarion Mine, Agios Konstantinos, Kamariza, Lavrion district, Attiki Prefecture, Greece. It was formed in the oxidation zone of a sulfide-rich orebody in association with goethite, gypsum, bukovskyite, jarosite, melanterite, chalcanthite, allophane, and azurite. Hilarionite occurs as light green (typically with an olive or grayish tint) to light yellowish green spherulites (up to 1 mm in size) and bunches of prismatic to acicular “individuals” up to 0.5 mm long that are in fact near-parallel or divergent aggregates of very thin, curved fibers up to 0.3 mm long and usually lesser than 2 μm thick. The luster is silky to vitreous. The Mohs’ hardness is ca. 2. Hilarionite is ductile, its “individuals” are flexible and inelastic; fracture is uneven or splintery. D(meas) = 2.40(5), D(calc) = 2.486 g/cm3. IR spectrum shows the presence of arsenate and sulfate groups and H2O molecules in significant amounts. The Mössbauer spectrum indicates the presence of Fe3+ at two six-fold coordinated sites and the absence of Fe2+. Hilarionite is optically biaxial (+), α = 1.575(2), γ = 1.64(2), 2V is large. The chemical composition (electron microprobe, average of 7 point analyses; H2O determined by modified Penfield method) is as follows, wt %: 0.03 MnO, 0.18 CuO, 0.17 ZnO, 33.83 Fe2O3, 0.22 P2O5, 18.92 As2O5, 22.19 SO3, 26.3 H2O, total is 101.82%. The empirical formula calculated on the basis of 15 O is: (Fe1.903+Cu0.01Zn0.01)Σ1.92[(SO4)1.24(AsO4)0.74(PO4)0.01]Σ1.99(OH)1.01 · 6.03H2O. The X-ray powder diffraction data show close structural relationship of hilarionite and kaňkite, Fe23+(AsO4)2 · 7H2O. Hilarionite is monoclinic, space group C2/m, Cm or C2, a = 18.53(4), b = 17.43(3), c = 7.56(1) Å, β = 94.06(15)°, V = 2436(3) Å3, Z = 8. The strongest reflections in the X-ray powder diffraction pattern (d, Å-I[hkl]) are: 12.66–100[110],