Mineralogy, composition and PGM of chromitites from Pefki, Pindos ophiolite complex (NW Greece): evidence for progressively elevated fAs conditions in the upper mantle sequence     

Argirios Kapsiotis  Tassos A. Grammatikopoulos  Basilios Tsikouras  Konstantin Hatzipanagiotou  Federica Zaccarini  Giorgio Garuti 《Mineralogy and Petrology》2011,101(1-2):129-150

The Pindos ophiolite complex, located in the northwestern part of continental Greece, hosts various chromite deposits of both metallurgical (high-Cr) and refractory (high-Al) type. The Pefki chromitites are banded and sub-concordant to the surrounding serpentinized dunites. The Cr# [Cr/(Cr?+?Al)] of magnesiochromite varies between 0.75 and 0.79. The total PGE grade ranges from 105.9 up to 300.0?ppb. IPGE are higher than PPGE, typical of mantle hosted ophiolitic chromitites. The PGM assemblage in chromitites comprises anduoite, ruarsite, laurite, irarsite, sperrylite, hollingworthite, Os-Ru-Ir alloys including osmium and rutheniridosmine, Ru-bearing oxides, braggite, paolovite, platarsite, cooperite, vysotskite, and palladodymite. Iridarsenite and omeiite were also observed as exsolutions in other PGM. Rare electrum and native Ag are recovered in concentrates. This PGM assemblage is of great petrogenetic importance because it is significantly different from that commonly observed in podiform mantle-hosted and banded crustal-hosted ophiolitic chromitites. PGE chalcogenides of As and S are primary, and possibly crystallized directly from a progressively enriched in As boninitic melt before or during magnesiochromite precipitation. The presence of Ru-bearing oxides implies simultaneous desulfurization and dearsenication processes. Chemically zoned laurite and composite paolovite-electrum intergrowths are indicative of the relatively high mobility of certain PGE at low temperatures under locally oxidizing conditions. The PGM assemblage and chemistry, in conjunction with geological and petrologic data of the studied chromitites, indicate that it is characteristic of chromitites found within or close to the petrologic Moho. Furthermore, the strikingly different PGM assemblages between the high-Cr chromitites within the Pindos massif is suggestive of non-homogeneous group of ores.  相似文献   

Petrographic Investigation of Microcrack Initiation in Mafic Ophiolitic Rocks Under Uniaxial Compression     

Ioannis Rigopoulos  Basilios Tsikouras  Panagiotis Pomonis  Konstantin Hatzipanagiotou 《Rock Mechanics and Rock Engineering》2013,46(5):1061-1072

This paper investigates the influence of the petrographic characteristics of mafic ophiolitic rocks on the initiation and propagation of microcracks during uniaxial compression. The microcrack patterns of a troctolite and a diorite, collected from the Pindos and Othrys ophiolites (Greece), respectively, were analysed. Thorough observation and quantification of microcracks before and after the uniaxial compression test were conducted. Combined fluorescent and polarised microscopy of polished thin sections, together with digital image analysis, indicated that the intragranular microcracks are the dominating crack type in both loaded and unloaded specimens, only in terms of their total number and length. On the other hand, the intergranular and transgranular cracks seem to grow more readily compared to the intragranular cracks, implying that the longer microcracks grow more extensively under stress. The orientation of most of the newly formed intragranular and transgranular microcracks is nearly parallel to the loading direction; however, some of the randomly oriented transgranular cracks have probably been formed during the propagation of intergranular cracks. In the troctolite, the frequency of the intragranular microcracks decreases in the olivine crystals after the uniaxial compression test due to their partial serpentinisation, which increases their resistance to brittle deformation. In the plagioclase crystals of the troctolite, microcracks are often oriented parallel to the cleavage planes, implying that such crystallographic orientations act as planes of weakness. On the contrary, the plagioclase crystals of the diorite are mainly crossed by randomly oriented microcracks, presumably due to their high degree of alteration. In the diorite, the evolution of microcracks is substantially controlled by the two perfect cleavages of amphibole. The failure of a rock occurs as a result of the growth, interaction and coalescence of a great number of pre-existing and newly formed intragranular and transgranular microcracks. The petrographic and microcrack analysis may assist in the selection of the most suitable rock type for various construction applications.  相似文献   

Petrogenetic evolution of the Koziakas ophiolite complex (W. Thessaly, Greece)     

P. Pomonis  B. Tsikouras  K. Hatzipanagiotou 《Mineralogy and Petrology》2007,89(1-2):77-111

Summary The Koziakas ophiolitic complex is situated in W. Thessaly and is interpreted as an incomplete ophiolite sequence, thrust over the Western Thessaly Unit (W.T.U.). Two tectonically distinct units represent it: (1) the lower unit with a metamorphic sole and a tectonosedimentary mélange and (2) the upper unit including mantle peridotites and basaltic lavas. The mantle peridotites are composed by harzburgite, lherzolite and plagioclase lherzolite intruded by a sparse network of gabbroic, plagiogranitic and doleritic dykes. The volcanic sequence of the upper unit can be geochemically subdivided into four groups of basalts with: (1) tholeiitic N-type MORB affinities, (2) low-Ti boninitic affinities, (3) subalkalic E-MORB type affinities and (4) alkali characteristics displaying a different petrogenetic evolution with respect to the other groups. The magmatic history of the Koziakas ophiolite is in agreement with extensive fractional crystallization and variable degrees of partial melting of a heterogeneous mantle source, yielding, magmas mainly of MORB composition. Modal and cryptic metasomatic phenomena of the mantle peridotites as well as Sr-Nd isotopic ratios imply that this melt contained also a hydrous component derived from melting of a subducted lithosphere. The above geochemical characteristics and the correlation with the adjacent ophiolite suites of Pindos, Othris and Vourinos indicate that Koziakas ophiolitic complex formed in a small backarc basin situated at the eastern margin of the greater Pindos Ocean, between the Western Thessaly Unit (W.T.U.) and the Pelagonian continent.  相似文献   

A window to the operation of microplate tectonics in the Tethys Ocean: the geochemistry of the Samothrace granite,Aegean Sea     

K. St. Seymour  V. Tsikouras  K. Kotopouli  K. Hatzipanayiotou  G. Pe-Piper 《Mineralogy and Petrology》1996,56(3-4):251-272

Summary The island of Samothrace, northeastern Aegean Sea, consists of five main geological units: (i) A basement unit consisting of low grade metamorphic rocks (metapelites, marbles, metavolcanic rocks, and a metaconglomerate); (ii) an ophiolitic complex with K-Ar hornblende date of 154 ± 7 and 155 ± 7 Ma; (iii) A granite intrusion with biotite K-Ar dates of 14.5 ± 0.3 and 14.5 ± 0.5 Ma, and a contact metamorphic event dated at 40.9 + 2.2 Ma; (iv) a unit of Cenozoic volcanic rocks: orogenic volcanism apparently occurred in two cycles with Upper Eocene tholeiitic to calc-alkaline volcanic rocks and post-Eocene high-K andesites to trachytes. (v) Quaternary clastic sedimentary rocks which occur around the peripheral parts of the island. The granitic intrusion is predominantly a hornblende-biotite granite, granodiorite or quartz monzonite, with porphyritic variants and mafic enclaves. The pluton is cut by granophyre, aplite and rare granodioritic veins. All lithological units of the Samothrace intrusion show smooth and continuous major element trends and similar chondrite- and Ocean Ridge Granite-normalized incompatible element profiles. ORG-normalized incompatible element contents of Hf, Zr, Sm are explained with fractionation close to the normalizing values Y and Yb contents combined with high K/Yb ratios; Rb and Th are significantly enriched relative to Nb and Ta. In Y-Nb and Rb-SiO₂ space most samples of the Samothrace granite, plot in the volcanic arc and the syn-collisional granite fields. In Y + Nb-Rb space they are equally distributed within and transgress these two domains. The geochemical and regional data suggest a subduction or collision environment but biotite mineral data do not support a collisional setting for magma genesis. The Samothrace granite was probaby associated with a post-collisional domain after the closure of the Axios section of the Tethys Ocean.

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Ein Einblick in das Wirken von Mikroplattentektonik in der Tethys—Die Geochemie des Samothrake Granites, Agäisches Meer

Zusammenfassung Die Insel Samothrake in der nordöstlichen Ägäis besteht aus fünf geologischen Haupteinheiten: (i) einem schwach metamorphen Basement (Metapelite, Marmore, Metavulkanite und Metakonglomerate); (ii) einem ophiolithischem Komplex, der mit K-Ar Datierungen an Hornblende ein Alter von 154 ± 7 und 155 ± 7 Ma ergab; (iii) ein granitischer Intrusionskörper mit K-Ar Altern an Biotit von 14.0 ± 0.3 und 14.5 ± 0.5 Ma und einem kontaktmetamorphen Ereignis, das mit 40.9 ± 2.2 Ma datiert ist; (iv) eine Abfolge känozoischer Vulkanite, wobei der orogene Vulkanismus offensichtlich in zwei Zyklen ablief mit tholeiitischen bis kalkalkalischen Vulkaniten im oberen Eozän und high-K Andesiten bis Trachyten im post-Eozän; (v) quartären klastischen Sedimentgesteinen, die im Randbereich der Insel auftreten. Die Granitintrusion setzt sich hauptsächlich aus Hornblende-Biotitgraniten, Granodioriten oder Quarzmonzoniten mit teilweise porphyrischen und mafischen Enklaven enthaltenden Varietäten zusammen. Der Pluton wird von Granophyren, Apliten und seltener von granodioritischen Gängen durchschlagen. Alle lithologischen Einheiten der Samothrake Intrusion zeigen kontinuierliche Hauptelementtrends und ähnliche Chondrit und ORG-normalisierte inkompatible Elementprofile. Die Gehalte an den inkompatiblen Elementen Hf, Zr, Sm sind sehr ähnlich denen von ozeanischen Graniten (ORG). Die niedrigen Y und Yb-Gehalte und die hohen K/Yb Verhältnisse werden durch Fraktionierung erklärt. Rb und Th sind signifikant angereichert im Vergleich zu Nb und Ta. In Y-Nb und Rb-SiO₂ Diagrammen plotten die meisten Proben des Samothrake Granites im Feld der vulkanischen Inselbogen- und Synkollisionsgranite. Im Y + Nb-Rb Diagramm zeigt sich eine gleichmäßige und überlappende Verteilung. Die geochemischen und regionalen Daten weisen auf einen Subduktions- oder Kollisionsbereich hin, obwohl die Biotitzusammensetzungen nicht für eine Bildung der Magmen in einem Kollisionsbereich sprechen. Die Bildung des Samothrake Granites steht möglicherweise mit post-Kollisionstektonik nach dem Schließen der Axioszone in der Tethys in Zusammenhang.

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With 7 Figures  相似文献   

Platinum-Group Mineral Characterization in Concentrates from High-Grade PGE Al-rich Chromitites of Korydallos Area in the Pindos Ophiolite Complex (NW Greece)     

Argyrios Kapsiotis  Tassos A. Grammatikopoulos  Basilios Tsikouras  Konstantinos Hatzipanagiotou 《Resource Geology》2010,60(2):178-191

The Pindos ophiolite complex, located in the north-western part of continental Greece, hosts various podiform chromite deposits generally characterized by low platinum-group element (PGE) grades. However, a few locally enriched in PPGE + Au (up to 29.3 ppm) chromitites of refractory type are also present, mainly in the area of Korydallos (south-eastern Pindos). The present data reveal that this enrichment is strongly dependant on chromian spinel chemistry and base metal sulfide and/or base metal alloy (BMS and BMA, respectively) content in chromitites. Consequently, we used super-panning to recover PGM from the Al-rich chromitites of the Korydallos area. The concentrate of the composite chromitite sample contained 159 PGM grains, including, in decreasing order of abundance, the following major PGM phases: Pd-Cu alloys (commonly non-stoichiometric, although a few Pd-Cu alloys respond to the chemical formula PdCu₄), Pd-bearing tetra-auricupride [(Au,Pd)Cu], nielsenite (PdCu₃), sperrylite (PtAs₂), skaergaardite (PdCu), Pd-bearing auricupride [(Au,Pd)Cu₃], Pt and Pd oxides, Pt-Fe-Ni alloys, hollingworthite (RhAsS) and Pt-Cu alloys. Isomertieite (Pd₁₁Sb₂As₂), zvyagintsevite (Pd₃Pb), native Au, keithconnite (Pd₂₀Te₇), naldrettite (Pd₂Sb) and Rh-bearing bismuthotelluride (RhBiTe, probably the Rh analogue of michenerite) constitute minor phases. The bulk of PGE-mineralization is dominated by PGM grains that range in size from 5 to 10 µm. The vast majority of the recovered PPGM are associated with secondary BMS and BMA, thus confirming that a sulphur-bearing melt played a very important role in scavenging the PGE + Au content of the silicate magma from which chromian spinel had already started to crystallize. The implemented technique has led to the recovery of more, as well as noble, PGM grains than the in situ mineralogical examination of single chromitite samples. Although, the majority of the PGM occur as free particles and in situ textural information is lost, single grain textural evidence is observed. In summary, this research provides information on the particles, grain size and associations of PGM, which are critical with respect to the petrogenesis and mineral processing.  相似文献   

Newtonian tidal theory and PPN metric theory. Variable G and earthquakes,II   总被引：1，自引：0，他引：1  

D. D. Dionysiou  G. A. Papadopoulos  E. N. Sarris  P. G. Tsikouras 《Earth, Moon, and Planets》1994,64(1):1-29

The tidal accelerations due to the Moon and the Sun can be computed at the same time and place of the earthquake events. The search for an earthquake-tide correlation has been referred to small Greek regions and in certain magnitude range. The events correlating with the tides are functions of the angles of the place with respect to the sublunar and subsolar points. A significant correlation has been obtained between the earthquake events and the gravity tide which means that tides are one of the most important triggering mechanism, or in other words, triggering appears as function of Newtonian or Post-Newtonian gravity. Any tidal correlation of earthquakes, which has been examined in this paper, is explicitly given by Tables and Figures as a function of Newtonian and Post-Newtonian gravity.The present paper completes the previous one.This work has been supported by the Greek General Secretariat of Research and Technology under Grant No. 89E160.  相似文献   

Newtonian and post-newtonian tidal theory: Variable G and earthquakes     

D. D. Dionysiou  E. N. Sarris  P. G. Tsikouras  G. A. Papadopoulos 《Earth, Moon, and Planets》1993,60(2):127-140

A mathematical analysis of the Newtonian and parametrized Post-Newtonian tidal stresses is applied to tidal triggering mechanism for earthquakes. We investigate a possible correlation.The procedure used for calculating the solid earth tidal stress is described by the Newtonian theory and the parametrized Post-Newtonian metric. We calculate tidal stress histories for earthquakes between 1908 and 1991 in Greek area. Although no significant tidal correlation was found for the entire data set, which contained earthquakes of magnitudeM 6.0, a fairly striking correlation was observed for an earthquake-tide correlation by computing tidal functions at the time and place of the earthquake events. A successful correlation as used in this paper means that: earthquake events occur during a certain part of the tidal cycle.Also, we have studied a variation of the gravitational constant according to the variation of the velocity of the Earth and the uniform velocity of the Solar System with respect to a preferred Universal rest frame.This work has been supported by the Greek General Secretariat of Research and Technology under Grant No. 89e160.  相似文献   

Triassic rift-related komatiite, picrite and basalt, Pelagonian continental margin, Greece     

Basilios Tsikouras  Georgia Pe-Piper  David J.W. Piper  Konstantin Hatzipanagiotou 《Lithos》2008,104(1-4):199-215

A wide range of Triassic rift-related volcanic rocks is preserved on the extensional continental margin of the central part of the Pelagonian micro-continental block of Greece, in the region of northern Evia Island and Othris. This part of the continental margin is unusual for the abundance of lavas, the wide range of lava types, and the presence of komatiitic lavas. The predominant rock types are subalkaline basalt and basaltic andesite, mildly alkaline basalt and picrite, and minor komatiite. Four groups of mafic rocks are distinguished from the 32 samples analyzed on the basis of variation in incompatible trace elements, with all but one group including both picritic and basaltic rocks. The geochemical character of the volcanic rocks suggests derivation from inhomogenous, spinel-bearing, lithospheric mantle. This mantle source was variably depleted and moderately refertilized by subduction-related fluids during earlier Hercynian subduction. The picrites were generated by variable degrees of partial melting, which are inferred to inversely correlate with pressure, thus suggesting decompression melting of their source. Evidence is lacking for a major mantle plume, but the enhanced magmatism in this sector of the rifted margin suggests that melting was induced by a large amount of hydration, which is appropriate to generate melts at lower temperatures. Consequent fractionation coupled with crustal assimilation generated the wide range of subalkaline to mildly alkaline mafic rocks.  相似文献   

Microcracks in ultrabasic rocks under uniaxial compressive stress     

Ioannis Rigopoulos  Basilios Tsikouras  Panagiotis Pomonis  Konstantin Hatzipanagiotou 《Engineering Geology》2011,117(1-2):104-113

The initiation and propagation of microcracks under stress are highly dependent upon the mineralogical and textural characteristics of the various lithotypes. Detailed observation and quantification of microcracks before and after uniaxial compression test were conducted. A fresh olivine-rich harzburgite and a serpentinized dunite were analyzed, collected from the Pindos and Vourinos ophiolites (northern Greece) respectively, in order to compare their microcrack patterns. Quantitative analysis indicated that during uniaxial compression the intragranular microcracks, which are the dominating crack-type, are gradually transformed or organized into transgranular cracks. Some of the newly formed transgranular cracks may also be a result of the growth of existing grain-boundary cracks. The new intragranular microcracks in the olivine-rich harzburgite are oriented predominantly parallel to the compressive stress direction, while those in the serpentinized dunite show a scattered orientation presumably due to the mesh texture of this rock-type. The new transgranular cracks of both peridotites tend to be subparallel to the compressive stress direction, however, many of them show a random orientation due to the fact that they have been formed as a result of the propagation of grain-boundary cracks. The occurrence of the soft serpentine along fracture surfaces of olivine, when it is in assemblage with orthopyroxene, tends to absorb the applied stress hampering the development of microcracks in olivine. On the other hand in serpentinized peridotites, the microcracks are enhanced in olivine because it is surrounded by large amounts of the much softer and flexible serpentine. Microcracks are usually formed along the cleavage planes of orthopyroxene porphyroclasts, indicating that such crystallographic preferred orientations act as planes of weakness, controlling the direction of the crack paths. Cr-spinel grains likely comprise locations of initiation of microcracks due to their very hard nature and different mechanical behaviour relative to the surrounding silicate phases. Knowledge of the mineralogical and textural characteristics may assist in the prediction of potential development of failure surfaces of an ultrabasic rock in-service.  相似文献