Panagiotis Pomonis

Origin, evolution and radiometric dating of sub-ophiolitic metamorphic rocks from the Koziakas ophiolite (W. Thessaly, Greece)

The Koziakas ophiolitic complex is part of the western Greek ophiolitic zone of the Mesozoic Tethyan orogenic belt. Amphibolites and metasedimentary rocks that represent a sub-ophiolitic metamorphic sole underlie it. The amphibolites include MORB-type protoliths with IAT influence, suggesting a back-arc basin environment for their origin. Two amphibole K-Ar radiometric dates of the sub-ophiolitic metamorphic sole of the Koziakas ophiolite yielded ages of 174 ′ 3 Ma and 161 ′ 1 Ma. These ages are similar to those reported from the adjacent Pindos, Vourinos and Othrys ophiolitic complexes. Geothermobarometric calculations showed that the metamorphic evolution of the amphibolites occurred at around 530-650°C and 5-7kbars.

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

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.

The impact of petrographic characteristics on the engineering properties of ultrabasic rocks from northern and central Greece

This paper investigates the influence of petrographic characteristics on the engineering properties of ultrabasic rocks from the Pindos, Vourinos and Koziakas ophiolites. The studied lithologies include dunites, olivine-rich harzburgites, typical harzburgites and plagioclase-bearing lherzolites. Quantitative petrographic analysis indicates that the samples display various percentages of secondary minerals. The peridotites are tested to determine the following engineering parameters: water absorption, apparent density, total porosity, point load index, Schmidt hammer value, aggregate abrasion value and polished stone value. The relationships between the petrographic features and the engineering parameters are determined using regression analysis. The micropetrographic index Irep and the ratios of secondary to primary (Se/P) and soft to hard (So/H) minerals provide significant information about the impact of ocean-floor metamorphism on the strength and durability of peridotites. The correlations indicate that the formation of secondary minerals results in mechanically weaker rocks. The polished stone value is the only engineering parameter that is positively affected by the presence of low contents of alteration products. The results also imply that Cr-spinel grains are a major determinant for the preservation of the microroughness of aggregate particles owing to their very hard nature and different mechanical behaviour compared with the surrounding silicate minerals.

Correlations between petrographic and geometrical properties of ophiolitic aggregates from Greece

The study investigated the influence of petrographic characteristics on the geometrical properties of ultrabasic and basic ophiolitic aggregates from Greece. Quantitative petrographic analysis indicates that the studied lithologies display various percentages of secondary mineral phases. Statistical analyses indicate that the geometrical characteristics of ultrabasic rocks tend to improve as the micropetrographic index Irep, the degree of serpentinisation, the ratio of soft to hard minerals and the ratio of secondary to primary minerals decrease. The aggregate particles of the basic lithotypes tend to be more elongated and flaky when the ratio of soft to hard minerals increases. The higher flakiness and elongation index values of the ultrabasic samples are attributed to the high percentage of serpentine, the local preferred orientation of the olivine grains and the existence of oriented transgranular microcracks.RésuméL’étude a examiné l’influence des caractéristiques pétrographiques sur les propriétés géométriques des granulats d’ophiolites basiques et ultrabasiques de Grèce. Des analyses pétrographiques quantitatives ont montré la présence de pourcentages variables de phases minérales secondaires. Des analyses statistiques ont indiqué que les caractéristiques géométriques des granulats issus de roches ultrabasiques ont tendance à s’améliorer avec une décroissance de l’indice micro-pétrographique Irep, le degré de serpentinisation, le rapport minéraux tendres sur minéraux durs et le rapport minéraux secondaires sur minéraux primaires. Les granulats issus des roches basiques ont tendance à être plus allongés et aplatis lorsque le rapport minéraux tendres sur minéraux durs augmente. Les valeurs plus élevées des indices d’aplatissement et d’allongement des échantillons issus de roches ultrabasiques sont attribuées au fort pourcentage de serpentine, aux orientations préférentielles des grains d’olivine et à l’existence de microfissures trans-granulaires orientées.

Erratum to: Correlations between petrographic and geometrical properties of ophiolitic aggregates from Greece

Ophiolitic complexes are among the most important sources of aggregates as they include lithotypes with good engineering properties. They are remnants of the Earth’s oceanic crust and upper mantle, derived from mid-ocean ridges or marginal basins. A complete ophiolite suite consists from bottom to top of ultrabasic, basic, hypabyssal and extrusive rocks. Ocean-floor metamorphism usually obscures primary petrographic characteristics of ophiolitic rocks, leading to a modification of their strength properties (e.g. Ramana et al. 1986; Marinos et al. 2006; Rigopoulos et al. 2010). Aggregate particle shape appears to be a major determinant for the quality of aggregates, as it affects the properties of mixtures (Kaplan 1958; Bloem and Gaynor 1963). The effects of particle shape on the properties of both fresh and hardened concrete tend to be beneficial where the predominant shape is generally equidimensional and detrimental when it is flaky and/or elongated (Kaplan 1958, 1959; Neville 1995). As the proportion of flat particles in hydraulic concrete increases, the compression strength decreases, the cement consumption increases and the workability decreases (Frazao and Sbrighi Neto 1984). The shape characteristics of aggregates reflect intrinsic petrographic properties, together with production factors. Thus, petrography is of fundamental importance in understanding the geometrical properties of rocks; its use in the evaluation of aggregates dates back to the early part of the 20th century (Knight and Knight 1935; Rhoades and Mielenz 1946). The engineering parameters of rocks are a function of their mineralogy, texture, degree of chemical alteration, weathering and deformation (Hartley 1974; Å ́ kesson et al. 2001; Zorlu et al. 2004; Rigopoulos et al. 2011). As previously noted, ophiolitic complexes include a significant variety of lithotypes, which are often modified by hydrothermal metamorphism and tectonic deformation. Consequently, these suites commonly present a wide range of petrographic features and thus the existence of lithotypes with variable geometrical properties is very common, even in the same suite. It is generally recognised that aggregate shape significantly depends on the rock type and its microscopic characteristics, but only a few studies have The online version of the original article can be found under doi:10.1007/s10064-013-0486-3.

Geochemical and petrogenetic features of plagiogranite dykes in the Koziakas ophiolitic mélange (W. Thessaly, Greece)

Plagiogranite and dolerite dykes with boninitic affinities intrude thrust-bounded blocks of serpentinized harzburgite at the eastern part of the Koziakas ophiolitic melange. Geochemically they are characterized as oceanic plagiogranites with trondhjemitic composition and metaluminous affinities. Low ΣREE and trace element contents indicate derivation from a depleted mantle source. Geochemical modelling shows that the Koziakas plagiogranites could have originated by 5–20 % partial melting of a gabbro precursor. Locally, an early-stage high-T hydrothermal alteration event seems to have caused the anorthitization of magmatic plagioclase. This hydrothermal event was overprinted by a low-T retrograde alteration. The overall field data and geochemical affinities suggest a supra-subduction zone origin for the studied plagiogranite dykes, which is compatible with the geotectonic environment of the Koziakas ophiolite.