The ophiolitic rocks of the Attic-Cycladic crystalline belt are considered of key importance for understanding the Mesozoic tectonic evolution of the Aegean region. Unresolved questions concern their tectono-stratigraphic relationships across the region. The mode of occurrence of the Cycladic ophiolites varies, as they appear as: (a) dismembered blocks (olistoliths) within the supra-detachment units of Paros and Naxos; (b) mélange formations in the upper structural unit of western Samos and Skyros; and (c) meta-ophiolitic mélange in the Cycladic Blueschist Unit (CBU) from central Samos. The trace element geochemistry and Pb-Sr-Nd isotopes of the mafic ophiolitic rocks indicate four geochemical groups: (a) the upper unit metabasites from Paros and western Samos (Kallithea) display an evolved basaltic composition (Mg# 40.2–59.6), with low Zr/Nb values (5–16) and high Ce/Y values (1.3 to 2.6) compared to MORB, indicating island-arc tholeiite affinities; (b) Naxos upper unit metabasalts show spider diagrams patterns indicating ocean island basalt (OIB-type) affinities; (c) Central Samos metagabbros (CBU) are primitive rocks with Back-Arc Basin basalt affinities; (d) the Skyros metadolerites and Tinos (Mt Tsiknias) and S. Evia (CBU) metagabbros, cluster as a separate geochemical group; they exhibit high MgO values (>10 wt %), very low TiO₂ values (0.1–0.2 wt %), Y and Yb, and depleted trace element N-MORB normalized patterns, similar to volcanic rocks formed in modern oceanic fore-arc settings, such as boninites. A combination of the Pb- and Sr-isotopic compositions of Cycladic metabasites indicate that the Pb and Sr incorporated in the Cycladic ophiolites correspond to mixtures of magmatic fluids with seawater (²⁰⁶Pb/²⁰⁴Pb = 18.51–18.80; ²⁰⁷Pb/²⁰⁴Pb = 15.59–15.7; ²⁰⁸Pb/²⁰⁴Pb = 39.03–39.80 and initial ⁸⁷Sr/⁸⁶Sr80 = 0.705–0.707). Furthermore, peridotite relicts from Samos, Paros, and Naxos—irrespective of the structural unit—display chemical affinities of ocean floor peridotites formed in a supra-subduction zone. The characteristics of harzburgite relicts in Cycladic serpentinites and Skyros indicate a highly residual nature of the mantle source. Geochemical data from this study shed further light on the complex structure of the oceanic lithosphere from which the Cycladic ophiolites originated.

Underground geological energy and CO₂ storage contribute to mitigation of anthropogenic greenhouse-gas emissions and climate change effects. The present study aims to present specific underground energy and CO₂ storage sites in Greece. Thermal capacity calculations from twenty-two studied aquifers (4 × 10⁻⁴–25 × 10⁻³ MJ) indicate that those of Mesohellenic Trough (Northwest Greece), Western Thessaloniki basin and Botsara flysch (Northwestern Greece) exhibit the best performance. Heat capacity was investigated in fourteen aquifers (throughout North and South Greece) and three abandoned mines of Central Greece. Results indicate that aquifers present higher average total heat energy values (up to ~6.05 × 10⁶ MWh_(th)), whereas abandoned mines present significantly higher average area heat energy contents (up to ~5.44 × 10⁶ MWh_(th)). Estimations indicate that the Sappes, Serres and Komotini aquifers could cover the space heating energy consumption of East Macedonia-Thrace region. Underground gas storage was investigated in eight aquifers, four gas fields and three evaporite sites. Results indicate that Prinos and South Kavala gas fields (North Greece) could cover the electricity needs of households in East Macedonia and Thrace regions. Hydrogen storage capacity of Corfu and Kefalonia islands is 53,200 MWh_(e). These values could cover the electricity needs of 6770 households in the Ionian islands. Petrographical and mineralogical studies of sandstone samples from the Mesohellenic Trough and Volos basalts (Central Greece) indicate that they could serve as potential sites for CO₂ storage.

This case study investigates the influence of the mineralogical composition of ultramafic rocks derived from two ophiolite complexes from Greece (Veria-Naousa and Gerania) on their mechanical, physical and physicochemical properties. The investigated lithologies include lherzolite, harzburgite, dunite and olivine-orthopyroxenite with variable degrees of alteration. The ratio of secondary minerals to primary minerals (SEC/PR) of the studied ultramafic rocks shows good correlations with their physical, physicochemical and mechanical properties, suggesting that alteration has a negative effect on the engineering performance of the ultramafic rocks. Among the secondary minerals, serpentine plays the most critical role in determining the moisture content, the total porosity and hence the soundness of the host rocks, due to its phyllosilicate structure, which allows more water/solutions to be captured. The high percentage of serpentine creates surfaces of weakness, and as a result, it decreases the rock strength. The low microtopography of highly serpentinized rocks results in their reduced mechanical performance.

This study focuses on the use of petrology as a useful tool in construction applications (i.e., concrete). More specifically, this study investigates how the petrogenetic characteristics of ultramafic rocks derived from ophiolite complexes (Veria–Naousa, Gerania) can act as a key tool for the prediction of the final behaviour of ultramafic aggregates as concrete aggregates. For this reason, their petrographic, chemical and mineralogical characteristics were examined and correlated with their engineering properties for evaluating their suitability as concrete aggregates. This study had come up, for the first time, that the genesis environment of the ultramafic rocks is the determinant factor for their physico-mechanical characteristics. Their suitability is relevant to the impact of their mineralogical and structural characteristics both from the two different ophiolite complexes (Veria–Naousa and Gerania). Except serpentinization, the basic alteration process-index of ultramafic rocks, there are also other chemical indices which can be used for ultramafic rocks that may determine their properties. In this context the term ‘fertility rate’ (FR) was introduced which may characterize ultramafic rocks as fertile or not. Furthermore, the Ultramafic Rock Health Index (U.R.H.I.) as well as the Normalized U.R.H.I. (U.R.H.I._N) was also introduced and correlated with the engineering properties of the investigated aggregate rocks. The last index aims to assess and quantify the overall health conditions, encompassing the two major modifying factors that include removal of primary mineral phases, as well as the extent of serpentinization. The main conclusion of this paper is that the genesis environment of the ultramafic rocks is the critical factor that determines their mineralogical, petrographic and chemical characteristics which consequently determines the basic engineering properties of rocks that determine their suitability or not as concrete aggregates.