## 5 Discussion
All in all, the studied ophiolites are geochemically different, and this is due to the contrasting geotectonic settings that they have been subjected to. The Veria-Naousa ophiolite is dominated by a harzburgite mantle section (forearc region), whereas the Gerania's mantle section is Iberzolitic and thus more fertile (back-arc region). These characteristics are also expressed through the trace element chemistry of the studied rocks, as well as the chemistry of their hosted spinel group minerals. Geochemically wise, serpentinization related to fore arc peridotites is characterized by relatively elevated fluid mobile elements (FME: Pb, As, Sb, Th, U, Sr) [34]; in the present study the fore arc related peridotites (Veria-Naousa ophiolite) concentrate FME > 2.8 ppm coupled with low U.R.H.I.N values (<0.2) (Figure 9b). Concerning the MOR-related mantle sections of the Gerania ophiolite, their FME concentrations are <2.4 ppm. The above geochemical features are verified by the chemical compositions of the spinel-group minerals of the examined peridotite in both ophiolithic complexes (see Table S5 and Figure 10). The spinels analyses were plotted into two distinctive groups on the Mg# vs. Cr# diagram (Figure 10). Spinel analyses from the Gerania periodtites are characterized by Cr#! 40% [Cr# = 100 \(\times\) Cr/(Cr + Al)] and Mg#! 60% [Mg# = 100 \(\times\) Mg/(Mg + Fe\({}^{2+}\))] being plotted within the field of abyssal peridotite (lherzolites) and partially in the interception with the lower end of the forearc periodtites (harzburgites). On the contrary, the spinel group minerals hosted within the Veria-Naousa periodtites (both lherzolite and harzburgite) resemble forearc periodtites, characterized by Cr#! 40% and Mg#! 60%. The spinel grains hosted within the Veria-Naousa lherzolite consist of two groups; the Cr# richer are hosted within serpentinized and the Cr# depleted within fresh lherzolite. In view of their unique chemical traits, spinel group minerals can be used as a petrogenetic tools indicative of the tectonomagmatic history of the host peridotite. Spinel from Gerania ophiolite displays low Cr# (! 40%), typical of oceanic ophiolites, i.e., residues after low to moderate degrees of partial melting in a spreading environment. On the contrary, spinel in Veria-Naousa periodites shows high Cr# (?40%), which can only be generated in a mantle wedge above a subduction zone. The studied ultramafic rock samples were evaluated for their suitability as concrete aggregates according to standards (EAOT 408, EN 12620). According to these standards, limits for LA (! 40%), S (! 35%) and UCS (! 65%) are set for the evaluation of tested rocks for their suitability as concrete aggregates. The more serpentinized samples presented as unsuitable regarding the values of their uniaxial compressive strength and their soundness test resistance in contrast to those containing lower amounts of serpentine which were shown to be suitable concerning their total parameters. More specifically, ultramafic aggregates derived from the fertile Gerania ophiolite complex are presented as more suitable for concrete aggregates and less certain ultramafic rocks from the rather depleted mantle section of the Veria-Naousa ophiolite complex. Furthermore, great fluctuation in the values of physical properties is observed, which was related to the mechanical properties of rocks such as LA and UCS [12, 16, 17]. The variation in the values of the studied samples and consequently their suitability for use as concrete aggregates seems to be directly dependent on the environment of their genesis. In general, it appears that samples born in an abyssal environment (high F.R.) show more satisfactory results than those born in a forearc environment (low F.R.