Magmatic and post-magmatic processes recorded in Cr-PGE ores and mantle peridotites, Chalkidiki ophiolites, Greece : a mineralogical, geochemical and petrological investigation

• Main Author: Σιδερίδης, Αλκιβιάδης
• Other Authors: Sideridis, Alkiviadis
• Language: English
• Published: 2023
• Subjects: Chromitite; Chalkidiki ophiolites; Mantle processes; Post-magmatic processes; Ophiolite petrology; Χρωμιτίτης; Οφιόλιθοι Χαλκιδικής; Μανδυακές διεργασίες; Μετά-μαγματικές διεργασίες; Πετρολογία οφιόλιθων

Along the Chalkidiki peninsula there are two separate ophiolite zones, the West and the East Chalkidiki, both comprise of well exposed mantle sections which host podiform chromitite bodies. The West Chalkidiki (occurrences of Vavdos and Gerakini-Ormylia) comprises a highly depleted mantle section, which, based on the present research, consists of highly depleted harzburgite, transitional to dunite harzburgite, massive dunite and Cr-rich chromitite - dunite envelope assemblages. Using the chemistry of magnesiochromite hosted within the Cr-rich chromitite, the calculated parental melts demonstrate boninitic affinities, generated after high partial melting degrees in a forearc above a subducted slab. This led to the depletion of the mantle source and subsequent melt-rock interaction consumed orthopyroxene, leaving dunite residues. Dunite acted as pathways for the melt percolation and subsequent formation of chromitite. The PGE concentrations are typical of Cr-rich podiform Tethyan chromitites and their distribution is controlled by partial melting. The primary PGM revealed a) initial crystallization temperatures of ~1200 ◦C under low sulfur fugacity (Os-Ir alloys and Ru-rich laurite) and b) subsequent cooling of the system with increasing sulfur fugacity (erlichmanite and Ru-poor laurite). The high temperature PGM are hosted in the massive chromitite and the lower temperature in non-massive counterparts. These results demonstrate a common mantle evolution in the Vardar ophiolites. Apart from the described primary features, it was deemed important to study a) the effect of post-magmatic processes and b) the geotectonic settings that these took place. The major post-magmatic process is listwaenitization, which has led to complete silicification (silica listwaenite) of the peridotite protoliths which are dominated by a rusty silica pseudo-mesh texture mass. The listwaenite hosted chromitites do not preserve any primary silicate, just serpentine relics and chlorite. This supports a serpentinization effect prior to listwaenitization. The effect of listwaenitization upon chromitite has never been addressed. Study of the major and trace element chemistry of magnesiochromite showed that there were no significant chemical variations due to the listwaenitization. The encountered PGM were desulfurized and listwaenitized chromitite display Pd-Au enrichments. The listwaenitization took place within a mantle wedge, where CO2-bearing fluids originating from dehydrated subducted Mesozoic sediments, interacted with the protoliths, forming serpentinite and finally silica listwaenite. The meta-ultramafic bodies of Gomati and Nea Roda (East Chalkidiki) are situated in the Serbomacedonian Massif. They demonstrate bimodal character in terms of chromitite chemistry with both Cr- and Al-rich chromitites outcropping in proximity, with no obvious tectonic structure intercepting those two varieties. Based on the trace element abundances in spinel grains, metamorphosis reached amphibolite facies, forming porous spinel. Chromitite-hosted chlorite and garnet chemistry correlates with greenschist facies temperatures and formation of zoned spinel grains. Despite the metamorphic overprint, some of the primary features of the chromitites have been preserved. The PGE contents demonstrate an increase in Pd/Ir ratios in some chromitites pointing to fractionation, whereas low ratios of mostly Cr-rich chromitites point to partial melting being the main mechanism that controls PGE mineralization. The normalized trace element patterns of spinel-group minerals revealed that Al-rich chromitites were generated in spreading settings in a back-arc and the Cr-rich counterparts in SSZ environment. The parental melts of Al-rich and Cr-rich chromitites demonstrate MORB and boninitic affinities, respectively. The meta-ultramafic protoliths were modified within a subduction zone, with significant input of a sedimentary source, as confirmed by the chemistry of serpentinite, diopside and Sb-mineralization. These results suggest common geotectonic processes within the Rhodope and the Serbomacedonian massif, that have affected the ultramafic bodies and chromitite occurrences.