Composition and alteration of Cr-spinels from Milia and Pefki serpentinized mantle peridotites (Pindos Ophiolite Complex, Greece)

Abstract

The Pindos Ophiolite rocks include variably serpentinized peridotites derived from a harzburgitic and subordinately dunitic mantle. In the serpentinized matrix of these rocks pseudomorphic (mesh, bastite) and non-pseudomorphic (interpenetrating, type-2 hourglass) textures were recognized. Chromian spinel (Cr-spinel) is anhedral to subhedral and often replaced by a porous opaque phase. Chemistry data show that Cr-spinel cores retain their original composition, having Cr#[Cr/(Cr + Al)] that ranges between 0.45 and 0.73, and Mg#[Mg/(Mg + Fe2+)] that varies between 0.52 and 0.65, accompanied by low content in TiO2 ( < 0.11wt. %). The relatively wide variation of their Cr# values reflects that the studied peridotites were produced by variable degrees of melting. It is likely that the Pindos peridotites represent mantle residues originally formed in a mid-ocean ridge (MOR) environment, which were subsequently entrapped as part of a mantle wedge above a supra-subduction zone (SSZ) regime. Cr-spinel adjacent to clinochlore systematically displays limited compositional and textural zoning along grain boundaries and fractures. However, the degree of peridotite serpentinization does not correlate with the abundance of zoning effects in accessory Cr-spinel. Thus, Cr-spinel zoning is thought to represent a secondary feature obtained during the metamorphic evolution of the host peridotites. Core to rim compositional trends are expressed by MgO and Al2O3 impoverishment, mainly compensated by Cr2O3 and FeO increases. Such chemical trends are produced as a result of Cr-spinel re-equilibration with the surrounding serpentine, and their subsequent replacement by ferrian (Fe3+-rich) chromite and clinochlore, respectively, during a brief, fluid assisted, greenschist facies metamorphism episode (T > 300 °C). The limited occurrence of ferrian chromite with high Fe3+# values suggests that elevated oxidizing conditions were prevalent only on a local scale during Cr-spinel alteration

Mineralogy, geochemistry and geotectonic significance of harzburgites from the southern Dramala upper mantle suite, Pindos ophiolite complex, NW Greece

“…They supported a typical MORtype origin for the Dramala harzburgites, as well as their later modification by melt-rock interaction and impregnation processes. Kapsiotis (2014) studied Crspinel compositions from the Pindos serpentinized peridotites and supported that these rocks represent MOR-type mantle residues that were later entrapped and modified in the wedge above a SSZ. According to Saccani et al (2011) the severely depleted harzburgites cropping out along the Albanide-Hellenide ophiolitic belt actually represent residues originally associated with the release of boninite melts from the Mirdita-Pindos sub-oceanic mantle.…”

“…According to Saccani et al (2011) the severely depleted harzburgites cropping out along the Albanide-Hellenide ophiolitic belt actually represent residues originally associated with the release of boninite melts from the Mirdita-Pindos sub-oceanic mantle. Kapsiotis (2014) studied Crspinel compositions from the Pindos serpentinized peridotites and supported that these rocks represent MOR-type mantle residues that were later entrapped and modified in the wedge above a SSZ. Sergeev et al (2014) examined the lithological and isotopic variations of ancient pyroxenite layers intercalated with harzburgites ('mantle layering') in the northeastern part of the Dramala massif.…”

“…2c). Spinel grains may be infrequently replaced by ferrian chromite-clinochlore intergrowths across cracks and boundaries (Kapsiotis, 2014). In the most clinopyroxene-poor harzburgite samples, orthopyroxene clasts may host olivine inclusions.…”

Alteration of chromitites from the Voidolakkos and Xerolivado mines, Vourinos ophiolite complex, Greece: implications for deformation-induced metamorphism

“…Solvus determined at 600, 550 and 500°C by Shack and Ghiorso (1991) for chromian spinel coexisting with olivine containing 90% forsterite. Contrastingly, the serpentinized peridotites from the adjacent Pindos massif were affected by a later brief heating event during their exhumation route (Kapsiotis, 2014). The Vourinos peridotites also show evidence for metamorphic reworking followed by subsequent serpentinization .…”

Alteration of mélange-hosted chromitites from Korydallos, Pindos ophiolite complex, Greece: evidence for modification by a residual high-T post-magmatic fluid

“…Such elemental exchanges are driven by the subsequent dissolution-precipitation reaction, given by Gervilla et al (2012) Kamenetsky et al (2001) values (0.05-0.32), which implies considerable discrepancies in the oxidation conditions during the alteration process, varying from low to moderately oxidizing. Therefore, it can be inferred that although the metamorphic incident responsible for Cr-spinel alteration was short-lived, it was quite intense to allow the formation of compositionally mature ferrian chromite (e.g., Kapsiotis 2014). Nevertheless, in addition to ferrian chromite, magnetite was also found as a minor alteration product of Cr-spinel in the high-Al chromitite pod.…”