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

Abstract

Chromitites, associated with upper mantle spinel peridotites from the Voidolakkos and Xerolivado districts, located in the Vourinos ophiolite complex, northwestern continental Greece, were re‐investigated with respect to their structural and textural mode of occurrence, as well as their compositional signatures. They include variably deformed banded and podiform chromitite bodies made up of massive, semi‐massive, nodular, anti‐nodular, schlieren and disseminated chromian spinel. Chromitites have suffered intense shearing that was more severe in all but disseminated textured ore. Deformation has partly produced elongation of chromian spinel nodules and widespread protocataclastic zones within chromitites. The examined deposits are composed of magnesiochromite that shows a quite restricted range of Cr# [Cr/(Cr + Al)] values varying between 0.76 and 0.83, whereas Mg# [Mg/(Mg + Fe2+)] ranges from 0.55 to 0.67 accompanied by relatively low content in TiO2 (<0.15 wt.% on average). Compositional data indicate that these high‐Cr chromitite bodies crystallized from melts of boninitic affinities that have been compositionally modified after reaction with depleted harzburgite, followed by interaction with relatively undifferentiated low SiO2 melts within an intertwined system of dunite channels in the mantle wedge below the fore‐arc region of a supra‐subduction zone (SSZ). Magnesiochromite displays limited textural modification, being scarcely transformed to an opaque spinel phase along grain boundaries and fracture walls. The opaque spinel phase is characterized by elevated Cr# (0.76–0.97), relatively low Mg# (0.33–0.63) and low Fe3+# (≤0.14) and corresponds to ferrian chromite. Microscopic studies revealed that ferrian chromite is paragenetically associated with clinochlore even in unaltered chromitite specimens and the degree of serpentinization does not correlate with the frequency and abundance of alteration effects on magnesiochromite. Therefore, it is supported that regional metamorphism prior to serpentinization was responsible for the formation of the ferrian chromite–clinochlore association. In addition, magnesiochromite alteration was systematically recorded only in variably sheared chromitites displaying protocataclastic brecciation, thus it can be claimed that metamorphism was mainly governed by deformation mechanisms, which took place during the transition from ductile to semi‐brittle conditions. Ferrian chromite can be locally erratically enriched in MnO and ZnO, which is attributed to a Mn‐ and Zn‐bearing, slightly acid post‐magmatic fluid that invaded the chromitites across weakness zones. Overall, the data suggest that after magnesiochromite equilibration with the intergranular olivine, both phases were partly replaced by ferrian chromite and clinochlore, respectively, during a brief, fluid assisted episode of retrogade metamorphism that took place within a temperature interval between 700 and 300 °C, before ocean‐floor alteration. Copyright © 2014 John Wiley & Sons, Ltd.

Hydrothermal alteration of chromitite-dunite pairs from the Sabzevar ophiolite (NE Iran): Chemical and nano-textural evolution of Cr-spinel

“…However, as outlined from the bulk rock dataset, the extent of fluid-rock interaction seems to have been somehow limited. Additional fluid rock interaction in shear zones where, in addition, spinel alteration can be promoted by deformation (e.g., Kapsiotis, 2015;Satsukawa et al 2015;Qiu and Zhu, 2018;Gervilla et al, 2019) is required to reach ferritchromite stage and ultimately magnetite ores. This is rather consistent with our field observation showing serpentinite-hosted magnetite ores occurred in a locus of fault system (Fig.…”

Genesis and geotectonic setting of podiform chromitites from the Zhob Valley Ophiolite, Pakistan: inferences from chromite composition

“…As a result of petrographic investigations, any decomposition and lateritization process was not observed in Zhob valley chromitites. Therefore, the main cause of ferritchromite formation can be explained as serpentinization and/or metamorphism (Kapsiotis, 2015).…”

Hydrothermal Chromitites from the Oman Ophiolite: The Role of Water in Chromitite Genesis

“…The mantle diopsidite contains thin segregations or pods of chromitite up to several centimeters in thickness [21,22], and the crustal diopsidite encloses micro-pods, less than 1 cm across, of chromitite [21]. Those two chromitites are of clearly hydrothermal origin that has never been described before, although many works have dealt with hydrothermally altered or metamorphosed chromitites of primary igneous origin [17,26,27]. We would like to describe these really hydrothermal chromitites to clarify the petrological and geochemical characteristics of the chromitite formed from hydrothermal fluids.…”