Different Tectonic Evolution of Fast Cooling Ophiolite Mantles Recorded by Olivine-Spinel Geothermometry: Case Studies from Iballe (Albania) and Nea Roda (Greece)

Mg-Fe2+ diffusion patterns in olivine and chromite are useful tools for the study of the thermal history of ultramafic massifs. In the present contribution, we applied the exponential modeling of diffusion patterns to geothermometry and geospeedometry of chromitite ores from two different ophiolite contexts. The Iballe ophiolite (Northern Albania) hosts several chromitite pods within dunites. Primary and re-equilibrated Mg#, estimated by using an exponential function, provided re-equilibration and primary temperatures ranging between 677 and 996 °C for chromitites and between 527 and 806 °C for dunites. Cooling rates for chromitites are higher than for dunites, suggesting a different genesis for the two lithologies, confirmed also by spinel mineral chemistry. Chromitites with MORB affinity formed in a SSZ setting at a proto-forearc early stage, explaining the higher cooling rates, while dunites, with boninitic affinity, were formed deeper in the mantle in a more mature subduction setting. At the Nea Roda ophiolite (Northern Greece) olivine in chromitites do not show Mg-Fe variations, and transformation into ferrian chromite produced “fake” diffusion patterns within chromite. The absence of diffusion patterns and the low estimated temperatures (550–656 °C) suggest that Nea Roda chromitites were completely re-equilibrated during an amphibolite-facies metamorphic event that obliterated all primary features.

PETROGENETIC SIGNIFICANCE OF SPINELS FROM SERPENTINISED PERIDOTITES FROM THE VERIANAOUSA OPHIOLITE

The Veria-Naousa ophiolitic complex represents a dismembered ophiolite unit, which is superimposed on a basement consisting of rocks belonging to the Pelagonian and Axios (Almopias subzone) isopic zones in northern Greece. Mantle peridotites are composed of variably serpentinised lherzolite and harzburgite intruded by a sparse network of pyroxenitic dykes. The serpentinised lherzolite and harzburgite contain Alspinels (Cr#=38.83-42.52 and Mg#=58.94-64.77), Cr-spinels (Cr#=43.37-64.92 and Mg#=49.20-58.66) and magnesiochromites (Cr#=53.93 57.13 and Mg#=55.73- 61.71). All of them display commonly richer-in-Cr cores rimmed by secondary ferrian chromite and magnetite. Whole-rock geochemicall compositions and primary spinel chemical composition of these peridotites are analogous to peridotites that formed in a suprasubduction zone. Ιt is supported that the Mantle peridotites of the VeriaNaousa ophiolitic complex formed in a back-arc basin

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

The peridotites from the area of Korydallos, in the Pindos ophiolitic massif, crop out as deformed slices of a rather dismembered sub-oceanic, lithospheric mantle section and are tectonically enclosed within the Avdella melange. The most sizeable block is a chromitite-bearing serpentinite showing a mesh texture. Accessory, subhedral to euhedral Cr-spinels in the serpentinite display Cr# [Cr/(Cr + Al)] values that range from 0.36 to 0.42 and Mg# [Mg/(Mg+ Fe2+ )] values that vary between 0.57 and 0.62, whereas the TiO2 content may be up to 0.47 wt.%. The serpentinite fragment is characterized by low abundances of magmaphile elements (Al2 O3 : 0.66 wt.%, CaO: 0.12 wt.%, Na2 O: 0.08 wt.%, TiO2 : 0.007 wt.%, Sc: 4 ppm) and enrichment in compatible elements (Cr: 2780 ppm and Ni: 2110 ppm). Overall data are in accordance with derivation of the serpentinite exotic block from a dunite that was formed in the mantle region underneath a back-arc basin before tectonic incorporation in the Korydallos melange. Two compositionally different chromitite pods are recognized in the studied serpentinite fragment, a Cr-rich chromitite and a high-Al chromitite, which have been ascribed to crystallization from a single, progressively differentiating MORB/IAT melt. Although both pods are fully serpentinized only the Al-rich one shows signs of limited Cr-spinel replacement by an opaque spinel phase and clinochlore across grain boundaries and fractures. Modification of the ore-making Cr-spinel is uneven among the Al-rich chromitite specimens. Textural features such as olivine replacement by clinochlore and clinochlore disruption by serpentine indicate that Cr-spinel alteration is not apparently related to serpentinization. From the unaltered Cr-spinel cores to their reworked boundaries the Al2 O3 and MgO abundances decrease, being mainly compensated by FeOt and Cr2 O3 increases. Such compositional variations are suggestive of restricted ferrian chromite (and minor magnetite) substitution for Cr-spinel during a short-lived but relatively intense, low amphibolite facies metamorphic episode (temperature: 400-700 °C). The presence of tremolite and clinochlore in the interstitial groundmass of the high-Al chromitite and their absence from the Cr-rich chromitite matrix imply that after chromitite formation a small volume of a high temperature, post-magmatic fluid reacted with Cr-spinel, triggering its alteration.

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

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

ON THE METAMORPHIC MODIFICATION OF CR-SPINEL COMPOSITIONS FROM THE ULTRABASIC ROCKS OF THE PINDOS OPHIOLITE COMPLEX (NW GREECE)

Serpentinites and serpentinised ultramafic rocks from the Pindos ophiolite complex, northwestern Greece, contain Cr-spinel grains that are usually altered. The extent of alteration differs among Cr-spinels and two alteration trends can be distinguished. The most dominant is characterised by Cr-spinel overgrown by Cr-magnetite, while the second shows gradual replacement of Cr-spinel by ferrian chromite locally combined with Cr-magnetite development. Compared to cores, the altered rims are enriched in Fe and show elevated Cr# in both types of alteration, while they are impoverished in Mg and Al only at the second one. The common association of Crmagnetite with serpentine and ferrian chromite with chlorite provides insights to the metamorphic context of their formation through processes that include metasomatism by cation diffusion exchange