Origin of mantle peridotites from the Vourinos Ophiolite Complex, Greece, as deduced from Cr-spinel morphological and chemical variations

Abstract

The Vourinos Ophiolite Complex in north-western Greece represents a fragment of oceanic lithosphere emplaced after the Late Jurassic closure of the Neotethyan Pindos Ocean. This Ophiolite Complex consists mainly of a well-preserved mantle sequence dominated by peridotite and chromitite. Based on petrographic features, four distinct peridotite varieties can be distinguished in the Vourinos mantle suite: i) normal harzburgite (Opx ≥ 15 vol. %), ii) transitional harzburgite (Opx < 15 vol. %), iii) coarse-grained dunite and iv) fine-grained dunite. Chromian spinel morphology, based on DR# (degree of roundness) measurements, varies systematically from harzburgitic to dunitic rocks. Anhedral chromian spinel occurs in normal harzburgite (DR# < 0.40), whereas in the other studied peridotite types this mineral is more euhedral (DR# > 0.40). The Cr# [Cr/(Cr + Al)] in chromian spinel increases from normal harzburgite to coarse-grained dunite, varying between 0.47 and 0.84. Such a variation in chromian spinel composition indicates a multi-stage melting evolution for the Vourinos mantle suite. Chromian spinel from the fine-grained dunite bears similarities in terms of Cr# and Mg# [Mg/(Mg + Fe 2+ )] with chromian spinel from the neighboring chromitites, which implies a common origin for both lithologies. Chromian spinel morphological and compositional data indicate that after chromitite and fine-grained dunite formation a remnant boninite melt could have invaded the adjacent peridotites, modifying their accessory chromian spinels or even crystallizing new chromian spinel grains. Further evidence for that episode of melt percolation is provided by a few harzburgitic rock samples, which may be locally enriched in LREE exhibiting U-shaped chondrite-normalized REE patterns. Such patterns are characteristic of interaction between the depleted mantle peridotite and hydrous boninitic melt. The studied peridotites are interpreted as refractory residues whose initial composition was locally modified by mantle metasomatism in the fore-arc region of a supra-subduction zone.

Petrogenetic Significance of Spinels From Serpentinised Peridotites From the Verianaousa Ophiolite

“…The textural features and composition of Cr-spinels (including magnesiochromites combined with the highly depleted nature of the harzburgite are consistent with evolution in a back-arc basin. The analysed spinel-group minerals from the Veria-Naousa serpentinised peridotites display similar chemical characteristics from the Othrys and Vourinos ophiolite complexes of analogous SSZ origin (Karipi et al, 2006;Kapsiotis, 2013).…”

Physiognomy and timing of metasomatism in the southern Vourinos ultramafic suite, NW Greece: a chronicle of consecutive episodes of melt extraction and stagnation in the Neotethyan lithospheric mantle

“…Analyses of spinel grains in peridotites from the entire Vourinos mantle suite were presented in a complementary study by Kapsiotis (2013). Therefore, in the current research merely spinel analyses of ultramafic rocks derived from the southern branch of the Vourinos ophiolite complex will be taken into account for petrogenetic considerations.…”

Genesis and tectonic implications of cumulate pyroxenites and tectonite peridotites from the Nagaland–Manipur ophiolites, Northeast India: constraints from mineralogical and geochemical characteristics

“…The composition of Cr‐spinel in peridotites reflects magma chemistry, degree of partial melting, fractional crystallization and variation in oxygen fugacity ( f O 2 ) (Dick and Bullen ; Barnes ; Kapsiotis ) or even the tectonic discrimination between mid‐ocean ridge and supra‐subduction zone settings (Dick and Bullen ; Arif and Jan ; Oh et al ; Singh ). It is also considered that Al content of Cr‐spinel is sensitive to mantle melting processes and that systematically decreases with the degree of peridotites depletion (Zhou et al , ; Uysal et al ).…”