Origin of the pleonaste-bearing mafic-ultramafic rocks from the Armutlu peninsula, NW Turkey

Authors: MUTLU ÖZKAN, ÖMER FARUK ÇELİK

Abstract: Al-rich spinels were rarely reported compared to Cr-spinels, which were mostly observed in ophiolitic rocks. Pleonaste [(Mg, Fe²⁺)Al₂O₄], which is an Al-rich spinel, was observed in the ophiolitic mafic-ultramafic rocks tectonically located in the Early Cretaceous accretionary complex, at the eastern part of the Armutlu peninsula, NW Turkey. The ophiolitic mafic-ultramafic rocks have cumulate character, and most of them are represented by peridotite and pyroxenite. Pleonaste was observed in pyroxenites and gabbros of the ophiolitic rocks. Pyroxenites consist mainly of clinopyroxene + orthopyroxene + amphibole ± olivine + spinel. Gabbros are composed of clinopyroxene + orthopyroxene + amphibole + plagioclase + spinel. Pleonaste in these rocks lies parallel to the magmatic layers and is distinguished by its emerald greenish color under the microscope. Pleonastes have high Al₂O₃ (59.65-62.24 wt.%) and low Cr₂O₃ (0.05-1.32 wt.%) contents with Mg# and Fe³⁺# ranging from 54.23 to 59.77 and 3.83 to 4.28, respectively. Petrographical observations and the pressure?temperature (P-T) pseudosection modelling suggest that pleonaste in the mafic-ultramafic rocks from the study area crystallized during magmatic processes. Presence of amphibole and Ca-rich (An % 85-88) plagioclase in these rocks suggests that the ophiolitic rocks, located in the Early Cretaceous accretionary complex at the eastern part of Armutlu peninsula, formed from an arc-related hydrous magma source.

Keywords: Cumulate, ophiolite, pseudosection models, pyroxenite, spinel

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