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

Abstract

Al-rich spinels were rarely reported compared to Cr-spinels, which were mostly observed in ophiolitic rocks. Pleonaste [ $(Mg,;Fe^{2+})Al_2O_4$], which is an Al-rich spinel, was observed in the ophiolitic mafic–ultramafic rocks tectonically located in the Early Cretaceousaccretionary complex, at the eastern part of the Armutlu peninsula, NW Turkey. The ophiolitic mafic–ultramafic rocks have cumulatecharacter, and most of them are represented by peridotite and pyroxenite. Pleonaste was observed in pyroxenites and gabbros of theophiolitic rocks. Pyroxenites consist mainly of clinopyroxene + orthopyroxene + amphibole ± olivine + spinel. Gabbros are composedof clinopyroxene + orthopyroxene + amphibole + plagioclase + spinel. Pleonaste in these rocks lies parallel to the magmatic layers andis distinguished by its emerald greenish color under the microscope. Pleonastes have high $Al_2O_3$ (59.65–62.24 wt.%) and low $Cr_2O_3$(0.05–1.32 wt.%) contents with Mg# and $Fe^{3+}$ # ranging from 54.23 to 59.77 and 3.83 to 4.28, respectively. Petrographical observationsand the pressure–temperature (P-T) pseudosection modelling suggest that pleonaste in the mafic–ultramafic rocks from the study areacrystallized during magmatic processes. Presence of amphibole and Ca-rich (An % 85–88) plagioclase in these rocks suggests that theophiolitic rocks, located in the Early Cretaceous accretionary complex at the eastern part of Armutlu peninsula, formed from an arcrelated hydrous magma source.