Chemical composition of detrital chrome spinel and clinopyroxene from the Middle Eocene to lowermost Oligocene sandstones of central Iran (Nain area): Constraints on ophiolite provenance

Abstract

Studies of relict Upper Cretaceous obducted ophiolites provide information on the evolution of the Neotethys suture belts in central Iran. Studies of the sediment derived from their erosion will complement this information by addressing questions related to sandstone provenance, ophiolite emplacement history, and, perhaps most importantly, the mode and timing of their erosion. The ca. 3200 m thick Middle Eocene to lowermost Oligocene fine- to medium-grained siliciclastic beds of the Akhoreh Formation are superbly exposed along the western border of the Central-East Iranian Microcontinent. They cover the northeastern flank of the Cretaceous Nain ophiolite mélange massif and lie adjacent to the Paleogene Urmieh–Dokhtar Magmatic Arc. The common occurrence of chrome (Cr)-spinel in the sandstones indicates the presence of mafic to ultramafic rocks in the source region. However, the timing, extent and nature of contributions of the potential source rocks to the sedimentary succession are not well understood. We conducted a petrographic and chemical compositional study of Cr-spinel in samples of Akhoreh Formation sandstones to outline the type of ocean lithospheric source. Our results document variable Cr# (0.09 to 0.79) and Mg# (0.21 to 0.88) while Fe3+ and TiO2 (average 0.12 wt%) are generally low. The wide range of Cr# and Mg# indicates provenance from a variety of (ultra-)mafic rock types, including supra-subduction peridotites and partially volcanic rocks of an arc-forearc setting. Cr-spinels are chemically similar to those of the lherzolites, harzburgites and dunites of the Nain ophiolite. While 70 % of the Cr-spinel have Cr# > 0.6, suggesting a harzburgitic peridotite provenance, the minimum Cr# < 0.4, observed in the samples from the base of the succession, were likely sourced from lherzolite. The chemical composition of detrital clinopyroxene grains shows high SiO2 (52.76 wt%), CaO (20.15 wt%), and MgO (16.36 wt%), while FeO has moderate (6.03 wt%), Al2O3 has low (2.23 wt%), and TiO2 has very low (0.25 wt%) concentrations (commonly augite and diopside), indicative of crystallization from subalkaline magma in an arc setting. The provenance data trace the erosion history of the obducted ophiolite and indicate that the Nain supra-subduction ophiolite supplied detritus to the Akhoreh Basin northwards from the Middle Eocene to the earliest Oligocene.