Petrogenesis of ultramafic rocks from the Neoproterozoic Bou Azzer Ophiolite (Morocco): new insights into a long-standing geodynamic question

Abstract

The Neoproterozoic Bou Azzer Ophiolite, located in the Pan-African orogen of the Anti-Atlas, Morocco, exposes a well-preserved mantle section of the ophiolitic sequence. Although ultramafic rocks are the most common rock types in the sequence, they have received less attention in previous studies. In this work, we investigate the petrogenesis of these ultramafic rocks in detail, including their petrology, mineralogy, and whole rock geochemistry. The mineral assemblages are mainly formed by antigorite with minor lizardite, chlorite, and accessory Cr-spinel. The only preserved primary mantle mineral is Cr-spinel, which shows unaltered cores with Cr# between 0.61 and 0.71 and alteration rims to ferrian chromite and magnetite. Whole rock analyses indicate almost total serpentinization and, locally, strong carbonation (mass loss of ignition between 11.94 and 22.20 wt%). The non-carbonated samples preserve protolith signatures, with decreased MgO/SiO2 suggesting metasomatic processes related to serpentinization. Immobile trace elements compare well with fore-arc peridotites, while melting modelling indicates intense and polyphasic melting events. We propose polyphase melting in a subduction-initiation setting, with a first stage of 14–23% anhydrous melting, forming fore-arc basalts, and a second stage of 15–25% melting with 1 wt% H2O, forming boninites. The Bou Azzer ultramafic rocks represent the fore-arc region of a supra-subduction zone, challenging previous interpretations that suggested a back-arc position.