The earliest stage of mantle-melt evolution during subduction initiation: Evidence from the Neo-Tethyan Mirdita Ophiolite, Albania

Mantle-melt evolution processes in the earliest stage of subduction initiation (SI) are still ambiguous. Here, we report the systematic petrogeochemical and physical records of the SI processes demonstrated by the Gomsiqe massif, Mirdita ophiolite. The Gomsiqe ultramafic body consists of basal lherzolites, upper harzburgites con-taining lentoid dunites and disseminated chromitites, displaying transition of petrological (Cr# of spinels range from 12.7 to 47.1) and geochemical signatures. Clinopyroxene trace element modeling indicates that lherzolites and harzburgites have undergone variable garnet-facies melting prior to spinel-facies melting. Harzburgites encountered higher degrees of spinel-facies melting compared with those of lherzolites (-8-10% vs. -3-5%), and late-stage MORB melt metasomatism. Lherzolites escaped metasomatism. The dunites and chromitites formed by interactions between the harzburgitic lithosphere and MORB-like transitional magmas continued, but with limited hydrous fluid involvement during subduction. The transitional characteristics of metasomatic melt composition and physical condition (i.e., oxygen fugacity [fo2]) of different lithologies are consistent with the corresponding archive in the Izu-Bonin-Mariana forearc. Lherzolites and harzburgites have comparable fo2 to that of the abyssal peridotites. The dunites are apparently more oxidized compared to the associated chromitites, further supporting a hypothesis that sulfur-saturated MORB melts were responsible for the formation of chro-mitites and reduced Cr# of chromitite spinels. Thermometry showed that lherzolites preserved higher equili-bration temperatures than harzburgites. Both experienced a later lower-temperature re-equilibration process. Thus, the Mirdita ophiolite preserves multi-stage mantle-melt evolution processes, which are the comprehensive physicochemical responses of the earliest stage of SI.

Automated summary

This study investigates the mantle-melt evolution processes during the earliest stage of subduction initiation using the Gomsiqe massif in the Mirdita ophiolite. The results show transitions in petrological and geochemical signatures and provide insights into the physicochemical responses of subduction initiation.