Geochemistry of basaltic blueschists from the Deyader Metamorphic Complex (Makran Accretionary Prism, SE Iran): New constraints for magma generation in the Makran sector of the Neo-Tethys

The North Makran Domain in the Makran Accretionary Prism consists of an imbricate stack of Mesozoic Neo Tethyan ophiolitic and metaophiolitic units. The Deyader Complex is an important metamorphic unit of this Domain and includes tectonic slices of HP-LT blueschists derived from upper oceanic crust protoliths. The volcanic protoliths consist of basalts with Mg#=75.1-62.7, SiO2 = 41.59-48.02 wt%, and TiO2 = 0.79-1.53 wt%. Based on incompatible and REE contents and ratios, three geochemical types can be distinguished. Group 1 basalts show trace element compositions, and LaN/YbN and LaN/SmN ratios <1 resembling those of normal-type (N-) mid-ocean ridge basalts (MORB). Compared to N-MORBs, Group 2 basalts show slight enrichment in Th, Ta, Nb, as well as LaN/YbN and LaN/SmN ratios = ~1 similar to those observed in enriched-type (E-) MORB. Group 3 basalts show a transitional-alkaline nature (Nb/Y = 0.7-1.0), marked Th, Ta, Nb enrichment and high LaN/YbN (4-7) and LaN/SmN (2-3.5) ratios resembling those of plume-type (P-) MORB. Trace element and REE petrogenetic models show that N-MORB protoliths were generated from a depleted MORB mantle source (DMM), whereas E-MORB and P-MORB protoliths were generated from partial melting of a DMM source that was metasomatized to variable extents by OIB-type chemical components. The Deyader blueschists protoliths show close geochemical and petrogenetic similarities with all the North Makran ophiolites, suggesting that they were formed in a large Late Jurassic - Cretaceous oceanic basin that was strongly affected by mantle plume activity and different extents of plume-ridge interaction.