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.