New geochemical and age data on the Bajgan Complex (Makran Accretionary Prism, SE Iran): Implications for the redefinition of its tectonic setting of formation from a Paleozoic continental basement to a Cretaceous oceanic domain.

The Makran Accretionary Wedge (SE Iran - SW Pakistan) includes several tectonic domains of different ages. Among them, the North Makran Domain (NMD) consists of a stack of tectonic units, which record the Cretaceous tectonic evolution of the Neotethys – Eurasia realm. According to the existent interpretations, the Cretaceous tectonic evolution of the NMD implies the subduction of the Neotethyan lithosphere beneath the Lut block and the formation of an Early Cretaceous volcanic arc on its southern rim. In this time, the opening of a back-arc basin (North Makran Ocean) led to the separation of a continental ribbon (the Bajgan-Durkan microplate) from the southernmost edge of the Lut Block. In this view, the Bajgan Complex has been considered as remnants of a Paleozoic continental basement. However, our field observations have revealed that the Bajgan Complex largely consists of metaophiolitic tectonic slices including metaperidotites, metagabbros, metaplagiogranites, and metabasaltic lavas. The aim of this study is, therefore, to present new geochemical and geochronological data on mafic metaophiolites in order to define the age and tectono-magmatic setting of formation of their magmatic protoliths. Based on geochemical data, three main groups of mafic protoliths can be identified: 1) rocks showing normal (N-) MORB affinity with low contents of Nb (1.3-3.3 ppm) and Th (0.10-0.24 ppm), Nb/Yb ratios and LREE depletion compared to Yb (LaN/YbN=0.5-0.7); 2) rocks showing enriched (E-) MORB affinity with moderate enrichment in Nb (9.7-11.4 ppm), Th (1.04-1.07 ppm), and Nb/Yb ratios, coupled with LREE enrichment compared to Yb (LaN/YbN=2-3); 3) rocks showing a clear alkaline affinity with high contents of Nb (19-65 ppm), Th (2.9-9.6 ppm), and TiO2 (1.8-2.3 wt%), high Nb/Yb ratios and significant enrichment in LREE compared to Yb (LaN/YbN=8-20). U-Pb dating on zircons indicates magmatic age of the protoliths ranging from 156 to 112 Ma (Late Jurassic-Lower Cretaceous). Petrogenetic models based on REE, Th, Nb, and TiO2 show that the different rock types were generated by partial melting of distinct sub-oceanic mantle sources: a) N-MORBs: ~15% partial melting of a depleted MORB-type mantle in the spinel facies; b) E-MORBs: ~10% partial melting of a depleted MORB-type mantle metasomatized by OIB-type components in the spinel facies; c) OIBs: ~6% partial melting of an enriched (plume-type) mantle that started to melt in the garnet facies and continued in the spinel facies. In conclusion, our data indicate that the interpretation of the Bajgan Complex as a Paleozoic continental ribbon should be abandoned. Rather, this Complex represents a long-lived (>50 My) oceanic lithosphere. The chemically composite nature of the Bajgan metaophiolites with N-MORBs, E-MORBs, and OIBs suggests that they represent an oceanic lithosphere characterized by mantle plume activity and plume–ridge interaction processes. Similar rock associations and processes have been documented in many other Cretaceous ophiolites of the NMD suggesting that the magmatic protoliths of the Bajgan metaophiolites were most likely formed in the same oceanic basin