The Makran Accretionary Wedge in southeast Iran 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 (McCall and Kidd, 1982). According to the existent interpretations, the Cretaceous tectonic evolution of the NMD implies the subduction of Neotethyan lithosphere beneath the Lut block and formation of an Early Cretaceous volcanic arc on its southern rim. In this time, the opening of a back-arc basin (known as the North Makran Ocean) triggered the separation of a continental ribbon (the Bajgan-Durkan microplate) from the southernmost edge of the Lut Block (Burg, 2018). In this view, the Bajgan and Durkan Complexes in the North Makran have been considered as remnants of a Paleozoic continental basement and its unconformable Early Cretaceous continental platform, respectively (Burg, 2018). However, recent studies have shown that the Durkan Complex consists of volcano-sedimentary sequences formed in a seamount setting (Barbero et al., 2021a, b), whereas the Bajgan Complex largely consists of metaophiolitic tectonic slices including metaperidotites, metagabbros, metaplagiogranites, metabasaltic lavas, metavolcaniclastic deposits, and metasedimentary pelagic rocks. U-Pb dating on zircons indicates age of the magmatic protoliths ranging from 156 to 112 Ma (Pandolfi et al., 2021). The aim of this study is, therefore, to present new geochemical data on metaophiolites in order to define petrogenesis and tectono-magmatic setting of formation of their magmatic protoliths. Based on geochemical data, three main groups of protoliths can be identified: 1) Ultramafic rocks showing variable compositions (e.g., MgO= 27-41 wt%; Cr=650-3500 ppm; V=29-265 ppm) that, together very depleted chondrite-normalized REE patterns strongly support a cumulitic nature; 2) mafic (gabbros and basalts) and acidic (plagiogranites) rocks showing subalkaline affinity with Nb/ ratios < 0.6. The overall geochemical features point out for a general MORB affinity. Nonetheless, based on many incompatible elements and REE, two subgroups of samples can be recognized. Subgroup 2a shows 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). In contrast, Subgroup 2b shows 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 ocean island basalt (OIB) affinity with Nb/Y ratios >1. They show 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). 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. Many incompatible element ratios (Zr/Y, Zr/Nb, TiO2/Yb, Nb/Yb) indicate plume-ridge interaction processes and residual garnet in the source of alkaline OIBs (Pearce, 2008). In fact, N-MORBs primary melts derived from ~15% partial melting of a depleted MORB-type mantle in the spinel facies. E-MORBs primary melts derived from ~10% partial melting of a depleted MORB-type mantle metasomatized by OIB-type components in the spinel facies. The composition of alkaline OIBs primary melts is compatible with ~6% partial melting of an enriched (plume-type) mantle that started to melt in the garnet facies and continued to melt largely 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 (e.g., Band-e-Zeyarat: Barbero et al., 2020; Durkan: Barbero et al., 2021a, 2021b; Coloured Mélange: Saccani et al., 2018) suggesting that the magmatic protoliths of the Bajgan metaophiolites were most likely formed in the same oceanic basin in which other NMD ophiolites were formed.
Geochemical and age data on the Bajgan Complex metaophiolites (Makran Accretionary Prism, SE Iran): New evidence for their magmatic formation in a Cretaceous oceanic domain
Barbero E.Conceptualization
;Saccani E.Validation
2021
Abstract
The Makran Accretionary Wedge in southeast Iran 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 (McCall and Kidd, 1982). According to the existent interpretations, the Cretaceous tectonic evolution of the NMD implies the subduction of Neotethyan lithosphere beneath the Lut block and formation of an Early Cretaceous volcanic arc on its southern rim. In this time, the opening of a back-arc basin (known as the North Makran Ocean) triggered the separation of a continental ribbon (the Bajgan-Durkan microplate) from the southernmost edge of the Lut Block (Burg, 2018). In this view, the Bajgan and Durkan Complexes in the North Makran have been considered as remnants of a Paleozoic continental basement and its unconformable Early Cretaceous continental platform, respectively (Burg, 2018). However, recent studies have shown that the Durkan Complex consists of volcano-sedimentary sequences formed in a seamount setting (Barbero et al., 2021a, b), whereas the Bajgan Complex largely consists of metaophiolitic tectonic slices including metaperidotites, metagabbros, metaplagiogranites, metabasaltic lavas, metavolcaniclastic deposits, and metasedimentary pelagic rocks. U-Pb dating on zircons indicates age of the magmatic protoliths ranging from 156 to 112 Ma (Pandolfi et al., 2021). The aim of this study is, therefore, to present new geochemical data on metaophiolites in order to define petrogenesis and tectono-magmatic setting of formation of their magmatic protoliths. Based on geochemical data, three main groups of protoliths can be identified: 1) Ultramafic rocks showing variable compositions (e.g., MgO= 27-41 wt%; Cr=650-3500 ppm; V=29-265 ppm) that, together very depleted chondrite-normalized REE patterns strongly support a cumulitic nature; 2) mafic (gabbros and basalts) and acidic (plagiogranites) rocks showing subalkaline affinity with Nb/ ratios < 0.6. The overall geochemical features point out for a general MORB affinity. Nonetheless, based on many incompatible elements and REE, two subgroups of samples can be recognized. Subgroup 2a shows 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). In contrast, Subgroup 2b shows 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 ocean island basalt (OIB) affinity with Nb/Y ratios >1. They show 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). 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. Many incompatible element ratios (Zr/Y, Zr/Nb, TiO2/Yb, Nb/Yb) indicate plume-ridge interaction processes and residual garnet in the source of alkaline OIBs (Pearce, 2008). In fact, N-MORBs primary melts derived from ~15% partial melting of a depleted MORB-type mantle in the spinel facies. E-MORBs primary melts derived from ~10% partial melting of a depleted MORB-type mantle metasomatized by OIB-type components in the spinel facies. The composition of alkaline OIBs primary melts is compatible with ~6% partial melting of an enriched (plume-type) mantle that started to melt in the garnet facies and continued to melt largely 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 (e.g., Band-e-Zeyarat: Barbero et al., 2020; Durkan: Barbero et al., 2021a, 2021b; Coloured Mélange: Saccani et al., 2018) suggesting that the magmatic protoliths of the Bajgan metaophiolites were most likely formed in the same oceanic basin in which other NMD ophiolites were formed.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
