1,354,601 research outputs found
Field relations, geochemistry and geodynamic implications of chromitites from the Bandan Mine, Eastern Iran
No full textThe Bandan chromite mine is the main chromite deposit in Sistan suture zone, eastern Iran. The chromite deposits are structurally tabular to lens-shaped bodies hosted by dunitic to harzburgitic mantle peridotites. Similar to Alpine type podiform chromites, chromitite pods are enclosed within dunitic envelops. The chromites show mainly massive to disseminated and also brecciated textures. Chemically, the Cr/Fe ratio is higher than 2 and TiO2 content in accordance with ophiolitic chromites is low (< 0.2 wt. %). As a result of low Cr# (Cr# = Cr x 100 / (Cr + Al)) ranging from 50 to 52 the Bandan chromite deposit is high-Al type. Calculated parental melt chemistry shows MORB (Mid-ocean ridge basalt)-type signature with Al2O3 and FeO/MgO ratio contents of 15-16 and 1.1-1.2, respectively. Although the geodynamic setting of high-Al podiform chromites have been debated but petrographical - geochemical characteristics of ophiolitic mantle-crust sequences may relate chromite genesis to supra-subduction zone setting
Geochemistry and petrogenesis of mantle peridotites from the Nehbandan Ophiolitic complex, Eastern Iran
No full textOphiolitic exposures from the East and Northeast of Nehbandan. Eastern Iran, are remnants of a Cretaceous oceanic basin between Lui and Afghan continental blocks. Based on petrographic studies, they contain harzburgite. clinopyroxene-harzburgite and Iherzolite equilibrated in spinel peridotite fades. Chemistry of clinopyroxene in Iherzolite is characterized by low Na20 and TiO2, contents suggesting sub-oceanic origin. Spinel is a ubiquities accessory phase yielding chromium number [Cr# = l00*Cr/(Cr+Al)] in the range of 31-37 and 15-21 for harzburgite and Iherzolite. respectively. Melting degrees on the basis of spinel chemistry is 5 to 8% for Iherzolite and 12.5 to 14.2% for harzburgite. Cr# of magmalic spinel is 50-51 which correlate with Al-rich type of podiform chromitites. Whole-rock chemistry shows that most of the peridotitic samples exhibit evident U-shaped primitive mantle-normalized rare earth element patterns. In addition, in multi-element diagrams, relative to neighboring ra..
Petrological and geochemical constraints on the origin of the Nehbandan ophiolitic complex (eastern Iran): Implication for the evolution of the Iranian sector of the Neo-Tethys
No full textPetrological and geochemical constraints on the origin of the Nehbandan ophiolitic complex (eastern Iran): Implication for the evolution of the Sistan Ocean
No full textThe Nehbandan ophiolite complex (NOC) crops out in the Sistan suture zone, which marks the boundary between the Lut and Afghan continental blocks. The NOC is composed of various ophiolitic lithotypes included in a tectono-sedimentary mélange, which are commonly interpreted as remnants of the oceanic lithosphere of the Sistan Ocean. Three different sequences (or associations) of ophiolitic rocks can be recognized in the NOC: (1) mantle peridotites consisting of clinopyroxene- (cpx-) rich harzburgites and depleted harzburgites; (2) olivine websterite–pyroxenite–gabbronorite sequence; (3) wehrlite–troctolite–cumulate gabbro–isotropic gabbro–basalt sequence. Petrographic observations, mineral chemistry, whole- rock chemistry, and rare earth element (REE) modelling carried out on the different rock associations led to the following conclusions: (1) the wehrlite–troctolite–cumulate gabbro–isotropic gabbro–basalt association represents a portion of oceanic crust generated in a mid-ocean ridge setting; (2) the cpx-rich harzburgites represent the residual mantle after 5–20% removal of mid-ocean ridge basalt-type (MORB) melt. This residual mantle was subsequently enriched in light REE (LREE) by subduction-derived fluids in a supra-subduction zone (SSZ) setting and it compositionally represents the typical mantle source for boninitic melts; (3) the olivine websterite–pyroxenite–gabbronorite association represents a portion of oceanic crust generated in an intra-oceanic arc setting; (4) the depleted harzburgites represent the residual mantle after 10–30% removal of boninitic melts in an intra-oceanic arc setting. The data presented in this paper provide new constraints for the tectonic evolution of the Iranian sector of the Neo-Tethys. In fact, in contrast with previous geodynamic models, the occurrence of SSZ ophiolites in the NOC implies that the phase of convergence between the Lut and Afghan blocks, which led to the closure of the Sistan Ocean, was accompanied by the development of an intra-oceanic arc setting
Petrology and tectono-magmatic significance of the Nehbandan ophiolite (Sistan Suture Zone, E. Iran): Implication for the evolution of the Iranian Neo-Tethys.
No full textAssociation of MORB and SSZ ophiolites along the shear zone between Coloured Mélange and Bajgan Complexes (North Maran, Iran): Evidence from the Sorkhband area
No full textOne of the largest worldwide accretionary wedges is exposed in the Makran region (SE Iran). The backstop of this accretionary wedge consists of an imbricate stack of continental and oceanic units, referred as North Makran domain. This domain is characterized by a km-thick shear zone, along which the metamorphic Bajgan Complex is thrust onto the Coloured Mélange Complex. Along this shear zone two slices of ophiolites have been identified in the Sorkhband area. The upper tectonic slice consists of gabbros, whereas the lower one consists of mantle peridotites associated with dunites and chromitite ore deposits. Petrography and geochemistry of gabbros clearly indicate an N-MORB-type affinity, suggesting that they were generated at mid-ocean ridge setting. In contrast, mantle peridotites consist of harzburgites and depleted harzburgites, both showing geochemical features suggesting their genesis in a SSZ setting. The new data presented in this paper indicate that the slices of ophiolites from Sorkhband area derived from two different oceanic domains representing two different geodynamic settings. This occurrence provides new evidence that the boundary between the Coloured Mélange and the Bajgan Complexes represents a first-order tectonic structure that played an important role in the geodynamic evolution of the Makran area
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.
No full textThe 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 basi
The oceanic units of the Sorkhband area (north Makran, Iran): evidence of an association of MORB and SSZ ophiolites at the top of the Coloured Mélange Complex
No full textIn the Makran region, SE Iran, one of the largest worldwide accretionary wedges is exposed. This accretionary wedge is regarded as the result of the northward subduction of the oceanic lithosphere of the Oman Sea beneath the Lut and Afghan continental blocks. To the North, the accretionary wedge is bounded by the north Makran domain that can be regarded as the backstop of the accretionary wedge. The North Makran domain is represented by an imbricate stack of continental and oceanic units including, the Coloured Mélange Complex, also referred as the Imbricate Zone.
The Coloured Mélange Complex is part of the North Makran domain (SE Iran) and consists of an assemblage of metric-to decametric-thick slices mainly represented by volcanic rocks, locally stratigraphically associated with radiolarian cherts. We had got new geochemical data on volcanic rocks and biochronological data on the associated cherts. Our data indicate the occurrence of a wide range of volcanic rocks-types, which are: 1) normal-type mid-ocean ridge basalts (N-MORB); 2) oceanic plateau basalts (OPB); 3) alkaline basalts; 4) calcalkaline basalts, basaltic andesites, andesites, and dacites; 5) volcanic arc tholeiitic basalts and dacites, and high pressure-low temperature metabasalts formed in deep levels of an accretionary wedge. The volcanic arc tholeiites range from Early (late Hauterivian-early Aptian) to Late (latest Cenomanian-lower late Campanian) Cretaceous, whereas the calcalkaline rocks and OPBs are Late Cretaceous in age (early Coniacian- Santonian and early Turonian-early Campanian, respectively). Alkaline basalts, OPBs, and N-MORBs represent remnants of the Mesozoic Neo-Tethys oceanic branch located between the Arabian plate and the Lut block. In this paper we document that this oceanic sector was characterized by the development of an oceanic plateau in the Late Cretaceous. In contrast, calc-alkaline and volcanic arc tholeiitic rocks represent remnants of a continental volcanic arc and forearc, respectively, developed onto the southernmost realm of the Lut block. The petrogenesis and age of volcanic rocks allow us to propose a new tectono-magmatic model for the evolution of the convergent margin developed in the northern sector of the Neo- Tethys from Early to Late Cretaceous. This model is basically constrained by the collision of the oceanic plateau with the continental arc, which led to the jump of the subduction toward the south, as well as to the formation of the imbricate pile of different units today observed in the North Makran are
New insights into the geodynamics of Neo-Tethys in the Makran area (2017). evidence from age and petrology of ophiolites from the Coloured Mélange Complex (SE Iran).
No full textIn the Makran region, SE Iran, one of the largest worldwide accretionary wedges is exposed. This accretionary wedge is regarded as the result of the northward subduction of the oceanic lithosphere of the Oman Sea beneath the Lut and Afghan continental blocks. To the North, the accretionary wedge is bounded by the north Makran domain that can be regarded as the backstop of the accretionary wedge. The North Makran domain is represented by an imbricate stack of continental and oceanic units including, the Coloured Mélange Complex, also referred as the Imbricate Zone.
The Coloured Mélange Complex is part of the North Makran domain (SE Iran) and consists of an assemblage of metric-to decametric-thick slices mainly represented by volcanic rocks, locally stratigraphically associated with radiolarian cherts. We had got new geochemical data on volcanic rocks and biochronological data on the associated cherts. Our data indicate the occurrence of a wide range of volcanic rocks-types, which are: 1) normal-type mid-ocean ridge basalts (N-MORB); 2) oceanic plateau basalts (OPB); 3) alkaline basalts; 4) calcalkaline basalts, basaltic andesites, andesites, and dacites; 5) volcanic arc tholeiitic basalts and dacites, and high pressure-low temperature metabasalts formed in deep levels of an accretionary wedge. The volcanic arc tholeiites range from Early (late Hauterivian-early Aptian) to Late (latest Cenomanian-lower late Campanian) Cretaceous, whereas the calcalkaline rocks and OPBs are Late Cretaceous in age (early Coniacian- Santonian and early Turonian-early Campanian, respectively). Alkaline basalts, OPBs, and N-MORBs represent remnants of the Mesozoic Neo-Tethys oceanic branch located between the Arabian plate and the Lut block. In this paper we document that this oceanic sector was characterized by the development of an oceanic plateau in the Late Cretaceous. In contrast, calc-alkaline and volcanic arc tholeiitic rocks represent remnants of a continental volcanic arc and forearc, respectively, developed onto the southernmost realm of the Lut block. The petrogenesis and age of volcanic rocks allow us to propose a new tectono-magmatic model for the evolution of the convergent margin developed in the northern sector of the Neo- Tethys from Early to Late Cretaceous. This model is basically constrained by the collision of the oceanic plateau with the continental arc, which led to the jump of the subduction toward the south, as well as to the formation of the imbricate pile of different units today observed in the North Makran are
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
No full textThe 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
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