Petrogenesis and geochemistry of the Eocene volcanic sequence in the northeast of Zanjan: Implications for active continental margin magmatism in the Alborz- Azarbaijan Zone

The Eocene volcanic sequence in NE of the Zanjan and in the Alborz-Azarbaijan zone is mostly intermediate in composition and is accompanied with pyroclastic tuff and breccia. Petrographic studies reveal that these volcanic rocks are andesite and trachy-andesite. The common textures are porphyritic and glomeroporphyritic and the phenocrysts are composed of plagioclase, pyroxene and amphibole minerals. Also, calcite, quartz, chlorite and epidote are the secondary phases. According to geochemical classification, Theses rocks are plotted on the fields of andesite and trachy-andesite. Geochemically, the study rocks are enriched in LREEs and LILEs relative to HFSEs. Petrographical observations along with geochemistry of rare earth and trace elements suggest calc-alkaline affinity of the rocks under discussion as well as crustal assimilation, fractional crystallization and derivation from a subducted-related environment. Accordingly, the studied rocks are analogous with tectonic features of active continental margin arc settings

Evidence for an early-MORB to fore-arc evolution within the Zagros suture zone: Constraints from zircon U Pb geochronology and geochemistry of the Neyriz ophiolite (South Iran)

Late Cretaceous Neyriz ophiolite, as a part of the Outer Zagros Ophiolitic Belt, represents a remnant of the southern Neo-Tethyan oceanic lithosphere exposed along the Zagros suture zone in the south of Iran. Neyriz ophiolite is composed of dismembered lithological units including variably depleted mantle peridotites as well as intrusive and extrusive rocks preserving a geochemical evolution from mid-ocean ridge (MORB) to subduction-related (boninitic) signatures. Initially, the southern Neo-Tethyan oceanic lithosphere formed in the mid-ocean ridge spreading centre between the Arabian plate to the south west and the Eurasian plate to the north east. The inception of intra-oceanic subduction in the northern margin of the Arabian plate caused a slightly metasomatised supra-subduction-zone (SSZ) mantle lithosphere, producing MORB-like melts with a high degree of partial melting (~25–30%), and complementary depleted residual clinopyroxene (Cpx)-rich harzburgite. The zircon U-Pb geochronology for plagiogranite and gabbro intrusions reveals formation ages of 100.1 ± 2.3 Ma to 93.4 ± 1.3 Ma, respectively (Cenomanian). At a later stage, the rapid slab rollback and associated fore-arc spreading led to asthenospheric diapirism and arc-wedge mantle corner flow, which produced boninitic-like melts with advanced degrees of shallow partial melting (~35%), and ultra-depleted residual Cpx-poor harzburgites. The pervasive interaction between boninitic-like melts and refractory Cpx-poor harzburgites produced dunites as replacive lenses. Thus, this study postulates that Neyriz ophiolite formed from the Neo-Tethyan intra-oceanic subduction system, and preserves evidence of the evolution from an early-MORB oceanic crust to an extending fore-arc basin during subduction rollback processes. This tectonic environment is common in the Eastern Mediterranean region, where SSZ ophiolites have resulted in extended protoarc-forearc settings above subducted slabs

Significance of Nain-Baft ophiolitic belt (Iran) : short-lived, transtensional Cretaceous back-arc oceanic basins over the Tethyan subduction zone

Four dismembered massifs belonging to the Nain-Baft ophiolitic belt (Central Iran) stretch in a NW-SE direction parallel to the fossil active margin of the Iranian Continental Block (Sanandaj-Sirjan Zone). They are separated by huge transcurrent faults. The Nain, Dehshir, Shahr-e-Babak and Baft massifs are composed of associated slices of harzburgites, small bodies of gabbros and dike swarm complexes, accompanied by various extrusives from basaltic-andesitic lava flows and breccias to dacites and rhyolites. Trace element geochemistry of these lavas displays calc-alkaline and arc-tholeiite signatures, suggesting a back-arc origin for these ophiolites. This is in accordance with the position of these massifs, to the North of the Mesozoic Magmatic Arc crosscutting the Sanadaj-Sirjan Zone. Conventional K-Ar datings on amphibole within amphibolite and gabbros deliver ages between 93 Ma and 67 Ma. These ages are in good agreement with the stratigraphic age of the conformably Cenomanian to Maastrichtian sedimentary cover of the extrusives. The closure of these back-arc basins occurred in the Middle Paleocene as testified by the presence of neritic limestones, sealing all the tectonic contacts. The general geological setting of the Nain-Baft belt suggests that these massifs generated in a transtensional small back-arc basins separated by transcurrent faults. These short-lived transtensional basins result from the oblique subduction of the Tethyan Ocean under the Iranian Continental Block.13 page(s

The Eastern Makran Ophiolite (SE Iran): evidence for a Late Cretaceous fore-arc oceanic crust

The nature, magmatic evolution, and geodynamic setting of both inner and outer Makran ophiolites, in SE Iran, are enigmatic. Here, we report mineral chemistry, whole-rock geochemistry, and Sr–Nd–Pb isotope composition of mantle peridotites and igneous rocks from the Eastern Makran Ophiolite (EMO) to assess the origin and tectono-magmatic evolution of the Makran oceanic realm. The EMO includes mantle peridotites (both harzburgites and impregnated lherzolites), isotropic gabbros, diabase dikes, and basaltic to andesitic pillow and massive lava flows. The Late Cretaceous pelagic limestones are found as covers of lava flows and/or interlayers between them. All ophiolite compo- nents are somehow sheared and fragmented, probably in Cenozoic time, during the emplacement of ophiolite. This event has produced a considerable extent of tectonic melange. Tectonic slices of trachy-basaltic lavas with oceanic island basalt (OIB)-like signature seal the tectonic melange. Our new geochemical data indicate a magmatic evolution from fore-arc basalt (FAB) to island-arc tholeiite (IAT)- like signatures for the Late Cretaceous EMO lavas. EMO extrusive rocks have high εNd(t) (+8 to +8.9) and isotopically are similar to the Oman lavas. This isotopic signature indicates a depleted mid-ocean ridge basalt (MORB) mantle source for the genesis of these rocks, except isotopic gabbros containing lower εNd(t) (+5.1 to +5.7) and thus show higher contribution of subducted slab components in their mantle source. High 207Pb/204Pb and 208Pb/204Pb isotopic ratios for the EMO igneous rocks also suggest considerable involvement of slab-derived components into the mantle source of these rocks. The variable geochemical signatures of the EMO lavas are mostly similar to Zagros and Oman ophiolite magmatic rocks, although the Pb isotopic composition shows similarity to the isotopic characteristic of inner Zagros ophiolite belt. This study postulates that the EMO formed during the early stages of Neo-Tethyan subduction initiation beneath the Lut block in a proto-forearc basin. We suggest subduction initiation caused asthenospheric upwelling and thereafter melting to generate the MORB-like melts. This event left the harzburgitic residues and the MORB-like melts interacted with the surrounding peridotites to generate the impregnated lherzolites, which are quite abundant in the EMO. Therefore, these lherzolites formed due to the refertilization of mantle rocks through porous flows of MORB-like melts. The inception of subduction caused mantle wedge to be enriched slightly by the slab components. Melting of these metasomatized mantle generated isotropic gabbros and basaltic to andesitic lavas with FAB-like signature. At the later stage, higher contribution of the slab- derived components into the overlying mantle wedge causes formation of diabase dikes with supra- subduction zone – or IAT-like signatures. Trachy-basalts were probably the result of late-stage magmatism fed by the melts originated from an OIB source asthenospheric mantle due to slab break-off. This occurred after emplacement of EMO and the formation of tectonic melange

Late Ediacaran iron formations in NW Iran: Origin, depositional age, tectonic and climatic significance

International audienceThe late Neoproterozoic Takab banded iron formation (BIF) occurs in one of the most important part of the Iranian plateau containing Precambrian crustal segments, now embedded in the Alpine-Himalayan orogenic system. The Takab BIF is hosted by low to medium grade metamorphic sequence including schist, quartzite and marble and its structure is dominated by bands or streaks. The ore body is mostly composed of quartz and iron oxides (magnetite, hematite, maghemite, goethite in different proportions), occurring as alternating iron- and silica-rich laminates. LA-ICP-MS U–Pb dating on zircons in schists from both the lower and upper layers of the orebody indicate a depositional age of ca. ∼560 Ma for iron deposits. Major, trace and rare earth elements of the ore body support the mixing of seawater and hydrothermal fluids through iron precipitation and up to ca. 20 % incorporation of terrigenous materials into the chemical precipitate of the original BIF. The provenance indicators as well Nd isotopic data, point towards the Neoproterozoic felsic-to-intermediate crystalline basement of the Takab area as the main sediment source. Fe isotope data together with Ce anomalies suggest that suboxic conditions prevailed in Ediacaran seawater at the time of deposition of the ore body. Combined with geochronological and geochemical data from other Neoproterozoic formations and crystalline rocks on the Iranian plateau, as well as supportive evidence for the occurrence of glacial sediments and bimodal volcanism in the studied area, we propose that the formation of the Takab BIF occurred in a back-arc basin environment, hence corresponding to a Rapitan-type iron ore. The co-occurrence of late Ediacaran BIFs and glaciogenic metasedimentary formations suggest the presence of active-magmatic marine anoxic basins at ca. ∼560 Ma, equivalent to the Ediacaran post-Gaskiers glaciation time

In Situ Trace Element and Fe-O Isotope Studies on Magnetite of the Iron-Oxide Ores from the Takab Region, North Western Iran: Implications for Ore Genesis

International audienceThe early Cambrian Takab iron ore deposit is situated in the northern part of the Sanandaj-Sirjan zone, western Iran. It consists of banded, nodular and disseminated magnetite hosted in folded micaschists. Trace element and Fe and O isotopic experiments reveal various hydrothermal precipitation environments under reduced to slightly oxidizing conditions. Disseminated magnetite has high Ti (945-1940 ppm) positively correlated with Mg + Al + Si, and heavy Fe (+0.76 to +1.86‰) and O (+1.0 to +4.07‰) isotopic compositions that support a magmatic/high-T hydrothermal origin. Banded magnetite has low Ti (15−200 ppm), V (≤100 ppm), Si and Mg (mostly ≤300 ppm) and variable Al. The ∂ 56 Fe values vary from −0.2‰ to +1.12‰ but most values also support a magmatic/high-T hydrothermal origin. However, variable ∂ 18 O (−2.52 to +1.22‰) values provide evidence of re-equilibration with lower-T fluid at ~200-300 • C. Nodular magnetite shows high Mn (≤1%), and mostly negative ∂ 56 Fe values (average, −0.3‰) indicative of precipitation from an isotopically light hydrothermal fluid. Re-equilibration with carbonated rocks/fluids likely results in a negative Ce anomaly and higher ∂ 18 O (average, +6.30‰). The Takab iron ore deposit has, thus, experienced a complex hydrothermal history

Resolving the source and ore-forming processes of the Takab Iranian BIF using Fe and O isotope pairs in magnetite.

International audienceIron ore deposits from Iran are spatially related to the main suture zones of the Iranian continental fragmented block. In western Iran, the Sanandaj-Sirjan structural zone (SSZ) hosts several iron ore deposits interpreted being of volcano sedimentary, hydrothermal or mixed volcano sedimentary-skarn origin. In the northern part of the SSZ the early Cambrian (~530 Ma) Takab iron ore deposit consists of disseminated, layered and nodular magnetite mainly hosted in folded micaschists, and also in calcschists or metavolcanics. Quartz may show grain boundary migration and feldspar is partly altered. Accessory minerals are Mn-Ba-oxides, barite, monazite &#177; uraninite and Mn-carbonate (in calschists) in the matrix or in cross-cutting veins.The low concentrations of Cr (1in disseminated (1.2) and nodular magnetite (2.2), but <1 in layered magnetite (0.5-0.7). Nodular magnetite shows a negative Ce anomaly, similar to that of the calcschists. These results indicate mixing of hot hydrothermal fluid and seawater during the precipitation of the Takab BIF.In nodular magnetite the average &#8706;56Fe of -0.3 &#8240; is typical of low T-hydrothermal environment, while the heavier &#8706;56Fe (1.4 &#8240;) in disseminated magnetite points to magmatic or magmatic-hydrothermal fluid. &#8706;56Fe data in the layered magnetite are variable (-0.2 to +1.12 &#8240;) but mostly in the magmatic-hydrothermal box of discrimination diagrams. &#8706;18O values are positive in disseminated and nodular magnetite (+2.15&#8240; and +5.30 &#8240; respectively on average), and vary from -2.52 &#8240; to +1.22 &#8240; in layered magnetite.Based on the trace elements and REE data it can be concluded that primary layered magnetite ore crystallized statically from a Fe-Si rich mixed seawater and hot hydrothermal fluid. Regional deformation induced dynamic recrystallization of quartz, and disruption of magnetite bands.The chemical and isotopic signature of the disseminated magnetite points to a predominant imprint of an ortho-magmatic fluid. However, post primary mineralization hydrothermal alterations complicate the signal recorded by magnetite and evidence a complex story: for example, the lighter &#8706;18O of layered magnetite suggests re-equilibration with low temperature fluid. Similarly, the low &#8706;56Fe of nodular ore results likely from the precipitation of magnetite from a light hydrothermal fluid that may have dissolved a primary magnetite with heavy iron isotope signature. Moreover, re-equilibration with carbonated rocks likely results in the observed negative Ce anomaly and higher &#8706;18O (up to 6.30 &#8240; on average)

Late Neoproterozoic Takab iron formation, NW Iran: Implication for BIF depositional setting

International audienceThe Takab banded iron formation occurred in one of the most important part of the Iranian plateau containing Precambrian crustal segments, now embedded in the Alpine-Himalayan orogenic system. The Takab BIF is hosted in low to medium grade metamorphic rocks including schist, quartzite and marble as well meta-basalt and meta-rhyolite interlayers. The structures of the iron formation are dominated by bands or streaks. The ore body is composed of alternating iron-and silica-rich laminates predominately composed of quartz and mainly magnetite that may be partly transformed into hematite, and goethite. The iron layers and lenses have followed the foliation and deformation of host rocks. All the Fe deposits display similar geological features, with variable trending and dipping, because of folding. The studied iron ore deposits occurred in forms of banded, disseminated and nodular ores. The ore laminas vary in thickness from a few mm to 4 cm. U–Pb dating results on detrital zircons of the associated schist rocks from lower and upper layers of the orebody bracket the age of 550 Ma for the iron deposition. Major, trace and rare earth elements support the contribution of seawater and hydrothermal fluids through iron precipitation and up to ca. 20% incorporation of terrigenous materials into the chemical precipitate of the original BIF. The geochemical as well as Nd isotopic data, suggest the Neoproterozoic felsic to intermediate crystalline basement in the Takab area as the precursor of sedimentary input. The Ce anomaly suggest suboxic condition for seawater mass during Ediacaran. Based on geochronological and geochemical data from the Takab iron formation together with those reported from other Neoproterozoic formations and crystalline rocks in the Iranian plateau, as well evidence from glacial sediments and the bimodal volcanism in study area, we propose that the Takab BIF formed in a back-arc basin environment, and can be classified as Rapitan-type. The occurrences of late Ediacaran BIFs and metasedimentary formations with glaciogenic records suggest the presence of active-magmatic marine anoxic basins at ca. ∼555 Ma, equivalent to the Ediacaran Sinian glaciation time interval (600-550 Ma)

Magnetite-Hematite Characterization at Micron Scale with Implications for Metallurgical Processing and Decarbonization †

International audienceMagnetite deposits represent important iron ore resources. Selective sorting of valuables from gangue and targeting of potential critical metals that can be recovered from waste streams must be implemented from the exploration and excavation steps onwards. Optical and scanning electron microscopy, electron microprobe analysis, dual-energy X-ray transmission, and computed tomography were applied to determine the mineralogy and classify the iron oxides of different iron ore types. These characteristics can be used for sorting at the exploration and extraction steps to reduce unvaluable materials at the loading and hauling steps, which contribute about 50% of the greenhouse gas emissions of the iron ore mining and mineral processing sector. These data also contribute to fine-tuning mineral processing parameters

Potential of Dual and Multi Energy XRT and CT Analyses on Iron Formations

International audienceDual and multi energy X-ray transmission imaging (DE-/ME-XRT) are powerful tools to acquire quantitative material characteristics of diverse samples without destruction. As those X-ray imaging techniques are based on the projection onto the imaging plane, only two-dimensional data can be obtained. To acquire three-dimensional information and a complete examination on topology and spatial trends of materials, computed tomography (CT) can be used. In combination, these methods may offer a robust non-destructive testing technique for research and industrial applications. For example, the iron ore mining and processing industry requires the ratio of economic iron minerals to siliceous waste material for resource and reserve estimations, and for efficient sorting prior to beneficiation, to avoid equipment destruction due to highly abrasive quartz. While XRT provides information concerning the thickness, areal density and mass fraction of iron and the respective background material, CT may deliver size, distribution and orientation of internal structures. Our study shows that the data provided by XRT and CT is reliable and, together with data processing, can be successfully applied for distinguishing iron oxide rich parts from waste. Furthermore, heavy element bearing minerals such as baryte, uraninite, galena and monazite can be detected