جستجوی مقالات مرتبط با کلیدواژه « serpentinite » در نشریات گروه « علوم پایه »

The south of Bavanat area is located in the high Zagros zone and it is considered a part of the Neyriz ophiolite. In this area, the ophiolitic complex is small coloured melanges include radiolarite cherts and serpentinites. The serpentinites composed of serpentine, spinel, opaque (as residual minerals from protholith), amphibole, rarely olivine and pyroxene. Olivines have been highly altered to serpentine and pyroxenes to bastite. Low values of Al2O3 and very low TiO2 against high values MgO, Cr, Ni and V in these serpentinites indicate the protholith of harzburgite- dunite tectonites. Also, very low values of CaO in these serpentinites indicate the low amount of clinopyroxene in the protholith and its harzburgite-dunite type. Based on the chemistry of the major, minor and rare elements, the primary peridotites that make up these rocks are formed in the suprasubduction-fore arc zone from a mantle affected by the reaction with the melt (melt-rock reaction) resulting from the subducted plate.

The subduction and closure of the vast Neo-Tethys ocean between the Arabian and Eurasian plates has left numerous ophiolitic traces, the unique position of Iran in its central part is noticeable. The lack of information, right on the border of Iran with Iraq and Turkey, due to security considerations, has so far prevented the overview of this suture zone in the northwestern border of Iran. Adding Gysian ophiolite in southern Urmia as a missing link in this stretch can partially cover this lack of information. A comparative study of whole rock chemistry of serpentinites in the central part of the Neo-Tethys ophiolites, considering several sectors from Iran (Kamyaran, Marivan and Gysian), Iraq (Penjwin and Mawat) and Turkey (Guleman and Osmanie) in this article, indicates that they belong to subducted serpentinites, whether they were originally formed in the fore-arc environment or the at abyssal oceanic environment. Composition of the serpentinites of the central part of the suture zone is similar to the average global serpentinites which have mostly lizardite/chrysotile. All of them show depletion of Mg resulting sea floor alteration during serpentinization. The mentioned point may be caused to data deviation from abyssal peridotites field. Considering that the transition metals contents the confirmed the above setting. Almost all of the studied serpentinites are from subducted type which indicates refertilization of LILE evidences as a result of rock/fluid interaction through serpentinization.

Some researchers supposed that investigating the formation of serpentineites can have important applications for understanding the large-scale geodynamics of that earth (Hattori and Guillot, 2003; Hilairet et al, 2007). Serpentineization occurs in a wide range of temperatures and pressures, low temperature serpentinite of alpine type ultramafic rocks occurs due to the penetration of atmospheric waters or from the infiltration of saline waters derived from sediments. This often refers to retrograde serpentineization. In contrast, progressive serpentineization occurs in many metamorphic processes during burial and warming of ultramafic rocks, which includes minimum water, during this process from the reaction of chrysotyl, antigorite and brucite, or antigorite reaction with brucite, forsterite + water were formed (Klein et al, 2014; Evans et al, 2013). The formation of anteatgorite may also occur from the reaction of chrysotyl/lyzardite with talc (Murzin et al, 2013; Lacinska et al, 2016).
After sampling and evaluation of microscopic sections, samples performed by X-ray micro-analyzer by Cansaran Binalood Company. With the aim of studying the whole rock geochemistry, 11 representative samples sent to the Zarazma Company of Tehran and ICP-MS analysis made for rare earth elements and transitional elements and XRF to obtain the major oxide elements.
The remaining olivines were ferro-hortonolite, spinels were chromite, clinopyroxenes were hedenbergite and orthopyroxenes left by serpentineization were ferrosilite. The geochemistry of these rocks is similar to the mantle wedge serpentinites studied along the Neo-Tethys. For example, with Voltri Massive serpentineites in the Italian Alps (Cannao, 2016) or with average subducted serpentinites (Deschamps et al, 2013). The serpentineization of these rocks start from the ocean floor, where the penetration of water in the fractures of the young oceanic crust and in hydrothermal veins, and spread with flat subduction in low temperature metamorphic facies. Since the main serpentine phase in them identified by XRD was chrysotyl, which is associated with the main mineral magnetite, so it is clear that their depth of subduction was low.
The various microstructures identified in these rocks are mesh, sieve, augen, strip, hourly glass, etc. and the effects of elastic (microfolds and kinks) and brittle alteration. The growth of lyzardite plates in the margins of low crystallized mass nuclei is irregular and the increase of serpentine veins and strong deformation reduces the resistance of serpentinites to shear and stress. With precision on the contents, ratios and diagrams, Gysian serpentineites evaluated as subducted type. The content of FME in these rocks is very high and is a sign of re-fertility resulting from the interaction of rock/fluid during subduction stages, which distinguishes it well from other types. In the absence of antigorites in these rocks, the depth at which Gysian serpentineites created was less than 50km. According to the above, Gysian serpentineites are subducted type in the northwestern part of Iran. Along the main Neo-Tethys sutur in Iran, other subduction serpentinites have been reported in Iraq and northwestern Anatolia in Turkey and Gysian serpentinites could be assumed a part of it.

The Kermanshah ophiolite Complex is located in the structural – tectonic zone of the western Iran in northern part of Zagros main thrust and is a part of highly dismembered Kermanshah – Panjvin ophiolite belt. The poly metamorphic zone in this region is mainly related to the development and tectonic emplacement of the ophiolite. Some of the metamorphic rocks (Jade and its simulants) have so ​​hardness, density and high specific gravity, containing tremolite, actinolite, plagioclase, clinochlore, grossular and hydrogrossular garnet. Metamorphic rocks containing very condensed and compact tremolite and nephrite found in the area, as well as very compact antigoritic serpentinite so called bowenite, clearly proof that the high-pressure metamorphism took place during subduction. Serpentinites also appear in the region which is characterized by the predominance of antigorite and Cr-magnetite, as well as by their relatively low LOI (10.9-12.1 wt.%), corresponding to medium or high temperature blueschist facies metamorphism. These metamorphic conditions are strongly similar to the high-pressure metamorphic appearing in this serpentinites that formed from serpentinization of highly depleted harzburgitic protoliths where they were subjected to extensive partial melting processes (>15%), consistent with a hydrous subduction-related environment. Gabbros in this ophiolite complex have been altered rodingitization, the altered rodingitized gabbros, in the continuation were underwent a de-rodingitization process too.

The study area, located in north Torbat Heydarieh, contains ophiolitic units comprised by chromium-bearing minerals such as Spinel. The results obtained from water samples analysis using XRF indicate high chromium concentrations in ophiolite complex as major source. Serpentinite, the dominant lithology in the area, comprises excess chromium concentration, being responsible for chromium-polluted water resources. As well as average chromium concentration severely decreases from serpentinite to other lithological units. Hydrogeochemical investigation of water resources that major cations and anions are in the order of Na>Mg2>Ca2>K and Cl->HCO3->HSO42-, respectively. Regarding piper diagrams, water samples indicate a variety of facies (water type) including Na-HCO3 (3 samples), Na-Cl (3 samples), Ca-HCO3 (3 samples), Mg-Cl (3 samples) and Mg-HCO3 (2 samples). Comparing with WHO (2011) permissible chromium limit, water resources of the study area indicate excess chromium concentration. Due to limited chromite mining in the area, the anthropogenic activity
could not be responsible for the elevated chromium level in the water resources, suggesting the dominant geogenic source of chromium. The abundance of Fe-Mn oxy-hydroxides in most sites of the area leads to chromium oxidation from Cr3 to Cr6. Average values of pH> 7.6 and DO> 7.67 mg/l of the water resources also reflect that Cr chiefly occurs as Cr6 and Cr2O72- and CrO4- deposits.

Alteration of ophiolitic ultramafic rocks، formation of listwaenite and associated mineralization are interesting phenomena of metallogeny of Khoy ophiolite. It seems that the hydrothermal circulations responsible for mercury mineralization were derived from geothermal systems caused by intermediate to acidic magmatism in Neogene-Pleistocene time. Reactivation of brecciated serpentinite with hydrothermal solutions led to the silica-carbonate hydrothermal alteration and formation of listwaenite. The importance of these alteration types is due to associated Hg-Au mineralization. Based on the microscopic mineralogical studies، XRD analysis، geochemical characteristics and field relationships، three types of listwaenite including silica، silica-carbonate and carbonate have been recognized. Tavreh mercury occurrence has formed in relation with silica type listwaenite which also named as birbirite. Alteration zones have structural controls and are restricted to shale (marl) /serpentinite fault type contacts. Mineralogically، silica type listwaenite mainly consists of quartz، chalcedony، opal and secondary iron hydroxides. Magnesite، dolomite، calcite and clay minerals are the minor phases. Toward silica-carbonate and carbonate listwaenites، silica minerals decrease while carbonate minerals increase. Geological field relations and existence of residual chrome spinel in the listwaenite of Tavreh area confirmed the ultrabasic host rock. Based on geochemical studies، average values of SiO2، Fe2O3، MgO and L. O. I as the main components of silica listwaenite are 82. 6، 6. 99، 1. 02 and 3. 74 wt %، respectively. Among rare elements، average values of Hg، Pb، As and S have been obtained as 646. 2 ppm، 517. 7 ppm، 329. 1 ppm and 281 ppm، respectively. Mass changes (gains and losses) of major and minor elements during the alteration process in the Tavreh area have considered، using Gresens’ equation in this study. Cinabar-bearing silica listwaenite is enriched in SiO2، Al2O3، K2O and Hg components and is depleted in MgO and MnO. Silica-carbonate type shows enrichment in CaO، Na2O، Al2O3، K2O and TiO2 and deplection in MgO، MnO P2O5، SiO2، Cr2O3 and Fe2O3. Carbonate type shows increase in CaO and Sr and reduction in SiO2، Na2O، MgO، Al2O3، Fe2O3، TiO2 and MnO.

Gushk chromite mine is one of the largest and the most important chromite mines in Iran that is located in southwest of Kerman Province, 5 km north of Baft city. In this open pit active mine, about 60 tons of chromite is extracted per day. The studied chromitites are podiform type and form discontinuous layers or lenses surrounded by completely serpentinized dunites. The chromites with average Cr2O3 = 62.8% and Cr# = 0.83 are classified as Cr rich chromitites or as the first grade type chromites in the world. Cr# and Mg# values indicate that the studied chromitites have been crystallized from boninitic magmas, probably in suprasubduction zone setting in a back-arc basin environment. It seems that the Baft chromitite ores have initially been formed in a primary ophiolitic complex within dunitic envelopes. In the next stage, due to serpentinization of the peridotites and ascending of the resulted serpentinite, the studied deposits have been emplaced along the shear zones of the Baft ophiolitic melange en route to the surface.

Three types of serpentine polymorphs (lizardite, chrysotile and antigorite) are found in serpentinites from the Baft ophiolite mélange which are different in the textural features and chemical compositions. Lizardite is seen as layered structures that set in the matrix of peridotites, while antigorite is formed in veins or sheared zones as blade crystals and chrysotile develops as shiny and golden fibers in small cracks. The studied peridotites, based on degree of serpentinization, can be divided into three subdivisions including relatively serpentinized peridotites (up to 50 vol. % serpentinization), mainly serpentinized peridotites (50-90 vol. % serpentinization) and completely serpentinized peridotites (90-100 vol. % serpentinization). We suggested three models for serpentinization of olivine in this area. First model contains serpentinization of olivine grain boundaries with unaltered centers. The process has developed in relatively serpentinized peridotites. Second model includes redeveloping serpentinization of inherited olivine grains from the first model and third model contains complete serpentinization of olivine grains. Second and third models have been developed in mainly and completely serpentinized peridotites. It seems that involved lizardite and chrysotile are the result of static alteration in an ocean floor like environment, while interpenetrative-blade antigorites have been produced by dynamic metamorphism. Production of antigorite can be related to subduction of Nain-Baft oceanic crust or it can be related to obduction of serpentinized pridotites in other times, it is possible that lizardite and chrysotile have formed before closure of Nain-Baft Small Ocean.

Huntite mineral with formula CaMg3(C03}4 is anatural carbonatic material with rhombohedral system that itscomposition is between dolomite (CaMg(C03h) and magnesite.For the first time Huntite was found in American Nevadaprovince, then in other places i,e. Poland, Hungary, France,Australia, Iran and Greece.According to thermOdynamic, Huntite forms in condition oflow pressure and tcmpreature in the surface or near surface area.Investigation in Iran shows the existence ofHuntite mineralwithin weathered ultramafic rocks. At outer pan of eastern Iran,a great bulk of this rocks in ophiOlitic zone is origineted. Insome of this area outcrop of magnesite is in adjacent withHuntite.