Jamshidzaei, A., Jamshidzaei, A., Khalili, K., Khalili, K., Torabi, G.
Journal of Economic Geology (20087306)16(2)pp. 61-94
The Godar-e-Siah Eocene monzodiorite stock is located in the southwest of the Jandaq area (NE of Isfahan) and northwest of the Central-East Iranian Microcontinent (CEIM). The main minerals in these monzodiorites are plagioclase, K-feldspar, clinopyroxene, phlogopite, and garnet, which are set in a fine-grained groundmass of feldspars. The main textures are granular, porphyritic, and poikilitic. In some cases, these rocks contain euhedral to subhedral garnet crystals with inclusions of the igneous clinopyroxene and groundmass minerals including feldspar and graphite. These garnets exhibit Ti-andradite and Ca-melanite composition from the solid solution series of andradite-grossular. The chemical zoning patterns of the studied garnets confirm that these garnets have a non-magmatic origin and metamorphic nature. In the investigated monzodiorites, the presence of euhedral garnet crystals with inclusions of igneous clinopyroxenes metasomatic scapolite, and metasomatic phlogopite shows that these garnets are of metasomatic origin, which formed due to the alteration of igneous clinopyroxenes. All geochemical and petrographic evidences from the studied garnets indicate that they have formed as a result of the intrusion of Eocene monzodiotites into the carboniferous limestones (or dolomites), leading to the creation of endoskarn or reactions skarn that can be distinguished at the millimetric scale in microscopic studies.
Introduction Garnet is a general mineral forms in metamorphic rocks derived from the sedimentary and igneous protoliths and at all metamorphic grades above the greenschist facies (Baxter et al., 2017). However, the presence of garnet in certain types of igneous rocks, such as peraluminous granite and ultramafic rocks in the upper mantle, introduces complexities in unraveling the petrogenesis of garnets in igneous and metamorphic rocks (Rong et al., 2018). Titanium-rich garnets are enriched in andradite, occur in various types of rocks, including a variety of igneous rocks, encompassing trachytes and phonolites (Dingwell and Brearley 1985), syenites and carbonatites, nephelinites and tephrites (Gwalani et al., 2000), as well as ultramafic lamprophyres, rodingites (Schmitt et al., 2019); high temperature metamorphic rocks and skarns. The composition of titaniferous garnets besides their paragenetic relationships is one of the significant petrology factors (Chakhmouradian and McCammon, 2005). The study area is situated in the northwestern part of the CEIM (northwestern part of the Yazd block), and southwest of the Jandaq City. The rock units of the Jandaq area are mainly include Paleozoic metamorphic rocks, Upper Paleozoic sedimentary rocks, Cretaceous and Paleocene sediments, Eocene intrusive rocks, Eocene subvolcanic (dikes) and volcanic rocks (Jamshidzaei et al., 2021), the Pis-Kuh upper Eocene sedimentary rocks (flysch), and Early Oligocene lamprophyric rocks, and alkali basalts (Torabi, 2010; Berra et al., 2017; Sargazi et al., 2019; Jamshidzaei et al., 2021). In this paper, the chemical characteristics of the monzodiorite stock and origin of garnet mineral are discussed.
Material and methods To determine the chemical compositions of minerals, JEOL JXA-8800 WDS at the Department of Earth Science, Kanazawa University, Kanazawa, Japan was used. Chemical analyses of minerals was performed under an accelerating voltage of 20 kV, a probe current of 20 nA, and a focused beam diameter of 3μm. The ZAF program was used for data correction. Natural minerals and synthetic materials with well-characterized compositions serve as standards for calibration and validation purposes. The Fe2+# and Mg# parameters of minerals are represented by the atomic ratios of Fe2+/(Fe2++Mg) and Mg/(Mg+Fe2+), respectively. To recalculate the FeO and Fe2O3 concentrations from Fe2O3*, recommended ratios of Middlemost (1989) is used. The mineral abbreviations used in this context are derived from Whitney and Evans (2010).
Results and discussion The monzodiorites of the Godar-e-Siah area mostly show fine to coarse-grained granular, porphyritic and poikilitic textures. These rocks are mesocrate in color, displaying massive and mineralogically homogeneous nature in their outcrops. K-feldspars are the primary minerals and garnet mineral is imposed on these rocks. The main minerals of this monzodiorite stock are plagioclase, K-feldspar, clinopyroxene, phlogopite, and garnet, set in a fine-grained matrix of feldspars. These rocks have mainly granular, porphyritic, and poikilitic textures. In some cases, these rocks contain euhedral to subhedral garnet crystals with inclusions of igneous clinopyroxene and groundmass minerals including feldspar and graphite. These garnets have a composition of Ti-garnet and Ca-melanite from the solid solution series of andradite-grossular. Based on the EPMA data, the clinopyroxenes show diopside to hedenbergite compositions, indicating that two types of clinopyroxene are in these rocks. The first group of this mineral contain MgO (6.9-10.20 wt.%), FeO*(11-16.88 wt.%), Al2O3 (2-5.84), and Na2O (1.1-1.9 wt.%), is reactive pyroxenes. The second category contains MgO (9.98-12.89 wt.%), FeO* (9.13-13.87 wt.%), Al2O3 (1.82-3.11 wt.%), and Na2O (0.7-1.42% Wt.%), is igneous pyroxenes. Chemistry of the pyroxenes reveals that reactive pyroxenes have higher concentrations of FeO* and Al2O3 than igneous pyroxenes. Chemistry of the feldspars indicates that the K-feldspars is orthoclase in composition. Also, chemical analyses of mica show that these minerals contain high concentrations of MgO (21.54-22.60 wt.%) and low values of Al2O3 (12.89-13.30 wt.%). The mica from the studied rocks of the Jandaq area plots in the phlogopite field. The garnet grains in these rocks contain 61.94-66.39 mol.% almandine (Fe2Al2Si3O12), 18.60-23.40 mol.% grossular (Ca3Al2Si3O12), 10.06-15.11 mol.% pyrope (Mg2Al2Si3O12), and 1.09-4.32 mol.% spessartine (Mn2Al2Si3O12). These garnets have a composition of Ti-garnet and Ca-melanite from the solid solution series of andradite-grossular. The chemical zoning patterns of the studied garnets confirm that these garnets have a non-magmatic origin and metamorphic nature. The presence of discontinuous chemical zoning and the pattern of variations in the end-member compositions of these garnets indicate that they were formed under disequilibrium conditions accompanied by changes in the environmental oxidation conditions. In the studied monzodiorites, the presence of euhedral garnet crystals with inclusions of igneous clinopyroxenes, metasomatic scapolite, and metasomatic phlogopite shows that these garnets are of metasomatic origin which formed due to the alteration of igneous clinopyroxenes. The geochemical characteristics and petrographic evidences from the studied garnets; including the presence of euhedral crystals with distinct boundaries to contact minerals, the occurrence of inclusions of background minerals and igneous clinopyroxenes in the garnets, as well as the presence of discontinuous chemical zoning, confirms that they have formed as a result of the intrusion of Eocene monzodiotites into the carboniferous limestones (or dolomites), leading to the creation of endoskarn or reactions skarn.
Acknowledgments The authors thank the University of Isfahan for financial supports.
Journal of Economic Geology (20087306)16(4)pp. 75-99
The Mesozoic ophiolitic mélange of Naein is located to the west of the Central-East Iranian Microcontinent (CEIM). In this ophiolite, the mantle peridotites cross cut by greenish, coarse-grained hornblendite dykes with up to 50 cm width. These dykes cross cut by carbonate veins with a few millimeters to a few centimeter width. Hornblendite dykes composed of Cr-spinel, magnesio-hornblende, chlorite, ilmenite, tremolite, calcite and dolomite. Hydrothermal spadaites (MgSiO2(OH)2·H2O) are formed in the late-stage phase. The chemical compositions of hornblendites indicate that hornblendes are magnesio-hornblende in composition (with a mean Mg# = 0.93) and chlorites are penninite and clinochlore, with a mean Mg# of 0.94. The Mg# and Cr# of Cr-spinels are 0.45 and 0.66, respectively. The presence of abundant hydrous minerals (hornblende and chlorite) and carbonate veins, as well as the chemical characteristics of hornblendes and Cr-spinels, indicates the non-magmatic origin of these dikes and veins, which were formed by the interactions of seawater-derived fluids with the uppermost mantle peridotites. The mineralogical and chemical characteristics of hornblendites demonstrate the mobility of elements such as Mg, Ca, Si, Al, Na, Cr, Fe, Ti and REE during the circulation of fluids derived from seawater within the uppermost mantle peridotites. This study suggests that the percolation of seawater ingression fluids in the uppermost mantle peridotites, resulted in the formation of hornblende dikes and, in the late-stage phase, the development of carbonate veins that contain calcite, dolomite and spadaite.
Introduction Petrological and geochemical studies indicate that the influence of seawater affects the mineralogy and chemistry of the oceanic crust and uppermost mantle peridotites (Berger et al., 2005; Python et al., 2007; Akizawa et al., 2011; Akizawa and Arai, 2014; Torabi et al., 2017). Diopsidite, hornblendite and hydrothermal chromitite have formed as a result of reaction between mantle peridotites and penetrating hydrothermal fluids (Python et al., 2007; Torabi et al., 2017; Arai et al., 2020). In the Naein ophiolites mantle peridotites, fractures and cracks within the uppermost mantle peridotites (Harzburgite and dunite) (Fig. 3) have been filled with hornblendites (Torabi et al., 2017). In the last stage, CO2, Mg, Si and Ca-bearing hydrothermal fluids formed the carbonate veins, cross-cuting the peridotites and hornblendites (Fig. 4). In this research, the formation of the hornblendite dikes, carbonate veins and the rare mineral spadaite (MgO.SiO2.2H2O), which were formed by circulating fluids in mantle peridotites of the Nain ophiolite, will discuss.
Materials and methods After the field studies, sampling and petrographic studies, polished thin sections of the selected fresh samples were used for point analyses by electron microprobe. Chemical analyses of mineral were performed at the Kanazawa University (Japan) using a wavelength-dispersive electron probe microanalyzer (EPMA) (JEOL JXA-8800R). The analyses were conducted at an accelerating voltage of 15 kV, a probe current of 15 nA (Table 1, 2 and 3) and counting time of 40 seconds. In addition to the microprobe, the minerals of the carbonate veins were investigated by scanning electron microscopy (SEM) (EDS-RONTEC) at an accelerating voltage of 20 kV in the Razi Metallurgical Research Center (RMRC) (Tehran) (Table 4). Discussion Hornblendite formation The petrographic, mineralogical and chemical specifications of the hornblendites indicate their non-magmatic origin (Torabi et al., 2017). These samples composed of primitive hydrous phases (such as Mg-hornblende and chlorite). Some of the primary Mg-hornblendes, have changed to tremolite due to retrograde metamorphism. These minerals indicate the penetration of hydrothermal fluids in the uppermost mantle section (Python et al., 2007; Torabi et al., 2017). The fluid composition is enriched in Cr, Mg, Fe, Si, Al, Ca, Na and HREE as a result of reacions with peridotites. The circulation of fluids through the fractures and veins of mantle peridotites has led to the formation of hornblendites (Torabi et al., 2017). In the hornblendites, the higher content of MgO contrasted to CaO reveals a considerable activity of Mg in circulation of hydrothermal fluids (Torabi et al., 2017). Carbonate veins formation After the formation of hornblendites in the upper mantle peridotites, carbonate veinswere formed in the last stage. The presence of carbonate veins in peridotites reveals that these veins formed under the influence of circulating hydrothermal fluids at lower temperatures. These fluids are enriched in elements such as Mg, Ca, Si, CO2 and H2O. The carbonate veins are composed of calcite, dolomite, and spadaite. These carbonate veins cross-cut the hornblendites and peridotites. The presence of dolomite and calcite in carbonate veins, and hornblende (Ca-rich mineral) in hornblendite dykes, shows in the study area, the fluids have passed through Ca -rich rocks (limestone, gabbros) before reaching the uppermost mantle, resulting in the enrichment of the fluids in Ca and CO2. These mineralogical and chemical specifications possibly confirm seawater origin for the fluids. Spadaite Formation The occurrences of magnesium silicate spadaite (MgSiO2(OH)2·H2O), along with calcite and dolomite, developed under the influence of fluid–rock interaction, serpentinization of olivine and orthopyroxene, and subsequent dissolution of serpentine by CO2-bearing hydrothermal fluids. This hydrous magnesium silicate forms under basic conditions, at low temperatures and in the last stage. The Mg and Si-bearing hydrothermal fluids play an important role in the formation of spadaite. The formation of carbonate minerals (calcite and dolomite) in the uppermost mantle peridotites indicates a high fugacity of CO2 in hydrothermal fluids. The kind of new minerals seem to be influenced by ion activities in hydrothermal fluids (Birsoy, 2002), and as well as indirectly by pH.
Mobility of Elements Seawater-derived fluids pass through the entire oceanic crust and extend to the uppermost mantle. The hornblendites in the Naein ophiolite were formed by a reaction between seawater ingression fluids and peridotites (harzburgite and dunite) at temperatures ranging from 700–850°C. The mineralogy and chemical characteristics of hornblendite dykes suggest that the circulation of hydrothermal fluids at high-temperatures helps the mobility of Cr, Mg, Ti, Fe, Ca, Si, Al, Na, and REEs (Torabi et al., 2017). The presence of hydrothermal chromite and ilmenite within the hornblendite dykes show mobility of Cr, Fe and Ti, in hydrothermal conditions during the circulation of high temperature silicate-rich fluids through mantle peridotites. The formation of hornblendites dykes (Torabi et al., 2017), diopsidites (Python et al., 2007; Akizawa et al., 2011; Akizawa and Arai, 2014) and hydrothermal chromitites (Arai et al., 2020), under The influence of metasomatic process, indicates that the activity of seawater ingression fluids alters the initial concentration of Ca, Mg, Cr and Si from the lower crust to the uppermost mantle section (Akizawa et al., 2011). Hydrothermal fluids change the chemical composition of minerals, lead to the decomposition of olivine and the formation of serpentine, modify the chemical composition of chromites and form chlorite and secondary chromites. The hydrothermal chromites of the hornblendites (Cr# 0.56 and Mg# 0.62) are chemically intermediate between to chromite found in the surrounding harzburgite (Cr# 0.56 and Mg# 0.62) and dunite (Cr# 0.79 and Mg# 0.41) (Fig. 6E and F), indicating dissolution of primitive chromite grains present in nearby peridotites and their reprecipitation in cracks and fractures during the formation of hornblendite dyke. Altered chromite grains in the hornblendites (Cr# 0.86 and Mg# 0.21) and peridotites (Cr# 0.91 and Mg# 0.17) suggest that hydrothermal fluids have leached Cr-spinel from the host rock and hornblendites (Fig. 6E and F).
Conclusions The mineralogical and chemical properties of the Naein mantle hornblendites and their associated carbonate veins indicate a non-magmatic origin, suggesting that they have a hydrothermal nature. The circulation of seawater-derived fluids through the uppermost mantle peridotites will cause to the mobility of Cr, Ti, Fe, and REE. The hydrotermal spadaite formed by H2O, CO2, Mg, Ca and Si-bearing hydrothermal fluids, in the last stage phase that developed in a low-temperature environment under basic conditions. Calcite, dolomite and spadaite are minerals of the carbonate veins.
Acknowledgments The authors thank the University of Isfahan and Kanazawa University for financial supports and laboratory equipments.
Ghadirpour, M., Ghadirpour, M., Ghadirpour, M., Ghadirpour, M., Torabi, G., Torabi, G., Torabi, G., Shirdashtzadeh, N., Shirdashtzadeh, N., Shirdashtzadeh, N.
Journal of Economic Geology (20087306)15(4)pp. 55-79
In central part of the Mesozoic Ashin ophiolite (Northwest of Anarak, Isfahan province, Iran), the Upper Eocene monzonitic stock cross cuts the Ashin ophiolite and Middle Eocene volcanic rocks. Amphibolite xenoliths are enclosed in the stock and associated Eocene volcanic rocks. Xenoliths are more abundant in the margin of the monzonitic stock. Rock-forming minerals of the stock are plagioclase with andesine to labradorite composition (An=34-60%), Alkali-feldspar with orthoclase composition (Or= 70.8 to 96.1%), diopsidic clinopyroxene with (Mg# =0.71-0.90), and phlogopite mica with (Fe#=0.3). Opaque minerals are magnetite and titanomagnetite (TiO2=1.6-4.4 wt.%). Main textures of samples from this intrusive body are granular, intergranular and poikilitic. Samples from the margin of this stock represent porphyritic texture.
Geochemistry of minerals and whole rock samples of this stock indicate that they belong to the calc-alkaline magmatic series and are similar to the samples from the continental magmatic arcs.
These magmatic rocks possibly were formed by subduction of the CEIM (Central-East Iranian Microcontinent) confining oceanic crusts (Ashin and Nain oceanic crusts) during Mesozoic and Early Cenozoic eras.
Introduction
Iran is a part of the Alpine-Himalayan orogenic system, including the Paleozoic to Cenozoic ophiolites, magmatic and metamorphic rocks (Takin, 1972; Berberian and King, 1981; Berberian et al., 1982; Dercourt et al., 1986; Alavi, 1994; Mohajjel et al., 2003; Shahabpour, 2007). The main pulse of the Paleogene and Neogene magmatic (volcanic and intrusive) activities of Iran can be attributed to the two Cenozoic subduction events, including the western Neo-Tethyan oceanic crust subduction beneath the Sanandaj-Sirjan block in the west and the eastern Neo-Tethyan oceanic crust subduction beneath the Central Iran (e.g., Shirdashtzadeh et al., 2022). The former subduction possibly caused to the formation of the Urumieh-Dokhtar Magmatic Arc, but the later subdution results is not well studied yet.
In the this research, the target region is located in the west of the Yazd block (Central Iran), where the Eocene volcanic and plutonic rocks represent subduction-related characteristics (Jamshidzaei et al., 2021). The investigated subduction-related monzonitic stock that cross cuts the central part of the Ashin ophiolite in the Kuh-e-Kalut-e-Ghandehari region, in the northwest of Anarak (Isfahan Province, Iran). The main lithologies in the Kuh-e-Kalut-e-Ghandehari are Mesozoic lithologies of Ashin Ophiolite, Paleocene limestone, Eocene volcanic rocks, monzonitic stock, Lower Red Formation, and Akhoreh Formation. Ashin ophiolite was formed in the mesozoic (Shirdashtzadeh et al., 2022) and emplaced in the Late Paleocene (~60 Ma; Pirnia et al., 2020; Shirdashtzadeh et al., 2022), before than Eocene volcanism and plutonism. The studied monzonitic stock of the Kuh-e-Kalut-e-Ghandehari intrudes the Mesozoic Ashin ophiolite and Middle Eocene volcanic rocks.
The calc-alkaline affinity of the volcanic and plutonic rocks of the area, tectonic activity of the Great Kavir fault caused to the crushing and mylonitization of the surrounding rock units, as well as the alteration evidences in the field studies point to suitable conditions for the ore deposit exploration in the area (e.g., copper). In this research, the petrology, mineralogy, and whole rock geochemistry of the Upper Eocene monzonitic stock are considered. This research will expand our understanding of the geochemical nature of subduction-related Cenozoic magmatism in Central Iran.
Materials and methods
After detailed field studies and sampling, the selected fresh samples were used for microscopic thin section and polished-thin section studies by the polarizing binocular microscope (Olympus BH-2). The microprobe analyses were performed at the School of Natural Systems, College of Science and Engineering, Kanazawa University (Kanazawa, Japan) using a wavelength dispersive electron probe microanalyzer (EPMA) (JEOL JXA-8800R). The mineral analysis was achieved under an accelerating voltage of 20 kV, a probe current of 20 nA, and a focused beam diameter of 3μm. 14 whole rock samples analyses were performed by Brucker S4 PIONEER XRF in the central laboratory of the University of Isfahan and 3 samples were analyzed in the Isfahan Nuclear Technology Center by neutron activation analysis (NAA).
Results
Based on the field relation ships, this gray to light gray pluton intrudes into the Middle Eocene volcanic rocks and belongs to the Upper Eocene. The Middle Eocene volcanic rocks and Upper Eocene monzonitic stock crosscut the Ashin Ophiolite. This Eocene stock and volcanic rocks contain amphibolite xenoliths with the same mineralogy and petrography. Xenoliths are more abundant in the margin of the monzonitic stock. Gradual decreasing of modal plagioclase content indicates that the xenoliths range from amphibolite (plagioclase + amphibole) to hornblendite (only amphibole) in composition.
Rock-forming minerals of the stock are plagioclase with andesine to labradorite composition (An = 34-60 %), alkali-feldspar with orthoclase composition (Or = 70.8 to 96.1%), diopside clinopyroxene with Mg# = 0.71-0.90, and phlogopite mica with Fe# = 0.3. Opaque minerals are magnetite and titanomagnetite with TiO2 = 1.6-4.4 wt%. The main textures of samples from this intrusive body are granular, intergranular and poikilitic. Samples from the margin of this stock represent porphyritic texture. The SiO2 value in the whole rock compositions ranges from 47.9 to 61.65 wt.% (basic to intermediate). The average content of alkalis is 9.75 wt.%). The Kuh-e-Kalut-e-Ghandehari rocks show sodic affinity by higher Na2O than K2O, based on the Na2O/K2O versus SiO2 and K2O/Na2O versus SiO2 diagrams (Jaques et al., 1985). The Eocene intrusive and volcanic rocks of this area are similar in terms of mineralogy and texture. Petrography and whole rocks chemical analyses indicate that the studied stock is geochemically composed of gabbro, monzodiorite to monzonite in composition with metaluminous affinity. Monzonite is the predominant rock.
Tectonic setting
Various tectonomagmatic discrimination diagrams are used to determine the tectonomagmatic setting of the Kuh-e-Kalut-e-Ghandehari stock. Mineral chemistry and whole rock geochemistry of the Kuh-e-Kalut-e-Ghandehari monzonitic stock indicate a calc-alkaline magmatic series similar to the subduction-related magmas in the normal continental magmatic arcs formed during the mantle metasomatism. According to the the temporal and geological situation, as well as the geochemical characteristics of the Kuh-e-Kalut-e-Ghandehari stock, it is considered as a part of an arc magmatism, related to the subduction of Neo-Tethyan oceanic crust beneath the CEIM (Central–East Iranian Microcontinent) during the Late Mesozoic and Early Cenozoic eras.
Acknowledgments
We are grateful to the University of Isfahan and the Department of Geology of Kanazawa University (Japan) for their supports. We are also grateful to anonymous reviewers for their useful comments and suggestions that improved the quality of this paper.
Journal of Economic Geology (20087306)15(4)pp. 137-158
The Lower Oligocene basic dikes are cropped out in the Chah-e-Alikhan area (Northeast of Isfahan province, North of the Daq-e-Sorkh desert). These dikes show NE-SW and NW-SE trends and cross cut the Eocene volcanic rocks and associated flysches. NW-SE dikes are younger and cut the NE-SW ones. These dikes are similar in petrography and are composed of plagioclase, clinopyroxene, olivine, sanidine, Cr-spinel and ilmenite. Zeolite, serpentine, calcite and magnetite are secondary minerals. These dikes represent the porphyritic, glomeroporphyritic, poikilitic and trachytic textures. Intergranular and granular textures can be seen at the center of the larger dikes. These basalts are enriched in alkalis (Na2O+K2O), LREE and LILE (Cs, Rb, Ba, Pb) and have high values of LREE/HREE ratio (La/Yb=8.9-10). In the classification diagrams, which are based on the incompatible elements and HFSEs, they are classified as alkali basalts. The primitive magma of these basaltic dikes has been produced by partial melting of a garnet-spinel lherzolite of the mantle previously suffered the carbonate metasomatism. The formation of the alkali basalt dikes of the Chah-e-Alikhan area can be ascribed to the former subduction of the Central- East Iranian Microcontinent (CEIM) confining oceanic crust and decompression melting induced by the extensional basin of the Anarak‒Jandaq area in Early Oligocene. The primary basaltic magma has been formed by low degree of partial melting of a metasomatised mantle lherzolite during continental crust extension episode in the lower Oligocene and has been ascent through the faults.
Introduction In the Northwest of CEIM (Central-East Iranian Microcontinent), along the Great Kavir fault, volumes of alkali basalts with the lower Oligocene age are outcropped as volcanic and subvolcanic (Dike) rocks. In this research, the subvolcanic exposures of this basic magmatism in the the Chah-e-Alikhan area is discussed. The Lower Oligocene basic dikes are cropped out in the Chah-e-Alikhan area (Northeast of Isfahan, Northeast of Zavareh, and Northwest of the CEIM). These dikes show NE-SW and NW-SE trends and cross cut the Eocene volcanic rocks and associated flysches. In this paper, the geological and petrological aspects, as well as the geodynamic setting of alkali basalt dikes of the Chah-e-Alikhan area are discussed. Study of these dikes, as a part of the Cenozoic alkaline magmatism from Northwest of the CEIM, will be useful in understanding the geodynamical evolution of the Central Iran.
Analytical method The method of study is including petrography (field, library and microscopic studies) and whole rocks geochemical analysis of rocks. 13 fresh whole rock samples of alkali basalts from the Chah-e-Alikhan area were selected for the major and trace elements chemical analyses. Whole rock geochemical analyses carried out by using a Bruker S4 Pioneer XRF at the Central Laboratory of the University of Isfahan. Trace element compositions of the selected samples were achieved by ICP-MS (Inductively coupled plasma-mass spectrometry) at the Zarazma Mineral Studies Company (Tehran, Iran).
Results and discussion The rock-forming minerals of the Chahe-e-Alikhan basic dikes are Cr-spinel, olivine, clinopyroxene, plagioclase, sanidine and ilmenite. Zeolite, serpentine, calcite and magnetite are secondary minerals which are formed as a result of the alteration of primary minerals. Petrographical characteristics indicate that these dikes are alkali basalt and represent the porphyritic, glomeroporphyritic and trachytic textures. Intergranular and granular textures can be seen at the center of the larger dikes. These basalts are enriched in alkalis (Na2O+K2O=4.5-5.4 wt%), LILEs (Cs, Rb, Ba, Pb) and have high values of LREE/HREE ratio (La/Yb=8.9-10). Trace elements ratio diagrams such as La/Nb versus La/Yb, Dy/Yb against La/Yb, Sm/Yb versus La/Yb (Bogaard and Worner, 2003) and Ce/Yb-Ce (Ellam, 1992) are used in order to determination of the depth, type and degree of partial melting of the source rock. Based on the geochemical characteristics and diagrams, the primitive magma of the Chah-e-Alikhan alkali basalts possibly have been produced by about 5 to 10 percent partial melting of a garnet-spinel lherzolite, which is located at the depth of about 105 km, as a part of a mixed asthenospheric–lithospheric mantle. The elevated values of the Zr/Hf ratio and the Na2O + K2O versus TiO2 diagram (Zeng et al., 2010) indicate that the primitive magma of the studied basic dikes previously suffered the carbonate metasomatism. The Chah-e-Alikhan alkali basalts show high values of the Alkalis (Na2O + K2O), enrichment in LREE, HFSE and LILE. The subducted oceanic slab is the source of carbon and LILEs are the mobile components of subduction (Shaw et al., 2003). Considering that Cs is a highly fluid mobile element, enrichment in Cs relative to Rb suggests that the fluid phases derived from a subducting slab are probably the metasomatic agents. The lower Oligocene alkaline magmatism in the Chah-e-Alikhan area and the enrichment of the mantle with incompatible elements (metasomatism) can be attributed to two oceanic crust subduction events: (1) Northeast ward Neotethys subduction along the Zagros Thrust Zone beneath the Central Iran from the Triassic to the Eocene (Torabi, 2010); and (2) Subduction of an oceanic crust along the Great Kavir Fault, which is situated to the western margin of the CEIM. The spreading of the last ocean crust started in the Triassic and ended in the Eocene. The remnants of this oceanic crust are found as ophiolitic melanges on the western side of the CEIM, such as the Nain, Surk, and Ashin ophiolites (Rajabi and Torabi, 2012; Torabi, 2010). The geological history and position of the Chah-e-Alikhan alkali basalt dikes suggests that the the carbonate metasomatism of the mantle peridotites can be attributed to the subduction of the CEIM confining oceanic crust. Several tectonic discrimination diagrams have been used for determination of the tectonic setting of the Chah-e-Alikhan basalts. The La/Yb versus Th/Nb (Hollocher et al., 2012), Ta/Yb against Th/Yb (Gorton and Schandl, 2000) and DF1 versus DF2 (Verma and Agrawual, 2011) diagrams suggest a within-plate (continental) tectonic setting. The activity of the major faults of the area such as Great Kavir, Chah Mishury and Chah Gireh Faults has been created a suitable inter-plate extensional system to ascending the Lower Oligocene alkali basalt magma in the Chah-e-Alikhan area.
Conclusion The Lower Oligocene alkali basalts of the Chah-e-Alikhan area is a part of the intra-continental alkaline magmatism crosscuts the Eocene volcanic rocks. The area provides a setting to study the Cenozoic alkaline magmatism of the northwest of the CEIM. These basalts are enriched in total Alkalis, TiO2, LREE and LILEs. They have been produced by about 5 to 10 percent degree of partial melting of a garnet-spinel bearing lherzolite of a mixed lithospheric-asthenospheric mantle which is previously metasomatised. The mantle enrichment can be ascribed to the subduction of the CEIM confining oceanic crust beneath the Central Iran from the Triassic to the Eocene. The Grate Kavir Fault and related faults have played an important role in the Lower Oligocene alkaline magmatism in northwest of the CEIM.
Acknowledgments The authors thank the University of Isfahan for financial support.
Pirnia, Tahmineh, Bahramnejad, Elham, Sharifi, Mortaza, Bahramnejad E., Bagheri S., Sharifi, M., Nurlu N., Shi Y., Torabi, G., Noghreyan, M.
GEOCHEMISTRY (00092819)84(1)
The Eastern Iranian Ranges are considered to be either a suture zone produced by closure of a Neo-Tethyan backarc basin between the Lut and Afghan blocks or an oroclinal buckling of multiple terranes accreted to the active margin of the Neo-Tethyan Ocean. Both models are based on the presence of Cretaceous ophiolite complexes and sequences of Eocene turbiditic sedimentary rocks. The Dumak ophiolitic melange, a significant ophiolitic assemblage that crops out in the western portion of the orogen adjacent to Lut, contains all the essential elements of a typical ophiolite in a matrix of serpentinite and clay-rich sediments. Ultramafic-mafic cumulates in the melange are characterized by medium, non-rhythmic bedding in limited outcrops where they are in tectonic contact with other ophiolitic units. The cumulates consist of plagioclase-bearing dunite, troctolite, Cpx-troctolite and gabbro composed chiefly of olivine, plagioclase, and clinopyroxene accompanied by rare orthopyroxene. This assemblage is very similar to the differential crystallization sequence of tholeiitic magma at modern midocean ridges. The clinopyroxene in these rocks is diopside (En = 47-49) and the plagioclase is bytownite (An = 71-77). Whole-rock geochemistry of samples from the crustal sequence of Dumak melange are characterized by low TiO2 (0.03-0.17 wt%). Investigation of this crystalline sequence and geochemical properties of the rocks suggests that they can be considered as the low-Ti ophiolite originated from mid-ocean ridges. Additionally, the positive Eu anomalies as well as comparison of frequency of LREE and HREE in the mafic and ultramafic samples indicate the cumulates formed by fractional crystallization and differentiation of mantle-derived magmas. According to petrographic, geochemical, and structural evidence, it is possible that the Dumak ophiolite, after being formed or displaced between Lut and Afghan blocks, was firstly accreted to the south of the Lut block and then re-mixed into Eocene sediments emplaced in the current position.
Detrital chromian spinels from adjacent sediment (recent sands and a Tertiary sandstone) were used to obtain a general view of the lithological and petrological characteristics of the Nain ophiolitic melange, Iran. They display a wide chemical range in terms of Cr# (=Cr/(Cr + Al) atomic ratio), from 0.10 to 0.92. Except for some grains (11% of the total), the spinels show low TiO2 contents (<0.25 wt%), typical for the spinels of mantle peridotites. Relatively high-Ti spinels (TiO2 content 10.25 wt%, up to 1.26 wt%) are relatively low in Mg# (=Mg/(Mg + Fe2+) atomic ratio), from 0.15 to 0.65, and were possibly derived from mantle peridotite that reacted with impregnating melt (e. g., dunite or plagioclase-bearing peridotites). The high-Cr# (0.55-0.92) spinels, which are dominant in the Tertiary sandstone, are chemically homogenous and clearly different from high-Cr# altered spinels (ferritchromite and Cr-magnetite), which are formed by low-T alteration, and are low in Mg# and high in Fe3+. The higher abundance of the high-Cr# spinels in the Tertiary sandstone than in the recent sands indicates higher abundance of refractory lithologies in the Paleo-Nain ophiolite than the present one. The refractory lithologies for the source of the high-Cr# spinels have not been reported from the Nain melange, which implies that the refractory dunite and harzburgite were effectively sheared to provide the melange matrix and then eroded during later stages of emplacement. This indicates uplift and intermittent protrusion of a vertical slice of heterogeneous upper mantle in the Nain since the late Cretaceous. This is in good agreement with the geological situation of the Nain melange located on a fossil transcurrent fault (Nain-Baft fault zone), which was involved in the opening of the Nain-Baft basin along an active margin of the central-east Iranian microplate (the Sanandaj-Sirjan block). The initial presence of the high-Cr# spinels implies an origin from a spreading center above a subduction zone (e. g., back-arc basin) for the Nain melange.
Revista Mexicana de Ciencias Geologicas (10268774)28(3)pp. 544-554
Anarak Paleozoic ophiolite is located in western part of the Central - East Iranian Microcontinent. This metaophiolite is covered by Paleozoic schist and marble. Blueschists of the Anarak ophiolite are exposed along the northern Anarak east-west main faults and are considered as remnants of the Paleo-Tethys suture zone in Central Iran. Anarak blueschists are formed by metamorphism of primitive basic lavas. In some cases, they preserve the primary pillow structure. Petrography and microprobe analyses show that they are composed of riebeckite, actinolite, plagioclase (albite), sphene, magnetite, white mica and apatite. Secondary minerals are chlorite (pycnochlorite), epidote, pyrite and calcite. Mineralogical assemblages are consistent with blueschist facies metamorphism, which is followed by a retrograde metamorphism in greenschist facies. Estimation of the metamorphic conditions suggests 300-450 °C and 4-9 kbar. Whole rock geochemical analyses show that these rocks can be classified as alkaline basalts. Chondrite-normalized rare-earth element (REE) patterns of the studied rocks display 10-150 times enrichment, high light REE and relatively low heavy REE contents. These geochemical characteristics are representative of mantle-derived magmas. Primitive mantle normalized spidergram of the Anarak samples exhibit negative anomalies of Ba, U, K and Sr, and positive anomalies of Cs, Rb, Th, Nb, Ta and Zr. Similar geochemical features of all analyzed rocks indicate that they were all derived by more than 12% partial melting of an enriched/carbonated garnet lherzolite and underwent similar degree of partial melting. Geochemically, the studied blueschists resemble intraplate alkali-basalts. The presence of Paleozoic ophiolitic rocks along the main faults of central and northern Iran are indicative of a multisuture closure of the Paleo-Tethys ocean.
The Mesozoic ophiolitic mélanges of Nain and Ashin-Zavar are located in the western part of the Central-East Iranian microcontinent (CEIM), along the major faults of Nain-Baft and Dorouneh. They contain two different groups of highly metamorphosed rocks (amphibolitic rocks, schists, marbles and quartzites) formed through metamorphism of oceanic basaltic and sedimentary units, and also some less metamorphosed rocks (sheeted dikes, pillow lavas, limestones and radiolarian cherts), that were tectonically melanged. These features show that they formed in two distinct phases. Geochemical data point to an island arc tholeiitic affinity for the amphibolitic rocks, and to a MORB nature for the pillow lavas and sheeted dikes that are related to a back-arc basin. Accordingly, oceanic crust extensional processes should have been active during two phases: a- In Early Jurassic, the Nain and Ashin-Zavar oceanic crust segments started spreading and producing diabasic dikes and pillow lavas, covered by pelagic sediments, then they suffered a high-grade metamorphism during the closure of this oceanic sector around the Middle Jurassic. b- During Early-Late Cretaceous to Paleocene, oceanic spreading produced sheeted dikes, massive basalts, and basaltic pillow lavas throughout the Austrian orogenic phase. There is no evidence of high-grade metamorphism as amphibolitic rocks. Radiolarian cherts and Globotruncana limestones of Late Cretaceous age cover the basaltic rocks.
JOURNAL OF MINERALOGICAL AND PETROLOGICAL SCIENCES (13456296)105(2)pp. 74-79
Plagioclase lherzolites of Nain melange, Iran, show peculiar textures that indicate melt impregnation (1) droplet or bleb-like grains of plagioclase distributed in the peridotite matrix, (2) plagioclase-hearing clinopyroxenite seams, and (3) trails of plagioclase crosscutting pyroxene porphyroclasts The textural characteristics show post-deformational igneous formation of plagioclase, and possibly, associated clinopyroxene, from the impregnating melt The melt has precipitated the clinopyroxenite seams and chemically modified all the peridotite minerals Highly refractory compositions of the precipitated minerals suggest involvement of a highly depleted MORB-like melt The melt was an increment of partial melt produced by 8% to 10% fractional melting from the MORB source This is in contrast to the involvement of ordinary MORB in melt impregnation in abyssal plagioclase peridotites Integration of increments of mantle partial melts to form MORB was possibly incomplete in the very incipient mid-ocean ridge as in the short-lived Nain back-arc basin
