Ghodrat Torabi, Ph.D. ,
Professor
Department Of GeologyFaculty Of Geology
Address
University of Isfahan
Azadi square
Isfahan, Iran
Postal Code : 8174673441
Research Output
Articles
2024
Publication Date: 2024/08/22
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.
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.
Publication Date: 2024/12/21
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 veins were 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.
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 veins were 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.
2025
Publication Date: 2025
Journal of African Earth Sciences (1464343X)228
The Eocene Kalut-e-Ghandehari (KG) pluton, located in the Central Eastern Iranian Microcontinent (CEIM), intrudes the Ashin Mesozoic ophiolite and Middle Eocene volcanic rocks. Petrographic and geochemical analyses reveal a calc-alkaline, metaluminous intermediate to mafic composition ranging from gabbro to monzonite. The rocks exhibit characteristic REE and HFSE patterns indicative of subduction-related magmatism. The KG pluton is composed of plagioclase (An = 34–60 %), Alkali-feldspar (Or = 70.8–96.1 %), diopside (Mg# = 0.71–0.90), phlogopite (Fe# = 0.3), and opaque minerals. Geochemical evidence (e.g., enrichment of LREE, LILE (e.g., Cs, Ba, Rb, Th, U), Zr, and Hf; depletion of HREE, Ti, Nb, and Ta, and Y) suggests partial melting of a lithospheric spinel lherzolite that had been previously enriched by an earlier subduction event. The geochemical similarities of parental magmas of the KG pluton and the Soheyl-e-Pakuh pluton (located in the neighboring ophiolite of Nain) indicate that both derive from a subduction-induced partial melting of a mantle peridotite. However, their magma sources temporality and spatially are in accord with eastern and western Neo-Tethys subduction-related magmatisms, respectively. Thus, the cross-cutting relationships between the pluton and the Ashin ophiolite, combined with geochronological data, support a pre-Upper Eocene closure of the eastern Neotethys oceanic crust. This finding provides valuable insights into the Cenozoic tectonic evolution of the Central Iran. © 2025 Elsevier Ltd
2024
Publication Date: 2024
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.
Publication Date: 2024
Neues Jahrbuch fur Mineralogie, Abhandlungen (00777757)199(1)pp. 37-54
Calc-alkaline to shoshonitic Eocene and alkaline Oligocene volcanic rocks are exposed in Godar-e-Siah and Toveiereh areas, respectively, northwest of the Central-East Iranian Microcontinent (CEIM). Granulitic xenoliths have been found in these volcanic rocks. The Godar-e-Siah xenoliths comprise the Ca-poor plagioclase (An33) + phlogopite + corundum + sillimanite + spinel ± garnet. This mineral assemblage corresponds to conditions characteristic of peak of granulite facies metamorphism. The Toveireh xenoliths consist of spinel and plagioclase as major minerals and corundum, rutile, ilmenite and magnetite as accessory ones. The presence of Al-rich minerals and the absence of quartz suggest the Al-saturated but Si-undersaturated nature for the xenoliths. Mineralogical characteristics, thermobarometry estimates and use of experimental petrogenetic grids indicate that the estimated P-T conditions for Toveireh (8 –10 kbar, 800 – 900 °C) and Godar-e-Siah (7.8 kbar, 780 °C) xenoliths are consistent with the granulite facies rocks near anatectic condition. The Toveireh xenoliths are Al-rich granulites (Al2O3 = 33 – 34 wt%) and have LREE-enriched patterns with large positive Eu anomalies (Eu/Eu* = 3 – 5). These patterns indicate that the plagioclase rich restites of the lower continental crust devel-oped as a result of the removal of Neoproterozoic-Cambrian S-type granitic magma. The Aeirakan S-type granites and the xenolith bearing rocks are located at the northwest of the CEIM along the Great Kavir Fault. The parental magma of the Aeirakan S-type granite which is located at northeast of the xenolith bearing sites (Toveireh and Godar-e-Siah) is formed by anatexis and dehydration melting of such Al-saturated Si-undersaturated crustal granulites during Pan-African orogeny. It is probable that some parts of these dehydrated materials are brought to the surface as granulitic xenoliths by Eocene and Oligocene volcanism in Godar-e-Siah and Toveireh areas, respectively. © 2024 E. Schweizerbart’sche Verlagsbuchhandlung, Stuttgart, Germany.