1.1 Geological background

Chromitite, a rock chiefly containing the chromite mineral, is the major source of chromium and a common rock-type in many ophiolite massifs. The mechanism of chromian spinel concentration in podiform chromitites has been discussed in many papers (e.g. [4,5,19,24,27,34]), but it has not yet been completely solved. Chromitites are able to record petrogenetic processes in the upper mantle. However, the study of many ophiolites reveals that they are not equally distributed among a mantle section of ophiolite complexes. In some cases, chromitites are never observed, contrasting with the majority of ophiolites in which chromitite is present in the transition zone [8,22,23]. This important observation has been elaborated in 1985 by F. Boudier and A. Nicolas, with the LOT-and HOT- ophiolite types [8], see also [19,23,24]. Chromitite is generally absent in ophiolites that are characterized by a lherzolite-dominant mantle section (Lherzolite Ophiolite Type [LOT]). On the contrary, it is present in ophiolites where this section is harzburgitic (Harzburgite Ophiolite Type [HOT]).

As the initial source of chromium is contained in clinopyroxene (diopside) [8], the degree of partial melting plays an essential role for the eventual formation of chromitites. In LOT-peridotites, which have suffered a low-degree of partial melting, chromium hosted by clinopyroxene is still largely retained in this mineral. In HOT-systems, on the other hand, the clinopyroxene of fertile mantle peridotite has been molten. As a result, chromium has been incorporated into the basaltic melt. This increases the possibility for the later segregation of chromium into chromitites during basalt crystallization [24].

Petrography of mantle peridotites of the Jandaq ophiolite shows that this ophiolite is of the LOT type, and based on the general trend, it should not contain the chromitite. Despite the general rule, some LOT ophiolites have chromitite (e.g. [8,15,24]), and paucity of chromite mineralization in some HOT is presented (e.g. [33]). These exceptions show the necessity of chromitite potential study on the Jandaq ophiolite as one of the important Iranian ophiolites.

Chromitites are present in most of Iranian ophiolites (e.g. Neyriz, Naein, Esfandagheh, Sabzevar, Faryab and Ashin), all belonging to the HOT-type [30,32]. However, they have not been found in other occurrences (e.g. Anarak, Posht-e-Badam and Jandaq, Bayazeh), which until now have not been studied in great detail.

The main aim of this paper is to check the chromitite potential of mantle peridotites in Jandaq ophiolite. This ophiolite presents good exposures of mantle peridotites in northwestern part of the Central-East Iranian Microplate (CEIM). This occurrence will then be compared with the Ashin ophiolite (Central Iran) which is a HOT with considerable amount of chromite deposits (Fig. 1). It is hoped that the comparison of Jandaq chromitite absent LOT with Ashin chromitite bearing HOT, will be useful to evaluate the chromitite potential in Jandaq ophiolite.

1.2 Geological setting

The study area (Jandaq ophiolite) is located on the West of central Iran, and the southern margin of Great Kavir with highly deserted geographical conditions (Fig. 2), considered to be Upper Proterozoic [1,3,28] or Paleozoic [10,11]. The Jandaq ophiolite is a remnant of Paleo-Tethys, transferred to central Iran by anti-clockwise rotation of Central-East Iranian microcontinent (CEIM) [6,10,11]. The ophiolite massif is composed of mantle peridotite and serpentinized mantle peridotite, metagabbro, basic and ultrabasic metamorphosed dikes, metapyroxenite, amphibolite, rodingite, and listwaenite [31]. It has been covered by Paleozoic metamorphic rocks (schist and marble) (Fig. 2). Chromitite has not been found yet. Mesozoic granite and mylonitic granite intrusions crosscut the Jandaq ophiolite and the covering metamorphic rocks. Chahpalang formation with Upper Jurassic age, and lithology of sandstone, siltstone and conglomerate, together with the Cretaceous limestone, have been covered by the Jandaq ophiolite, metamorphic rocks and granitic intrusions (Fig. 3A).

Analysis of minerals was carried out with a Cameca SX-100 electron-probe micro-analyzer (WDS) at the Institute of Mineralogy, Leibniz University, Hanover, Germany. The analyses were performed under an accelerating voltage of 15 kV and a beam current of 15 nA with 3-μm probe beam diameter. Natural and synthetic minerals of known composition are used as standards. The Fe⁺³ and Fe⁺² amounts of spinel were calculated assuming spinel (AB₂O₄) stoichiometry [13]. The Cr#, Mg# and Fe³⁺# are Cr/(Cr + Al), Mg/(Mg + Fe⁺²) and Fe³⁺/(Cr + Al + Fe³⁺) atomic ratio of minerals, respectively. Representative analyses of the minerals and calculated structural formulas are shown in Table 1. Mineral abbreviations in photomicrographs are from Kretz [18].

The major and trace element analyses were carried out on whole rocks at the Central laboratory of University of Isfahan by Bruker S4 Pioneer XRF, and Activation Laboratory of Isfahan (MNSR Department), by NAA method. Forty-four samples of mantle peridotites from the Jandaq and Ashin ophiolite were selected for analyses. Whole rock geochemical data are presented in Table 2.

The serpentine mineral was identified at the Central laboratory of the University of Isfahan as antigorite on the basis of its X-ray diffraction pattern by Bruker D8 Advance XRD machine.

In outcrop, these metamorphosed ultramafic bodies are grayish-green to dark-green. Generally, they are extensively serpentinized, metamorphosed and sheared, retaining very little primary relict texture. Main textures of the investigated rocks are porphyroblastic, granoblastic, nematoblastic, poikiloblastic and mesh texture. The textural relationships of mantle peridotites in Jandaq ophiolite demonstrate a prograde metamorphism from partially serpentinized peridotite to metaperidotite in response to a regional metamorphism in upper amphibolite facies conditions. A retrograde metamorphism has been developed after the progression. Therefore, the studied rocks have undergone three main stages of recrystallization. An initial retrograde hydration was followed by a progressive dehydration and a final retrograde hydration. The evidence of this complex hydration and dehydration reactions is presented by minerals description:

• - Clinopyroxenes (diopside with Mg# 0.91–0.97) and chromian spinels (Cr# 0.46–0.61) are relicts of the primary igneous mineralogy and are the most resistant minerals against the metamorphism and alteration. Inner part of spinels and most of clinopyroxenes are intact (Fig. 3B and 3G). The modal abundance of clinopyroxene in some rock samples reaches to 30% (very high) which partly are changed to tremolite during the metamorphism. Spinels altered to chromian magnetite in margin and their modal abundance in all samples is less than 3%. Cr₂O₃ contents of clinopyroxenes ranges from 0.71–1.31 wt% (Table 1).
• - Olivines (Fo₈₈–Fo₉₂) are metamorphic, produced during progressive metamorphism. This is suggested by very irregular grain boundary relations of olivines with other metamorphic minerals and fine magnetite inclusions [26]. Olivine grains show alteration to serpentine that is one of the evidences of last hydration. Concentrations of minor elements as TiO₂, Al₂O₃ and CaO are very low. MnO content ranges up to 0.14 wt.%.
• - Orthopyroxenes (enstatite with Mg# 0.89–0.96) are elongate and much coarser-grained than coexisting olivines. The margins of all orthopyroxens are replaced by serpentine. This texture indicates that the orthopyroxene retrograde serpentinization has occurred. Al₂O₃ and Cr₂O₃ contents of analyzed orthopyroxenes are 0.49–1.94, and 0.03–0.40 wt.%, respectively. Concentrations of minor elements are low. Compositionally, the studied orthopyroxenes are similar to the reported metamorphic orthopyroxenes from other ultramafic metamorphosed rocks [14]. Orthopyroxenes occur as granoblasts to bladed grains with scalloped borders in contact with olivine and interpreted to record the prograde reaction (Fig. 3E):

• - Tremolite (Mg# 0.89–1) is common in these ultramafic bodies and present prismatic crystals and some times jack-straw texture (Fig. 3C and D). Some tremolites are poikiloblastic, having inclusions of opaque minerals and chlorite that are common around igneous clinopyroxene relicts; others present intergrowth with olivine and serpentine. Tremolites and clinopyroxenes are present as the most important Ca-bearing silicates in most of these rocks. Main chemical characteristics of analyzed tremolites are low concentration of Na₂O (< 0.45% wt.%) and Al₂O₃ (< 1.62 wt.%), and high Mg# (> 0.89).
• - Anthophyllite (Fe³⁺# 0.52–0.53) is magnesian amphibole of these metamorphosed peridotites and is much less abundant than tremolite. Anthophyllite can be recognized by its parallel extinction and lack of twinning. This mineral makes sheaf-like aggregates (Fig. 3F). Concentrations of MgO ranges from 33 to 34.16 wt.%, and abundance of Na₂O is very low (< 0.03 wt.%). This orthoamphibole has a restricted stability field between approximately 600 and 800 °C at pressures below 12 kbar [29].
• - Chlorite (peninite with Mg# 0.90–0.91) is present with faintly green pleochroic in matrix and sometimes in veinlets of studied rocks. Presence of this mineral is another evidence of the last retrograde metamorphism in greenschist facies. Cr₂O₃ content of analyzed chlorites is 0.08 to 0.30 wt.%.
• - Antigorite (Mg# 0.89–0.98), which is colorless or less commonly pale green, is dominant constituent of Jandaq ophiolite metamorphosed mantle peridotites. It occurs mostly as fine-grained crystals. Antigorite has partially replaced and surrounded the metamorphic olivines by the last metamorphic episode. This mineral identified by microprobe analyses and XRD pattern. Antigorites have a wide range of Mg# (0.89–0.98) and Al₂O₃ contents (1.35–4.12 wt.%). The upper stability limit of antigorite is between 500 and 600 °C [31].
• - Talc (Mg# 0.98) is common in orthopyroxene bearing rock samples. It is not aligned with respect to a foliation. The Na₂O and Al₂O₃ contents of analyzed talcs range from 0.11–0.46 and 0.45–2.12 wt.%, respectively.

Based on the petrography and mineral assemblages, two rock types are distinguished in mantle peridotite section of Jandaq ophiolite:

• (1) Metalherzolites (clinopyroxene + spinel + olivine + tremolite + chlorite + serpentine + magnetite ± calcite);
• (2) Metaharzburgites (olivine + orthopyroxene + talc + anthophyllite + magnetite + spinel ± low amounts of chlorite and tremolite).

Table 2 lists the chemical compositions of 44 whole rock samples. Twenty-five samples from mantle peridotites of the Jandaq ophiolite and 19 samples from the Ashin ophiolite analysed to find the chemical nature of studied ultramafic system. The mean of analyzed samples from each ophiolite is presented in lower row.

All the analyzed samples from the Jandaq ophiolite are rich in MgO (23.48–40.47 wt.%), with wide range in the concentrations of Fe₂O₃^* (5.45–13.5 wt.%), Al₂O₃ (0.66–8.07 wt.%) and CaO (0.70–12.45 wt.%). Cr₂O₃ and TiO₂ contents are 0.04–0.75 wt.% and 0.06–0.23 wt.%, respectively. Considerable amounts of LOI confirm the extensively serpentinization of these rocks and presence of hydrous minerals. The lherzolite samples which contain considerable amount of relict clinopyroxene (J716, J719, J723 and J724), present the highest contents of CaO, Sc and V. Low-CaO lherzolites are essentially made by serpentine and fine-grained tremolite.

Study of whole rocks chemical composition shows similarity of all analyzed rock samples with the pattern of mantle peridotites from ocean basins. Comparison of whole rocks chemical composition from the Jandaq and Ashin ophiolites and using the mean of analyses reveals that the Jandaq ophiolite samples have lower contents of MgO, Cr₂O₃, MnO, NiO, Co, V and higher amounts of SiO₂, TiO₂, Al₂O₃, Fe₂O₃*, CaO, Na₂O, K₂O, Sc and REE. These chemical characteristics show the undepleted nature of mantle peridotites in Jandaq ophiolite.

5.1 Metamorphic condition of P-T equilibration

5.2 Petrogenesis

Highly variable contents of Al₂O₃ in analyzed samples (0.79–8.07 wt.%) is one of characteristics of orogenic lherzolites [9]. Low amounts of Cr₂O₃ associated with highly variable content in Al₂O₃ appear as characteristics of peridotite massifs [9], compared to cratonic and abyssal peridotites that show a large spread of Cr₂O₃ whole rock content correlated with Al₂O₃ content. The contents of other mildly incompatible elements as V, Sc and Yb, in whole rock chemical data of Jandaq mantle peridotites, and Cr₂O₃ content of clinopyroxenes, support this idea. It resembles that the Jandaq mantle peridotites provide us the Earth's mantle with unique exposure. Mineral chemistry of clinopyroxenes (diopsides) reveals the sub-oceanic crust mantle origin of the Jandaq peridotites (Fig. 4A and B).

5.3 Characterization of the ophiolite type

5.4 Implications for chromitite potential

In Jandaq meta-ophiolite, the predominant mantle peridotite is lherzolite, which composed of clinopyroxene, spinel, olivine, tremolite, chlorite, serpentine and magnetite. Geochemically, the minerals and whole rock samples appear as orogenic peridotite massifs. The main geochemical characteristics of these fertile peridotites are low amount of Cr₂O₃ and highly variable content of Al₂O₃. Based on the field and petrographical studies, chemistry of minerals and whole rock samples, the Jandaq peridotites belong to the LOT (Lherzolite Ophiolite Type), and have no chromite concentration potentiality, following the thermal structure and chemical composition.