Chemistry of accessory chromian spinel in serpentinites from the Penjwen ophiolite rocks , Zagros thrust zone , northeastern Iraq

Chromian spinel is a widespread accessory mineral in ultramafic rocks of the Penjwen ophiolite, occurring in two modes; fresh grains in dunite rocks that crop out in the eastern part and altered grains in serpentinized peridotites in the western part of the Penjwen ophiolite. Unaltered grains are generally primary phase optically and chemically homogeneous with uniform high Cr# at ~ 0.72 and low Mg# at 0.3. HighCr#, low-Al2O3, low-TiO2 chromites have the geochemical characteristic of chromites preserved in arc lavas and specifically overlap the field of chromites in boninites. . The cores of altered spinel grains from serpentinized harzburgite appear to retain their original igneous chemistry and are similar to fresh chromian spinel grains. The outer rim of these grains shows different degrees of alteration. Four criteria characterized the alteration process of Cr-spinel in serpentinised peridotite rocks of the Penjwen ophiolite. First, optical inhomogeneity on the scale of micrometers; chromian spinel is deep reddish brown in thin section, but opaque due to replacement by magnetite along cracks and grain boundaries. Second, chemical zonation with a decrease in Mg, Al, and Cr, increase in Fe and significant modification of minor elements (Si, Ti and Mn) towards the fractures and grain margins. Third, significant amount of SiO2 analyzed in Cr-spinel. Although Si does not go into the crystal structure of spinel, the microprobe analysis detected sub ordinary amount of SiO2. This suggests the presence of submicroscopic silicate phase situated at the vacancy of dissolution elements (Mg, Al, and Cr). Fourth, Ferritchromite rims and chlorite aureoles. Altered spinels deviate from primitive Cr-spinel, by sharp increase in Fe# and Cr#. The chemical changes reflect hydrothermal metamorphic reactions between spinels and the surrounding serpentinized silicate matrix. Introduction Chromiam spinel is common in small quantities in peridotites, and its compositions have often been used to determine the petrogenetic conditions of evolving mafic magmas (Irvine, 1965). In igneous rocks the chromiam spinel compositions are sensitive to melt composition, crystallization pressure, and the degree of partial melting in the mantle source region (Dick & Bullen, 1984). More recently it has also been recognized that


Introduction
Chromiam spinel is common in small quantities in peridotites, and its compositions have often been used to determine the petrogenetic conditions of evolving mafic magmas (Irvine, 1965). In igneous rocks the chromiam spinel compositions are sensitive to melt composition, crystallization pressure, and the degree of partial melting in the mantle source region (Dick & Bullen, 1984). More recently it has also been recognized that chromites formed in mafic melts in different tectonic environments have compositional differences and so chromite has also become a powerful indicator of the former tectonic setting of mafic and ultramafic rocks (Kamenetsky et al., 2001). The chromite (Mg 2+ , Fe 2+ ) (Cr 3+ , Al 3+ , Fe 3+ ) 2 O 4 has a wide range in composition reflecting their primary magmatic or secondary origins. In the case of completely serpentinized ultramafic rocks, that preserve no primary silicate minerals, the composition of unaltered accessory chromite may provide useful petrogenetic information. Altered chromiam spinel occurred in serpentinized peridotites and serpentinites rocks as a result of serpentinization and regional metamorphism processes (Eales et al., 1988;Burkhard, 1993 andMelline et al., 2005). The best-known feature for alteration patterns in chromiam spinel is Fe-rich rims (ferritchromit) with higher reflectivity and depletion in some major elements (Cr, Al, Mg), due to element exchange and modification with the surrounding silicate minerals such as olivine and pyroxene (Kimball, 1990;Lipin, 1984). Several mechanisms have been discussed by workers; the effect of metamorphism on spinel alteration (Springer, 1974;Lipin, 1984 andBliss &McLean, 1975); whereas others (Ulmer, 1974;Bukhard, 1993;Tamura &Arai, 2005 and concluded that the alteration is due to the serpentinization process, while Mellini et al. (2005), referred the process to hydrothermal fluids. Chromian spinel cores appear to retain their original igneous chemistry under metamorphic temperatures up to 500 0 C and even higher in some circumstances (Barnes, 2000). This paper discusses the petrology of ultramafic rocks and chemistry of accessory chromian spinel from serpentinized alpine peridotites of Penjwen ophiolite complex, Zagros thrust zone, northeastern Iraq to conclude the genesis and tectonic environments of these rocks. Also it aims at explaining the alteration patterns of chromian spinel as assigned to serpentinization and metamorphic processes, and giving the different criteria for these alterations.

Geological Background
The Iraqi Zagros thrust zone in northeastern Iraq (IZTZ) represents a suture zone between the Arabian and Iranian plates to the northeast and the Turkish plate to the north. It occupies an area of ~5000 km 2 along the Turkey-Iraq-Iran border. The Iraqi Zagros thrust zone (IZTZ) represents a part of the larger Zagros belt, which belongs to the Alpine-Himalayan orogenic belt, and extends southeastward to Iran and northwestward to Turkey. The study area is located in the northeastern end of the IZTZ (Fig.  1). The Penjwen igneous complex is situated to the southwest of the Penjwen village (Fig. 2) about 40 km to the east of Sulaimani City, representing an ophiolite segment within the larger Zagros belt and it has been well recognized that the ophiolite belongs to the Tethyan ophiolitic belt of Mesozoic age (Mohammad et al., 2007;Jassim &Goff, 2006 andAlavi, 2004). The Penjwen ophiolite complex is an elongated body about 16 km long and 5 km wide trending northwest-southeast parallel to the regional tectonic lineaments; a considerable part of the ophiolite lies within the Iranian territory.The complex is an incomplete ophiolitic sequence (Jassim & Goff,2006), composed of ultrabasic, basic and dioritic rocks, emplaced tectonically on to the volcanosedimentary rock sequences of Penjwen group. Later on this complex was thrusted to its present position over Mirga Red Bed series during major tectonic events of Miocene-Pliocene (Al-Hassan & Hubbard, 1985). The Penjwen ophiolite sequence consists of Alpine-type peridotites overlain by banded gabbros and diorite with a small dike of pyroxenite. The complex is a continuous sequence from dunite, harzburgite and pyroxenite at the bottom, through amphibolitized gabbro, to diorite at the top. It is separated from the Tertiary Red Bed series to the west by a high angle reverse fault. The complex is in contact with metamorphosed limestone of the Qandil unit of cretaceous age at the northeastern end, and the boundary passes just south of the town of Penjwen (Jassim & Joff, 2006)

Petrology of Ultramafic Rocks
The ultramafic rocks form about 70% of the complex and occur in two exposures, cropping out in the western and eastern limits of the complex. Dunite and peridotites form the major type of these ultramafic rocks with minor occurrence of pyroxenite dykes that are roughly oriented in an eastwest direction. The peridotites are chiefly composed of harzburgite and subordinate lherzolite. The dunites make up more than 60 % of the Penjwen ultramafic rocks which are mostly characterized by xenoblasticgranular texture. Ultramafic rocks are almost completely serpentinized in the western part of the complex, while partially serpentinized in the eastern part, which are early low-temperature serpentinization products (greenschist facies). The differences in serpentinization degrees between the two parts are perhaps due to the differences in the effect of the major thrust fault on each part. Mesh-textured chrysotile/lizardite are prominent minerals, and they sometimes exhibit wavy extinction indicating distortion of precursor orthopyroxene ). Clinopyroxenes or their pseudomophs are absent because the bastite is very uniform in texture and appearance. Elongated and fine-grained opaque minerals (mainly magnetite) are commonly observed along the margins of individual mesh cells of olivine pseudomorphs and cleavages in pyroxene bastites. Chromite crystals occur either as disseminated, anhedral to subhedral grains, which constitute less than 4 % of the rock, or as bands or schlieren. They also occur in clusters forming pods and lenses some are >2 m length and ~0.5 m width. These pods and lenses are distributed irregularly over the entire rock body. Modal proportions of chromite in these pods and lenses are frequently as high as 80 %. Chromian spinel is strongly fractured and reddish brown in thin section. Alteration products with high reflectivity and sharp optical boundaries with unaltered part were produced along rims and cracks of chromian spinel.

Sampling and Analytical Methods
Specimens from two localities (eastern & western) part of the Penjwen complex were investigated. Twenty representative samples were selected for a detailed study using electron microprobe, including 200 spots of accessory chromiam spinels from serpentinized ultramafic rocks, which were analysed on polished thin sections for major and minor elements using a wavelength dispersive microprobe (JEOL Superprobe JXA-8800) at the Cooperation Research Center, Kanazawa University. Raw data for each element were corrected using the ZAF program. This work used various natural and synthetic minerals as standards (olivine for Mg and Si; albite for Al; wollastonite for Ca; ilmenite for Fe and Ti; chromite for Cr; and V, Mn, Ni and Zn metal). Analytical conditions were, 15 kV for accelerating voltage, 20 nA for beam current, and 3μm for the beam diameter. The counting time was 20s on the peak of the characteristic X-ray for each element. Analytical errors were <0.5 wt.% and <0.1 wt.% for major and minor element oxides, respectively. In the analyzed minerals, Mg# and Cr# are Mg/(Mg + Fe2+) and Cr/(Cr + Al) atomic ratios, respectively. We assumed all Fe in silicates was ferrous. Ferrous and ferric (Fe +2 ; Fe +3 ) in chromian spinel was calculated from raw analyses assuming spinel stoichiometry. The same technique (microprobe) was used to draw six elemental distribution maps for Cr, Fe, Mg, Al, Ti and Mn elements of selected altered chromian spinel to clarify the alteration process. These maps draw as result of the 500 spot analysis in the same grain. Map measurement conditions were; 15 kV accelerating voltage, 3 um pixel size, 30 ms dwelling time.

Mineral chemistry
Chromites structure can be compared with the formula AB 2 O 4 , where A denotes divalent cations (Fe +2 , Mg +2 ) occupying tetrahedral sites, B denotes trivalent cations (Cr +3 , Al +3 , Fe +3 ) occupying octahedral sites. Two different occurrences of chromian spinel grains were found in peridotitic rocks of the Penjwen complex; fresh and altered grains. Most chromian spinel grains associated with dunites were fresh and contain no evidence of alteration which crop out in the eastern part of the complex, while in the western part of the complex, the grains show different degrees of alteration within harzburgite lherzolite rocks,. Chemical characteristics and representative microprobe analysis of the two types of grains and associated minerals are summarized in Tables 1,2 and 3. Unaltered grains are generally primary phase optically and chemically homogeneous with uniform high Cr# at ~0.72 and low Mg# at 0.3. Generally, TiO 2 content is low at approximately 0.2 wt%, Al 2 O 3 is 13wt %, FeO is 29wt% and no SiO 2 is detected. The cores of altered spinel grains from serpentinized harzburgite appear to retain their original igneous chemistry and are similar to fresh chromian spinel grains, which indicating that alteration has not reached the cores of the mineral. Alteration along rims and fractures show depletion of Cr, Al, and Mg, enrichment in Fe as major elements and for minor elements there was enrichment in Mn, Ti and Si (Table 3; Fig 8).

Genesis and Tectonic Setting
For a long time, chromites have been considered as petrogenetic indicators, capable of conveying information about magmatic features (Irivan, 1967;Arai, 1994 andIsmail et al., 2009). The peridotite rocks of Penjwen complex are characterized by the high Cr# (>0.70). In the relationship between Cr# of spinel and Fo of olivine, the harzburgites fall within the OSMA (olivine-spinel mantle array; Arai, 1994) indicating the residual mantle peridotite (Fig. 3) The Cr# of chromite is a good indicator of the degree of partial melting for the mantle-derived spinel peridotite (Dick & Bullen, 1984;Arai, 1994). It is higher in peridotite residue with a higher degree of melt extraction from partial melting, such as more deplete peridotite. The high Cr# nature of almost all chromites of studied rocks, suggests crystallization from a boninitic magma produced by second stage melting (Fig. 4).The Cr# is also useful to discriminate the tectonic setting of derived peridotite. After a long scientific debate, the diverse petrological and structural features of ophiolites are now ascribed to the various tectonic settings where oceanic lithosphere is generated although some authors attribute these differences to slow-or fastspreading ridge systems (Nicolas, 1989). Accordingly, the main magmatic features of ophiolites classified as high-Ti, and as low-and very low-Ti have been, respectively related to two distinct geodynamic settings, namely Mid-Ocean Ridges (MOR) and intraoceanic supra-subduction zones (SSZ) (Pearce et al., 1984).  Kamenetsky et al. (2001) have compiled a database of Al2O3 and TiO2 compositions of chromite and have identified fields with varying degrees of overlap that can be used to distinguish six different tectonic settings. On the basis of these parameters, chromite compositions of Penjwen samples plot in the arc and supra-subduction zone fields (Fig. 5,6).

Alteration of chromian spinel
Chromite alteration has been studied widely (Kimball, 1990, Burkhard, 1993Barnes, 2000 and, and has been related to hydrothermal alteration process before, during or after serpentinization (Mellini et al., 2005) or to magmatic alteration and to regional metamorphism (Jan & Windley 1990). Penjwen serpentinites occurring in the regionally metamorphosed Zagros belt have been subjected to two alteration processes; serpentinization and regional metamorphism which may or may not coincide. The relationship of these alteration processes to the alteration of accessory chromiam spinel is, therefore, often obscured (Burkhard, 1993). The chromites associated with serpentinized peridotitic rocks show differences in chemical composition from the fresh chromites. The altered chromites in the Penjwen serpentinite show a considerable variation in Cr# [Cr/(Cr + A1)], and Mg# [Mg/(Mg + Fe 2+ ], reflecting different degrees of the alteration, indicating the important role of serpentinization in the alteration of chromian spinel. The absence of plagioclase, the presence of relict olivine, and the occasional presence of augen pyroxene in serpentinites of this area leave little doubt as to the ultramafic precursor and petrogenetic sequence of the serpentinization reaction (Ulmer, 1974). Actually, two different spinel compositions have been distinguished within the Penjwen serpentinites: Cr-spinels, occurring as rounded anhedral grains, and altered spinels (ferritchromite rims), which consist of a complex association of Cr-magnetite + chlorite/lizardite rather than existing as a single phase (Fig 7). These altered spinels are typically surrounded by chlorite aureoles (Fig 9). Different chromian spinel stabilities can be inferred from their crystal structure as follows: a divalent ion in tetrahedral coordination yield the most stable arrangement according to the electronegativity principle of Pauling (Burkhard, 1993). This arrangement is achieved in a normal spinel where divalent ions accupy 1/8 of the tetrahedral holes and the trivalent ions 1/2 of the octahedral holes of a closed packed cubic lattice. Such an arrangement contrasts with that of inverse spinel structure where divalent ions are in octahedral site with 1/2 of the trivalent ion, the other half of which occupy the tetrahedral sites. Normal spinel is, therefore, expected to have a higher stability than the inverse one.
The alteration of chromian spinel associated with serpentine from the study area is caused by decomposition of the spinel structure of solid state in which Mg, Al and Cr diffuse out and Fe, Si, and other minor ions ( Ti, Mn and Ni ) diffuse in from serpentinized solution. As a result there is an increase in Cr# [Cr/(Cr + A1)] and/or Fe# [Fe 2+ /(Fe 2+ + Mg)] from core to rim (Table 3) . This chemical process is in agreement with the secondary hydrothermal reaction of chromite described by Kimball (1990). Fe content in chromian spinel control the degree of alteration, those Fe rich spinels are readily subjected to alteration. During hydrothermal alteration Al and Mg are mobilized to form amphibole and chlorite. The NiO content of the unaltered chromite core is low, being bellow 0.04 wt%, whereas that in outer rim of the ferritchromite reaches 0.28 wt%. This is attributed to the release of Ni from olivine during serpentinization. TiO 2 is correlated with NiO and reaches its highest values in outer rims of the chromite grains.

Criteria for alteration
Chromians spinel minerals of the Penjwen serpentinite rocks show different degrees of alteration, and some of these alterations may not be visible in thin sections. There are four criteria that characterized the alteration of chromian spinel in serpentinite rocks of the Penjwen ophiolite: Optical inhomogeneity on micron scale. Chromian spinel is deep reddish brown in thin section, but opaque due to replacement by magnetite along cracks and grain boundaries. On polished surfaces, the opaque secondary regions, have greater reflectivity with sharp optical boundaries in primary spinel. This feature is best known and well described by many authors, such as Gahlan, et al. (2006); Tamura and Arai (2005), Barnes (2000) and Burkhard (1993).
Chemical zonation. This is manifested by a decrease in Mg, Al, and Cr, increase in Fe and significant modification of minor elements ( Si, Ti and Mn ) towards the fractures and grain margins (Table 2) as a result of introduction of these elements from hydrothermal fluids that take roles in serpentinization processes (Barnes, 2000). For the elementary distribution color maps, the zonations are well recognized (Fig.8A), a decrease in Cr content from core (~52 wt %) to rim (~35) wt%. is shown by changing color from red in the core to yellow in the rim with changes in Cr concentration. Al color map changes from red to green (Fig. 8B), with a decrease in Al content from core (~12 wt %) to rim (<5 wt %). Fe content increases from core (~28 wt %) to rim (>40 wt %) and the colored map change from yellow in the core to red in the rim (Fig. 8C). Mg decrease from core (~5 wt %) to rim (~3 wt %) and color change from blue to deep blue (Fig. 8D). Slightly color changes is due to the increase in Ti and Mn content from core to rim (Fig 8E & F), the color changes from blue to green, and blue to spotty green blue respectively. These chemical changes form ferritchromite and magnetite which cannot be related to magmatic differentiation, is attributed to the regional metamorphism where access Fe content was driven from decomposing silicate minerals (olivine, opx, cpx) during serpentinization (Qasim Jan et al., 1985;Barnes, 2000). Color scale bar changes with concenteration of elements Aconcentration of Cr decrease from red in center to yellow in the margin, B-Al decrease from red to green, C-Fe increase from yellow to red, D-Mg decrease from blue to deep blue, E-Ti increase from blue to green, F-slightly change in Mn concentration pattern with color. Bar scale represent the elements concentrations.

Significant amount of SiO 2 analyzed in Cr-spinel.
Although Si is not present in the standard crystal structure of spinel, the microprobe analysis detected subordinary amount of SiO 2 ( Table-3). Variations in SiO 2 are not random but tend to be systematically related to negative variation of Cr and Al. The appearance of silica at the altered grain margin suggests the presence of submicroscopic silicate phase situated at the vacancy of dissolved elements (Cr and Al). The source of silica may be the host silicate rocks or fluids that took place during regional metamorphic serpentinization process (Burkhard, 1993).
Ferritchromit rims and chlorite aureoles. Altered spinels deviate from Cr-spinels, by sharp increase in Fe# and Cr#. The chemical change reflects hydrothermal metamorphic reactions between spinels and surrounding post-serpentinization silicate matrix (Mellini et. al, 2005). Al, Mg and Cr diffuses out of magmatic spinel, leaving behind a Fe-rich spinel (Crmagnetite) and promoting the formation of chlorite aureoles (Fig. 5). The formation of chlorite by spinel alteration has been described by Kimball (1990) as a fairly low to medium temperature process (> 400 o C) in the presence of MgO-and SiO 2 -rich fluids, according to the reaction MgAl 2 O 4 + 4MgO + 3SiO 2 + 4H 2 O Mg 5 Al Si 3 AlO 10 (OH) 8 .
According to petrography (upper greenschist facies), texture and chemical characteristic of Cr-spinel from Penjwen serpentinite, this study agrees with Kimball as regards the thermal environment of the hydrothermal reaction leading from magmatic spinels to chlorite. However, there are two stages of chlorite formation. The first one corresponds to the replacement of the Cr-spinel by the ferritchromit, whereas the second one affects the outer mesh serpentine, which is progressively transformed to a disordered chlorite/lizardite intergrowth. Thus, rather than a reaction of spinel with MgO-and SiO 2 -rich fluids, it is suggested here that spinel reacts with already formed serpentines, as confirmed by textural evidence indicating that the chlorite aureole overgrew mesh-textured serpentines preserved elsewhere.

Conclusions
1-Two different spinel compositions have been distinguished within the Penjwen serpentinites: Cr-spinels, occurring as rounded anhedral grains, more or less preserved and altered spinels (ferritchromite rims) with different degrees of alteration.
2-Unaltered grains are generally optically and chemically homogeneous with uniform high Cr# at ~ 0.72 and low Mg# at 0.3. TiO 2 content is generally low, about 0.2 wt%.
3-High-Cr#, low-Al 2 O 3 , low-TiO 2 chromitites have the geochemical character of chromitites preserved in arc lavas and specifically overlap the field of chromites in boninites. 4-Altered minerals show different degrees of alteration, which indicate the important role of serpentinization in alteration of chromiam spinel.
5-There are four criteria that characterized alteration of Cr-spinel in serpentinised peridotite rocks from Penjween complex: first, optical in homogeneity on the scale of microns; second, chemical zonation; third, significant amount of SiO 2 analyzed in Cr-spinel; fourth Ferritchromite rims and chlorite aureoles.