For the first time, albitite was found in the Iraq Zagros thrust zone near the village of Mlakawa, 60 km northeast of Sulaimani City, Kurdistan region, northeastern Iraq. It occurs as a white pod within the massive tectonized and serpentinized part of the Penjwin ophiolite sequence. Based on the preserved texture and mineralogical, petrological, and geochemical data from the core of the albitite pod, a plagiogranite protolith of Mlakawa albitite was inferred. It has undergone rodingitization and blackwall formation along its rim. The occurrence of barium aluminosilicate (celsian), cymrite, barium muscovite, and a high Na2O concentration (11 wt%) of albitite suggests that barium-sodium–rich fluid was involved during the albitization process of plagiogranite. Evidence of the progressive albitization includes the metasomatic replacement of Caplagioclase to albite and grossular, celsian to cymrite, replacement of tremolite by edenite, and newly formed sheaf-like barium muscovite. The presence of analcime and multiple generations of chlorite suggests that the albitite protolith was accompanied by chloritization and retrograde metamorphism before and after the albitization process. Ca-amphibole thermobarometry and the occurrence of strontium apatite and cymrite suggest that the albitization of plagiogranite occurred at <650 °C and 1.5 GPa.
Rock types including rodingite, jadeitite, and albitite have been closely associated with serpentinite, or found as inclusions in serpentinite, and are generally recognized as products of the serpentinization process (e.g., Coleman, 1980; Harlow, 1994; Dubińska et al., 2004). Although more studies have been conducted on both rodingite and jadeitite (e.g., Harlow, 1994; Hatzipanagiotou et al., 2003; Dubińska et al., 2004), albitite has received little attention because of its very rare occurrence. Albitite consists almost entirely of albite and contains subordinate amounts of muscovite and quartz. Albitite is found in several geological settings, one of which is in association with ultramafic rocks at amphibolite, granulite, and blueschist facies. Several origins have been proposed for albitite, including metasomatism of granitic rocks by hydrothermal fluids (Kovalenko, 1978; Cathelineau, 1988), direct precipitations from solution (Harlow, 1994; Johnson and Harlow, 1999), and direct crystallizations from Na-rich magma (Schwartz, 1992).
During the field work on serpentinite along the Iraq Zagros thrust zone (Mohammad, 2004), we found for the first time a pod of albitite within massive tectonized serpentinite of the Penjwin ophiolite sequence. This was on Mlakawa Mountain near the KaniManga village, 60 km northeast of Sulaimani City, Kurdistan region, northeastern Iraq. In this paper we record the occurrence of albitite in the Iraq Zagros thrust zone, describe the mineral assemblage and petrologic setting of this albitite locality, and identify the albitite protolith and the relationships between the albitization, rodingitization, and serpentinization processes during the evolution of the ultramafic part of the Penjwin ophiolite sequence.
Because the rock has undergone rodingitization and blackwall formation along its rim, samples were selected based on careful petrographical study. Five representative samples covering the Mlakawa albitite pod were selected for geochemical and mineral analysis. Analyses of major oxides and trace elements were performed on a PW 2404 Philips X-ray fluorescence spectrometer, using the LiBO4 fusion method at ALS-CheMEX Analytical Laboratory, Canada. We mixed 1 mg with 9 mg of lithium tetraborate flux and fused them in a furnace at 1100 °C. A flat disc was prepared from the resulting melt. Rare earth element (REE) measurements were done by Elan 6000 inductively coupled plasma–mass spectrometer (ICP-MS), using the LiBO2 fusion method (ALS-CheMEX Analytical Laboratory, Canada). A sample of 0.2 mg was mixed with 0.90 mg lithium metaborate and fused in a furnace at 1050 °C. The resulting melt was then cooled and dissolved in 100 mL of 4% nitric acid. For this study, the Canadian standards SY-4 and G2000 were used. Compositions of minerals were analyzed by using a JEOL-840A scanning electron microscope at Osaka Prefecture University, equipped with an energy dispersive detector analytical system (Link ISIS series L200I-S). Accelerating voltage and current were 15.0 kV and 0.5 nA, respectively, with corrections made using the ZAF method (Sweatman and Long, 1969). Suitable synthetic and natural mineral standards were applied for calibration. Total iron was measured as FeO and was recalculated by using ideal stoichiometry to give the FeO and Fe2O3 values.
GEOLOGY OF THE STUDY AREA
Little information has been published about the geology of the Kurdistan region of northern Iraq because of political instability in that area. Thus, the geology of the study area, as discussed here, was based on unpublished reports and the works of Mahmmod (1978) and Mohammad (2004). The Iraq Zagros thrust zone in north and northeastern Iraq 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 km2 along the Turkey-Iraq-Iran border. The Iraq Zagros thrust zone represents a part of the larger Zagros belt, which extends ∼2000 km from southeastern Turkey through northern Syria and Iraq to western and southern Iran. The studied area is located in the northeastern end of the Iraq Zagros thrust zone in Iraq (Fig. 1), and includes both the Shlair zone and the Qulqula-Kwakurk subzone; the latter includes the Penjwin igneous complex, Qandil unit, and Tertiary Red Bed series. The Penjwin igneous complex represents an ophiolite sequence within the larger Zagros belt. It is a northwest-southeast–trending elongated body (35 km2) within the Iraqi territories. Remnant parts were located within adjacent Iranian territories (Mahmmod, 1978). The Penjwin ophiolite sequence is bounded on the east by phyllite and calcschist of the Qandeil unit and on the west by the Merga Red Bed, of the Tertiary Red Bed series (Fig. 2). The Penjwin ophiolite sequence consists of alpine-type peridotite overlain by banded gabbro and diorite with a small dike of pyroxenite. Field relations show that the complex is a continuous sequence from peridotite (dunite, harzburgite, websterite, and bronzitite) at the bottom, through amphibolitized gabbro, to diorite at the top. A large part of the ophiolite extends into Iran territories. The Penjwin igneous complex 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 Qandiel unit at the northeastern end, and the boundary passes just south of the town of Penjwin.
During this study rodingite and albitite were found within the basal serpentinized ultramafic portion of the Penjwin ophiolite sequence in the Iraq Zagros thrust zone. Albitites occur as white pods or tectonic inclusions 2 × 3 m in dimension, and have sharp boundaries with the host serpentinite (Fig. 3). They underwent rodingitization and blackwall formation along the rims. Rodingite is a Ca-rich, SiO2-undersaturated rock, developed as a result of hydrothermal alteration and metasomatism between ultramafic rock and various igneous rocks, including granite, diorite, and gabbro, in an oceanic setting during the serpentinization process of ultramafic rocks (Dubińska et al., 2004). Blackwall is a thin rim around rodingite or albitite hosted in serpentinite, consisting of alternating bands of chlorite and carbonate minerals. In other localities within the same serpentinite body, rodingite also occurs as dikes 20–30 m long and 5 m wide. Based on the mineral assemblages and whole-rock composition of the rodingite, rodingite seems to have been derived from gabbroic dikes (Mohammad, 2004).
The Mlakawa albitite is a leucocratic, coarsely crystalline, and partly mylonitized rock. It underwent rodingitization and blackwall formation along its contacts with the host serpentinite. The overall texture of the rock is hypidiomorphic granular. Oscillatory and antiperthitic textures in plagioclase are common (Fig. 4A). The core of the albitite pod consists mainly of albite, analcime, chlorite, and amphibole. Allanite, apatite, strontium (Sr)-apatite, zircon, and ilmenite occur as accessory minerals. Albite occupies 80–90 vol% of the albitite and is found as highly fractured coarse crystals (0.1–5 mm grain size). It is commonly fragmented and shows kink bands. Locally, albite displays two sets of cleavage (Fig. 4B). Although feldspar with two sets of cleavage is very rare, this common phenomenon in albite suggests that it may be inherited from a jadeitic pyroxene. Zoned grains of albite contain fine inclusions of grossular garnet in the cores. Coarse-grained (0.3 mm) chessboard albite is also present, which is an unusual type of albite when viewed under crossed polars, showing alternating black and white rectangles resembling a chessboard (Moore and Liou, 1979). The volume percentage of albite decreases toward the rim of the pod. Analcime occurs either as inclusions in albite or as veins between and/or across albite grains. Amphibole (tremolite, edenite; see following) (10–20 vol%) occurs as aggregates of acicular crystals, 0.05–0.1 mm long. The amphibole aggregates rim the albite grains (Fig. 4D), and some acicular amphiboles penetrate the center of albite grains. The occurrence of thin amphibole rims on albite grains suggests that it is not the primary phase but a secondary product. Its volume increases from the center of the pod outward.
Diopside inclusions associated with chlorite and grossular garnet are found in coarse-grained albite (Fig. 4C). They are 0.02 mm in size. Allanite (1–5 vol%) occurs as brown coarse anhedral crystals (0.05 mm) (Fig. 4E), and as inclusions in apatite or ilmenite (Fig. 4F). It shows typical patchy zoning (Fig. 4G). Apatite (3 vol%) occurs as a subordinate phase with highly fractured coarse grains (0.1–0.5 mm), whereas Sr-apatite is found as fine-grained euhedral crystals within analcime (Fig. 4H), as inclusions in apatite (Fig. 5A), and as a thin rim around apatite grains (Fig. 5B). Zircon occurs as accessory euhedral crystals (0.01–0.05 mm) (Fig. 5C). They show oscillatory zoning, which is interpreted to indicate a magmatic origin. Ilmenite occurs as anhedral coarse grains, rimmed by rutile or metamorphic titanite. Celsian and cymrite are very fine grained and difficult to identify by optical microscopy. In backscattered electron (BSE) images, both celsian and cymrite occur as fine anhedral grains. Celsian is only found in the rim of pod and is associated with amphibole. Cymrite is found in the core of the pod and is associated with albite and analcime (Fig. 5D).
Rodingite Rim and Blackwall
The 10–50-cm-thick rodingite rim surrounding the albitite pod consists of the mineral assemblage of grossular + prehnite + calcite + chlorite + apatite ± amphibole, with minor disseminated fine-grained aggregates of titanite, celsian, and zircon.
Blackwall, the outermost zone of the albitite pod adjacent to the host serpentinite, is a thin irregular rim (5–10 cm thick) composed of alternating bands of clinochlor and calcite (Fig. 5E). It is cut by grossular and carbonate veins.
The host serpentinite is commonly massive and characterized by pseudomorphic textures. Olivine and pyroxene have mesh and baste textures, respectively. In mesh texture, the mesh center consists of olivine and serpentine with a rim consisting of serpentines, and is more common in dunite. Baste texture resemble mesh texture except that the mesh center consists of pyroxene, and it is more common in pyroxene-rich peridotite. Serpentine pseudomorphs consist of lizardite and chrysotile, which are early low-temperature serpentinization products. Antigorite is also found as a later-stage recrystallization product in the sheared parts of host serpentinites (Fig. 5F). In addition to serpentine, chromium chlorite, talc, tremolite, disseminated chromian spinel, and relict olivine and pyroxene are also found. 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 bustites. Based on the unaltered spinel chemistry (Cr# > 0.6) of the host serpentinites, the type III alpine peridotite of Dick and Bullen (1984) is a possible precursor of the serpentinites.
MINERALOGY AND MINERAL CHEMISTRY
Based on optical petrography and BSE analysis, four types of albite were distinguished. They were thought to reflect multiple sources and origins. Representative chemical analyses of various types of albite are given in 01Table 1.
1. Magmatic albite (Ab97An2Or1) occurs as large euhedral to subhedral crystals, typically with oscillatory zoning. It shows plastic deformation fabrics such as kink bands and undulatory extinction, and also underwent later brittle fragmentation. Chemical analysis of euhedral plagioclase with concentric zoning does not show a large compositional range; that is, Ca contents are 2–3 wt% richer in the core than in the rim. This zoning can be interpreted in three ways. First, direct crystallization from Na-rich magma could have occurred (e.g., Schwartz, 1992). Second, a primary zoned plagioclase could have changed to albite by metasomatism (e.g., Smith, 1974). Third, the occurrence of fine inclusions of grossular in albite may suggest the breakdown of calcic plagioclase to form garnet and albite.
2. Chessboard albite (Ab99An1) occurs as large euhedral to sub-hedral grains surrounded by magmatic albite (Fig. 5H). It has been reported from keratophyres (Battey, 1995; Carstens, 1966), granite (Gilluly, 1933; Anderson, 1982), and plagiogranite (Dubińska et al., 2004; Kaur and Mehta, 2005). In these occurrences, the chessboard albite is considered to have replaced potassium feldspar during metamorphism or metasomatism. The lack of either K-feldspar components or inclusions of K-feldspar in the chessboard albite suggests igneous origin.
3. Albite after Ca-bearing plagioclase (Ab97An3) occurs around ilmenite interpreted to have undergone titanitization during the albitization process because Ca-bearing plagioclase is the only source of Ca in titanite, and the titanite is the reaction product of Ca-bearing plagioclase with ilmenite, according to the following reaction (Harlov et al., 2006):
The albitites contain as much as 3 vol% apatite. Apatite occurs as large subhedral grains, and is commonly highly fractured. It contains inclusions of patchy zoned allanite and fine inclusions of Sr-apatite. Large apatite grains, characterized by blue, contain low SrO contents (<0.1 wt%), and lack SiO202(Table 2). Qualitative energy dispersive X-ray analysis suggests that a small amount of chlorine is present, possibly representing Cl-apatite of Deer et al. (1992). The Sr-apatite inclusions contain >50 wt% SrO and as much as 4 wt% SiO2. Because the presence of SiO2 in the apatite structure requires a coupled substitution of Si+4 for P+4 and Na+ or REE to Ca+2, the Sr-apatite contains a large amount of Na2O. Similar Sr-bearing apatite has been recorded in high-pressure metamorphic rock (e.g., Krenn and Finger, 2004) and kimberlite (e.g., Chakhmouradian et al., 2002).
Allanite and Analcime
Allanite occurs as large anhedral grains or as an inclusion in both ilmenite and apatite, and always shows patchy zoning (Fig. 4G). Allanite is of the Ce-allanite type; it contains >12 wt% of CeO, 8 wt% of LaO2, and 3 wt% of NdO203(Table 3). Analysis of X-ray maps and BSE images indicates significant REE substitution for Ca and Fe substitution for Al in allanite, as suggested by Gieré and Sorenson (2004) (Figs. 6A, 6B).
Analcime is found as a vein filling or as inclusions in albite. Compositions of analcime are given in 04Table 4. It is a hydrous equivalent to jadeite.
Barium muscovite, celsian, and cymrite are representative barian minerals found in albitite pods in the study area. The compositions of these minerals are given in 05Tables 5 and 066. Barium muscovite contains as much as 10 wt% BaO. It is typically zoned with BaO increasing and K2O decreasing from core to rim (Fig. 5G).
Celsian and cymrite are characterized by high BaO content (37–41 wt%) 06(Table 6). Cymrite is a hydrous high-pressure equivalent to celsian. It can be distinguished from celsian based on composition; cymrite gives totals of ∼100, whereas celsian gives totals <100 by 4–6 wt%.
The representative analyses of amphibole are given in 07Table 7. Amphiboles display compositional zoning, with Na2O increasing gradually from the core to the rim (Fig. 7), and FeO and MgO decreasing toward the rim. According to the classification of amphiboles by Leake et al. (1997), the cores of amphibole are tremolite-actinolite, whereas the rims are edenite (Fig. 8).
Compositions of pyroxene are given in 08Table 8. According to Morimoto et al. (1988), pyroxenes are classified into diopside. They are concentrically zoned (Fig. 9), and have a colorless core and pale yellowish-green rim.
Whole-rock analyses of five samples from the core of the albitite pod show that these rocks contain large amounts of SiO2 (average 63.8 wt%), Al2O3 (average 19.5 wt%), and Na2O (average 11.4 wt%) 09(Table 9). They show enrichment in all REEs (Figs. 10A, 10B). Spatially, light REE concentration is ∼100× chondrite, middle REE is 10× chondrite, and heavy REE is 7× chondrite. The Eu anomalies may relate to original Eu anomalies in the albitite protolith or mobility of this element during chessboard albite formation from feldspar or Ca-plagioclase. Similar REE patterns with Eu anomalies were recorded in Slavezines massif albitite in France and albitite deposits from central Sardinia, Italy (e.g., Boulvais et al., 2007; Castorina et al., 2006). For chemical classification, oxide weight percents were plotted in the CNK(CaO Na2O K2O) diagram (Fig. 11) of Glikson (1979) and normative albite (Ab), anorthite (An), and orthoclase (Or) were plotted on an An-Ab-Or ternary diagram (Fig. 12) (Barker, 1979). In both diagrams, the samples are located in the trondhjemite field. Typical plagiogranites worldwide plot in the fields of trondhjemite and tonalite (e.g., Coleman and Peterman, 1975; Rao et al., 2004; Kaur and Mehta, 2005). On the A/CNK-A/NK [A/CNK- Al2O3/(CaO Na2O K2O)] diagram (Fig. 13A) of Shand (1943), albitized plagiogranite is located in the peraluminous field and tends to be Al saturated with an A/CNK close to 1, which is the value of albite. In SiO2/Al2O3-A/CNK (Fig. 13B) of Kaur and Mehta (2005) and K2O-SiO2 diagram (Fig. 14) of Coleman and Peterman (1975), Mlakawa albitized plagiogranite is located in the field of high aluminous oceanic plagiogranite. Although the rock underwent loss and gain of Na2O, CaO, and Al2O3 compared with typical plagiogranite, differences are not large enough to shift the position of the Mlakawa albitite protolith from the plagiogranite field to other fields.
Euhedral zircon in the albitite pod is inferred to have been crystallized at a temperature of 1000 ± 100 °C based on Ti content (average Ti = 1500 ppm) obtained by laser ablation-ICP–MS. Based on calcic-amphibole thermobarometry of Ernst and Liu (1998), amphibole cores (actinolite-tremolite) represent low-temperature and low-pressure metamorphic conditions at the early prograde rodingitization stage of the plagiogranite, as shown in Figure 15. Toward the rim, the amounts of TiO2 and Al2O3 increase with progressive albitization or with increasing Na2O in the rock. Edenite was formed at the amphibole rim, representing the peak of albitization under high-pressure and high-temperature conditions (P = 1.5 GPa and T = 650 °C) (Fig. 15). This temperature is consistent with the results of the plagioclase-amphibole thermometry of Holland and Blundy (1994). Accordingly, the core is believed to have formed at T = 450 °C with a gradual increase to 750 °C at the rim.
DISCUSSION AND CONCLUSION
From the above reactions, it is clear that the serpentinization processes in the host rock, blackwall formation, and rodingitization of plagiogranite were contemporaneous. The lack of any vein or channel of albitite radiating from the albitite pod to the host serpentinite negates the possibility of precipitation from metasomatic fluids for the formation of Mlakawa albitite.
The albitite contains >63 wt% SiO2 and includes considerable amounts of analcime, up to 20 vol%. However, we did not find any quartz in or near analcime crystals, so the first reaction was preferred to the second. The analcime was suspected to be formed directly from the alteration of jadeite during retrograde metamorphism.
Based on the above interpretations, it can be concluded that the serpentinization, rodingitization, and albitization processes observed in the Mlakawa albitite are complementary (Fig. 16). Each process has a specific effect during a particular time in the evolution of the ultramafic-plagiogranite part of the Penjwin ophiolite sequence, leading to the formation of the Mlakawa albitite. Both serpentinization of peridotite and rodingitization of plagiogranite represent low-temperature and low-pressure conditions during the oceanic stage and the subduction stage before collision of the Arabian plate with the Iranian plate. In contrast, the albitization of plagiogranite represents high-pressure and high-temperature conditions at the collisional stage of the Arabian plate with the Iranian plate during the Late Cretaceous Period.
We thank F. Osman for his help during field work and Michael Williams and Callum J. Hetherington for their constructive reviews and comments. Discussion and comments by Y. Osada are also deeply appreciated. This work was supported by the Ministry of Higher Education and Scientific Research of the Kurdistan Regional Government.