Petrography and Mineral Chemistry of the Boroujerd Pluton (Sanandaj-Sirjan Zone, Western Iran)

MATERIALS AND METHODS

The major element compositions of the minerals were determined by electron microprobe analysis of polished thin sections. Some mineral compositions were determined using a Cameca SX-100 electron microprobe at the University of Hamburg (institute mineralogy and petrology), in June 2005 during 6 months, Germany. Also, the analysis were performed with JXA-8200 Super Probe at university of Huelva in March 2006 during 6 months, Spain, operated with an accelerating voltage 15 keV and a probe current of 5 nA. Silicate standards were Jadeite for Na, Wollastonite for Ca, Alkali Feldspar for K and Al, Enstatite for Mg, Fayallite for Fe and Mn and apatite for P. Chemical composition and structural formula of hornblende, biotite and feldspar are shown in Table 1-3.

RESULTS

Geological setting: The Boroujerd pluton is a NW-SE trending body covering an area of 600 km², approximately 60 km in length and 8-10 km in width, which lies between 33° 38’-34° N and between 48° 45’-49° 20’ E (Fig. 2). The Boroujerd area is characterized by the predominance of metamorphic rocks of Jurassic age (Baharifar et al., 2004) and the presence of the Boroujerd pluton. Metamorphic rocks subdivided in to 2 groups based on their setting: Dynamothermal and contact. Dynamotermal metamorphism has affected a vast area which is composed of slate, phyllite and schist (Ahmadi-Khalaji et al., 2007). Contact metamorphic rocks, consisting of spotted schists, cordierite-andalusite and cordierite-silimanite hornfelses, are evident only to the North of the pluton, because the southern margin of the complex is controlled by a fault system parallel to the contact and the granitoid rocks are thrusted onto the metamorphic rocks (Ahmadi-Khalaji et al., 2007).

Table 1:	Representative electron microprobe analysis of amphibole in quartz-diorite of the Boroujerd pluton (number of ions on the basis of 23 oxygen)

Table 2:	Representative electron microprobe analysis of biotite quartz- diorite, granodiorite and monzogranite from Boroujerd pluton (Number of ions on the basis of 11 oxygen

Table 3:	Representative electron microprobe analysis of plagioclase in quartz- diorite, granodiorite and monzogranite from Boroujerd pluton (Number of ions on the basis of 8 oxygen)

U-Pb zircon geochronological data from Boroujerd granitoid rocks indicate episode of magmatic activity 170 Ma ago during the middle Jurassic (Ahmadi-Khalaji et al., 2007). The compositional variation found in this major pluton, usually range from quartz-diorite-granodiorite to monzogranite. The granitoids occurring in Boroujerd show close similarities with those described elsewhere in the Sanandaj-Sirjan zone. In general, mineral assemblage in Boroujerd pluton is same to other calc-alkaline granites in Sanadaj-Sirjan zone.

Field description and petrography: Detailed mapping of the Boroujerd area (Fig. 2) distinguished three main rock types; include quartz-diorite, granodiorite and monzogranite which are locally associated with acidic dikes. The granodiorite is the most dominant rock in this pluton.

Fig. 2:	The Boroujerd area, western Iran

Quartz-diorite: The quartz- diorite and tonalite are exposed within the granodiorite and have gradual boundaries with them (Fig. 3A). These rocks have granular texture (Fig. 3B) to porphyritic with plagioclase megacrysts and composed predominantly of plagioclase (40-50 vol.%), amphibole (10-15 vol.%), biotite (15-20 vol.%), alkali feldspar (<5 vol.%), quartz (<15 vol.%). Plagioclase is anhedral to subhedral plates, zoned and altered to sericite, epidote and calcite. Biotite occurs as brown kinking flakes and altered to chlorite and prehnite (Fig. 3D). Amphibole shows a euhedral prismatic habit, green colour and altered to biotite, chlorite, epidote and prehnite. Quartz crystals occur as anhedral to subhedral with adulatory extinction and a late interstitial phase. Alkali feldspar is anhedral to subhedral crystals. Zircon, sphene, apatite are conspicuous accessory minerals.

Granodiorite: The granodiorites are medium to coarse-grained rocks and have a granular to hypidiomorphic texture.

Fig. 3:	Field and microscopic photos in quartz- diorite, granodiorite and monzogranite: Bt = Biotite; Amph = Amphibole; Prh = Prehnite; Mineral abbreviations are according to Kretz (1983)

The main minerals are plagioclase, (30-40 vol.%), are usually euhedral, more or less with variable degrees of sericitisation, shows zoning and the first felsic mineral to crystallize. K-feldspar (<20 vol.%) forms anhedral to subhedral crystals and includes microcline-perthites. Quartz (25-30 vol.%) forms anhedral crystals. Biotite, the most abundant mafic mineral (10-20 vol.%) appears in brown flakes. Apatite, zircon, allanite and opaques are common accessory minerals and some muscovite is present as a secondary mineral.

Monzogranite: The monzogranites are widely scattered as separate and small outcrops through the southern part of the area (Fig. 3B). These rocks are light in colour (Fig. 3C), fine to coarse-grained, with a granular texture. The mineral assemblages include perthitic alkali feldspar (30-35 vol.%), plagioclase (25-35 vol.%), quartz (30-35 vol.%), biotite (5-10 vol.%). Zircon, allanite and apatite are common accessory minerals. Plagioclase forms subhedral to euhedral plates and altered to sericite. It is commonly zoned and the first felsic mineral to crystallize. Quartz grains occur as anhedral crystals or interstitial and may be recrystallized. Biotite occurs as anhedral flakes. Most it has altered to chlorite. Euhedral zircon, with clear haloes and prismatic, needle-like apatite are abundantly contained in plagioclase and quartz.

Acidic dikes (aplites and pegmatites): A series of NW trending aplites and pegmatites (Fig. 3B), varying from a few meters to tens of meters in length and a few meters in width, occur in the area studied. The aplites are characterized by fine equigranular assemblage of quartz, alkali feldspar and some muscovite, tourmaline and opaque oxides. These rocks are the main manifestation of the final phase of magmatic activity. Pegmatites are mainly present in the granodiorites and its aureole. They show a simple mineralogy with graphic texture. These are characteristically composed of quartz, feldspar, muscovite, tourmaline, zircon and apatite with some andalusite and garnet in the aureole samples. Pegmatites are similar in age with the main units (Ahmadi-Khalaji et al., 2007) and again, could be final phase of magmatic activity.

Mineral chemistry
Amphibole: Amphibole is common in the quartz- diorites, but rare in the granodiorites and absent from the monzogranites. Representative major elements EPMA of unaltered magmatic hornblende from Boroujerd pluton (Fig. 4 A, B) is presented in Table 1. Major element EPMA compositions were calculated to an apfu 23 oxygen and normalized to total cations (Ca+Na+K) = 13, with Fe ³⁺/Fe²⁺ ratios calculated by charge balance.

Fig. 4:	Backscattered electron image (BSE) to representative samples, (A), (B), Amphibole, Plagioclase and Biotote in quartz- diorite, (C) Biotite in granodiorite and (D) Biotite in monzogranite

Based on the electron microprobe analysis, all amphibole are mainly magnesio-hornblende with a few actinolitic hornblendes on the (Si.P.f.u.) versus (Mg/Mg⁺Fe²⁺) (Fig. 4A). Amphiboles show (CaB> 1.5 (1.68-1.98), (Na+K) A<0.5 (0.03-0.29) and thus are calcic amphiboles occurring to Leake et al. (1997) classification (Fig. 5A), that usually in calc-alkaline granitoids.

Biotite: Biotite is the most abundant mafic mineral in the Boroujerd pluton and it is the only mafic silicate in the monzogranites. Electron microprobe analysis of selected biotites (Fig. 4C, D) are shown in Table 2. Electron micrpoprobe analysis of biotite mineral is a quick and easy method, allowing on this basis, the distinction between unaltered primary magmatic biotites and more or less reequilibrated, possibly neoformed ones by post-magmatic hydrothermal fluids. For use of the ternary diagram 10TiO₂-FeO+MgO-MgO (Nachit et al., 2005) is a necessary preliminary to the typological study of granitoids based on the biotite chemistry. All the analyzed biotites of the Boroujerd pluton are primary magmatic biotites (Fig. 5B).

According to the revision of mica classification (Rieder et al., 1999) the Boroujerd micas plot in the biotite field, at low to medium FeO/FeO+MgO ratios (0.56-0.77), (Fig. 5C). The low Fe/Fe+Mg ratio in biotites is described by Czamanske and Wones (1973) as reflecting increasing FO₂ conditions during magmatic evolution. The alumina saturation index of biotite (Altot/Ca+Na+K, ASI) is significantly increase quartz-diorite to monzogranite (1.56 to 2.26) and reflected increased alumina activity in the crystallizing magma of each area (Zen, 1988).

Nachit et al. (1985) developed some diagrams for identification of biotites belonging to rocks of different magma series (Fig. 5D). In general, biotites in quartz- dioritic rocks have compositions similar to those of subalkaline series, granodiorite in calc-alkaline series and monzogranite in calc-alkaline series to aluminopotassic field. The most important feature to note the biotites is the presence of prehnite (parallel to biotite cleavage) compositions in Boroujerd granitoids (Fig. 3D, 4D).

Growth of prehnite within biotite is interpreted as a secondary process, probably as a result of deuteric reactions. The biotite is intergrowth with prehnite and is partly altered to chlorite.

Fig. 5:

(A) Classification of Amphibole in quartz-diorite according to Leake et al. (1997). Showing crystallization of magnesio-hornblende and actinolite in quartz-diorite, (B) The ternary diagram TiO2-FeO*-MgO (Nachit et al., 2005). Showing all biotites of the Boroujerd pluton are primary magmatic biotites, (C) The mica classification (Rieder et al., 1999) for biotites of the Boroujerd pluton and (D) Total Al versus Mg diagram adapted from Nachit et al. (1985) for biotites of the Boroujerd pluton

Biotite cleavages and the margins of some crystals, locally bow around the pod like outline of the prehnite. About source of prehnite, Liou (1971) indicated that prehnite is unstable above 393°C at 5 Kbars; thus implies that a primary origin for prehnite in igneous rocks such as tonalites is unlikely. The possible reaction is (Phillips and Rickwood, 1975):

Feldspars: Locally plagioclase is oscillatory zoned in three main rock types of the Boroujerd pluton indicating probably disequilibrium melting in the generation of granitoid rocks (Castro, 2001). Plagioclase is the main mineral in the granodiorites, with the crystals occurring mainly as subhedral laths. Myrmekitic intergrowths with quartz are occasionally present in some granodiorites and monzogranites. Plagioclase generally appears as zoned crystals, often with a patchy core and overgrowths indicative of distinct crystallization periods. In the granodiorites, compositions span the whole andesine interval from An52 to An41, similar to the compositions in the quartz- diorites (An59 to An46). Alteration, mostly restricted to An-rich zones, is commonly to sericite, clay and saussuritic minerals. Plagioclase is less abundant in the monzogranites and shows less zonation than in the granodiorites. Compositions range from andesine to oligoclase (An48 to An26), partly straddling the compositions analyzed in the monzogranites. Representative analysis are given in Table 3.

Geothermobarometry hornblende and plagioclase: The recent application of Al (IV) and Al (tot) in hornblende, as both a geothermometer and geobarometer respectively, provides new information on the likely temperatures and pressures that exited during the emplacement of the granitic magma within the crust.

Wones (1989) suggested the chemical variability in amphiboles and also other mafic silicates to be potential indicators of intensive variables in granitic magmas.

None of the analyzed hornblendes are zoned, although only EPMA from amphibole rims were used for hornblende-plagioclase geothermometry. Al values for actinolitic-hornblende and actinolites were disregarded because of their probable post-magmatic nature. The primary phases of quartz, plagioclase feldspar, alkali feldspar, biotite, sphene and magnetite were present together with hornblende. Magnetite is a common mineral in the Boroujerd pluton. Many arc-related plutons crystallize at elevated FO₂ (magnetite series of Ishihara, 1977), whereas anorogenic plutons are often emplaced at low FO₂ (Anderson, 1983). Low FO₂ decreases the Mg/Fe and Fe³⁺/Fe²⁺ratios in hornblende (Czamanske et al., 1981). Generally it can be concluded, that hornblende crystallizing under high FO₂ gives better and more reliable geothermobarometry result than those growing under low FO₂ (Stein and Dietl, 2001).

Anderson and Smith (1995) concluded that temperature and, in particular, FO₂ are parameters that should be carefully evaluated before the application of a given geobarometer. The following considerations were made prior to the application of the Al-in-Hbl geobarometer to the analyzed rocks. The all of the magnesio-hornblende in quartz-diorites have Fe/ (Fe+Mg) <0.65, Si <7.5 and Ca> 1.6 (apfu). Thus there used for geobarometry (Hammarstrom and Zen, 1986).

There are several empirical Al-in-hornblende barometers that have been used to determine solidus pressures in calc-alkaline plutons (Hammarstrom and Zen, 1986; Holliste et al., 1987; Johnson and Rutherford, 1989). The most recent (Schmidt, 1992) was chosen in this study because of the smaller margin of error in the Eq:

where, P is in kbar and Al (T) is the total Al content of hornblende in atoms per formula unit.

From the all of the analyzed amphiboles of the Boroujerd pluton pressure is 1.09 ± 0.6 kbars in quartz- diorites.

For estimation of temperature in above rocks, Blundy and Holland (1990) first proposed a very simple, empirical thermometer on the basis of the edenite-tremolite reaction; which could be applied only to quartz-bearing, intermediate to felsic igneous rocks with plagioclase An < 0.92 and Si in hornblende < 7.8 atoms pfu. In this study for calculation temperature we used Holland and Blundy (1994) following thermometer,

It now is:

where, T is expressed in °C, R= 0.0083144 kJ K^-1 mol^-1, Yab = 0 for Xabplag > 0.5 or else Yab = 12.0(1-Xabplag) 2-3.0 kJ and various X terms (molar fractions) are defined in Holland and Blundy (1994). The estimated temperature is 785°C ± 40 in magnesio hornblende crystallized in quartz-diorite.

The approximate temperature of emplacement confirmed by the petrology of the hornfels zone that surrounds the Boroujerd pluton. Contact metamorphic rocks, consisting of spotted schists, cordierite-andalusite and cordierite-silimanite hornfelses.

Oxygen fugacity estimation: The oxygen fugacity of magma is related to its source material, which in turn depends on tectonic setting. Sedimentary-derived granitic magmas are usually reduced, while I-type granities are relatively oxidized. It is difficult to estimate the original oxygen fugacity of primary magmas from the study of granitoids, as magnetite usually becomes Ti free during slow cooling and ilmenite undergoes one or more stages or oxidation and exsolution (Haggerty, 1976). However, some inferences on the oxidation state of magma can be made using the rock mineral assemblage and mineral chemistry. The occurrence of magnesio-hornblende and Fe²⁺ biotite in Boroujerd rocks suggest relatively oxidized magma.

According to Wones (1989) the assemblages of titanite+magnetite+quartz in granitic rocks permit an estimation of relative oxygen fugacity. He made quantitative estimation of fugacity based on the equilibrium expression.

where, T is temperature (in Kelvin) and P is pressure (in bars). We used this equilibrium expression to estimate the prevailing oxygen fugacity in the Boroujerd pluton.

Temperature and pressure estimated from hornblende -plagioclase thermometry and aluminum in hornblende barometer were used in these calculations. Quartz-diorites analyzed have log FO₂-14.1 that show that magma crystallized in high FO₂.

DISCUSSION

The Boroujerd granitoids include quartz diorite, granodiorite and monzogranite. They cut by numerous acidic dikes. So, the Boroujerd granitoids consist of two different suites (types); a monzogranitic (more felsic, leucocratic type) and a quartz-dioritic to granodioritic (more mafic or mesocratic type).Whereas the mesocratic type occurs as an ellipsoid large intrusion and form elongated SE-NW trending complexes, leucocratic type as small intrusions show round shapes suggesting a change in the crustal stress field.

The results of microprobe analysis in different rocks and their minerals indicate that Boroujerd pluton have a metaluminous and slightly peraluminous character and I-type magma. The based on analyzed biotite and magnesio-hornblende, this pluton has calc-alkaline magma.

Coexisting mineral phases and their compositions from the granitoid rocks of Boroujerd in Sanandaj-Sirjan Zone were used to estimate the physicochemical parameters of their crystallizing parent magma. The samples contain the suitable assemblage for Al-in hornblende barometry (Hbl-Pl-Qtz-Kfs-Ttn-Fe, Ti oxide).

The aluminum in hornblende barometer, hornblende-plagioclase thermometer and estimation of FO₂, were used to calculate pressure, temperature and oxygen fugacity, respectively. The pressure and temperature is 1.093 ± 0.6 kbars and 785°C ± 40 in quartz-diorites respectively. The analyzed samples have log FO₂-14.1 that show this magma crystallized in high FO₂ and related to arc-magmatism.

ACKNOWLEDGMENTS

M. Mirmohammadi is thanked for his assistance in determination some mineral compositions at the University of Hamburg (institute mineralogy and petrology), Germany. The mineral chemical study was carried out in the University of Huelva (Spain) during a study leave of Z.T.