The Sanandaj-Sirjan zone, which is the host of the Boroujerd pluton, has a
length of 1500 km and a width up to 200 km from the Northwest to the Southeast
in Iran (Fig. 1). This tectonic zone is mainly composed of
Mesozoic and some Paleozoic rocks and separates the stable Central Iranian block,
from the Afro-Arabian plate (Stocklin, 1968).
The presence of a narrow arc-trench gap in this belt is an indication of steep
subduction (Isacks and Barazangi, 1977; Berberian
and Berberian, 1981). The Sanandaj-Sirjan calc-alkaline magmatic arc, including
the Boroujerd pluton, formed over a high angle subducting oceanic slab in the
Neotethyan subduction zone during Late Triassic to Late Cretaceous time (Berberian
and Berberian, 1981; Shahabpour, 2005).
So far exceptionally a few age determinations (Ahmadi-Khalaji
et al., 2007; Arvin et al., 2007),
no detailed studies especially mineral chemistry have been carried out on any
of the mesozoic plutonic rocks, in the Sanandaj-Sirjan zone.
The main aims of this study are to use petrography and mineral chemical
characteristics, as well as observe field relationships of the Boroujerd
pluton, to determine its origin and to shed light thermobarometer and
related magmatism in Iran, an area for which little information has been
available so far.
|| Geological map of Iran (Shahabpour, 1994),
showing major lithotectonic units
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.
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.,
|| Representative electron microprobe analysis of amphibole
in quartz-diorite of the Boroujerd pluton (number of ions on the basis
of 23 oxygen)
||Representative electron microprobe analysis of biotite
quartz- diorite, granodiorite and monzogranite from Boroujerd pluton
(Number of ions on the basis of 11 oxygen
||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
|| 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
Granodiorite: The granodiorites are medium to coarse-grained rocks
and have a granular to hypidiomorphic texture.
||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.
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.
||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.
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,
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.
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.
(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
-FeO*-MgO (Nachit et al.,
). Showing all biotites of the Boroujerd pluton are primary magmatic
biotites, (C) The mica classification (Rieder et
) 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):
Biotite+labradorite+water = prehnite+sericite+albite+
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
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
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:
P ( ± 0.6) kbar =-3.01+4.76 Al (T)
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
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.
Log FO2 = -30930/T+14.98+0.142 (P-1)/T
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.
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,
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.
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.