GeolCarp_Vol62_No2_171

www.geologicacarpathica.sk

GEOLOGICA CARPATHICA

, APRIL 2011, 62, 2, 171—180 doi: 10.2478/v10096-011-0014-y

The ammonium content in the Malayer igneous and

metamorphic rocks (Sanandaj-Sirjan Zone, Western Iran)

VAHID AHADNEJAD

, ANN MARIE HIRT

, MOHAMMAD-VALI VALIZADEH

and

SAEED JABBARI BOKANI

Geology Department, Payame Noor University (PNU), 19395-4697 Tehran, Iran; *ahadnejad@khayam.ut.ac.ir

Institute of Geophysics, ETH-Zürich, Sonneggstrasse 5, CH-8092 Zürich, Switzerland

School of Geology, University College of Science, University of Tehran, Tehran

Geological Survey of Iran (GSI), Azadi sq. Meraj St. P.O. Box 13185—1494, Tehran

(Manuscript received April 7, 2010; accepted in revised form October 11, 2010)

Abstract: The ammonium (NH

) contents of the Malayer area (Western Iran) have been determined by using the

colorimetric method on 26 samples from igneous and metamorphic rocks. This is the first analysis of the ammonium
contents of Iranian metamorphic and igneous rocks. The average ammonium content of metamorphic rocks decreases
from low-grade to high-grade metamorphic rocks (in ppm): slate 580, phyllite 515, andalusite schist 242. In the case of
igneous rocks, it decreases from felsic to mafic igneous types (in ppm): granites 39, monzonite 20, diorite 17, gabbro 10.
Altered granitic rocks show enrichment in NH

(mean 61 ppm). The high concentration of ammonium in Malayer

granites may indicate metasedimentary rocks as protoliths rather than meta-igneous rocks. These granitic rocks (S-types)
have high K-bearing rock-forming minerals such as biotite, muscovite and K-feldspar which their potassium could
substitute with ammonium. In addition, the high ammonium content of metasediments is probably due to inheritance of
nitrogen from organic matter in the original sediments. The hydrothermally altered samples of granitic rocks show
highly enrichment of ammonium suggesting external sources which intruded additional content by either interaction
with metasedimentary country rocks or meteoritic solutions.

Key words: Iran, Sanandaj-Sirjan Zone, Malayer, igneous rocks, metasedimentary rocks, ammonium.

Introduction

Recent research has revealed that geological nitrogen has an
important role in geological problems such as lithogeochem-
ical  explorations  (Ridgway  et  al.  1990;  Glasmacher  et  al.
2003),  biogeochemical  implications  (Boyd  2001;  Holloway
&  Dahlgren  2002),  environmental  studies  (Crews  et  al.
2001),  and  petrological  investigations  (Honma  &  Itihara
1981; Hall 1999).

Nitrogen is a rare element in igneous rocks which is signif-

icant  for  nutrition  of  soils  and  has  an  important  role  in  pe-
trology.  It  includes  an  inorganic  component  as  fixed
ammonium,  incorporated  into  potassium  sites  of  minerals
and,  generally,  concentrated  in  micas,  feldspars,  and  clay
minerals.  Nitrogen  in  low-grade  metamorphic  and  igneous
rocks  occurs  as  NH

, and in sediments and sedimentary

rocks as NH

(Wedepohl 1978; Halama et al. 2010). The

concentration of NH

in igneous rocks is generally related to

the kind and amount of the silicate minerals and the ammo-
nium contents that are available for fixation during the exist-
ence of the minerals (Stevenson 1962). The NH

ion has a

similar estimated ionic radius to that of K

(NH

– 1.66

– 1.59

), which tends to explain the presence of am-

monium in K-bearing minerals. Because of the stability of
(NH

) in high temperature conditions and its survival in

metamorphism, the concentration of geological nitrogen as a
geochemical tracer in the rocks would be important to crustal
processes. In the igneous rocks it is linked to their protolith;
if they originate from melting of metasedimentary rocks or

assimilated by crustal component, their NH

concentrations

could be high. The hydrothermal activity and hydrothermal
fluids could readily transport NH

from other systems into

rocks and cause the enrichment of altered samples in NH

Several researchers have reported different mean concentra-

tion  of  N  for  igneous  (e.g.  Wedepohl  1978;  Hall  1999)  and
low-grade metamorphic rocks (e.g. Juster et al. 1987). Wede-
pohl  (1978)  reported  an  average  N  concentration  of  around
20 ppm for granitoids and half of that amount for the gabbroic
and dioritic rocks, whereas Hall (1999) suggested 35 ppm for
the  granitic,  6  and  2 ppm  for  the  gabbroic  and  dioritic  rocks
respectively. Juster et al. (1987) pointed out that the low-grade
metamorphic rocks routinely contain 200—400 ppm NH

Hall (1993a) believed that the differences and uncertain-

ties on ammonium contents of rocks may have three reasons:
(1) Because analytical determination of ammonium and ni-
trogen in igneous rocks was developed recently and was not
easy in the past, few workers tend to measure this element in
their studies and there is not enough data from worldwide
rock types to gain a precise average content; (2) Determina-
tion of ammonium in igneous rocks and especially volcanic
types is difficult because of their very low ammonium con-
tent; (3) Geological processes such as alteration and country
rocks can affect enrichment of ammonium.

In this study, we performed the first ammonium measure-

ments on Iranian rocks which consist of the Malayer plutonic
and metasedimentary country rocks in the Sanandaj-Sirjan
Zone of Western Iran, for assessing its importance in the pet-
rological processes.

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GEOLOGICA CARPATHICA, 2011, 62, 2, 171—180

Geological setting

Sanandaj-Sirjan Zone

The investigated area is located within the Sanandaj-Sirjan

Zone (SSZ) which is a part of the Zagros orogeny. The SSZ
has  1500 km  length  from  northwest  (Sanandaj)  to  southeast
(Sirjan) in the western part of Iran and a width of 150—200 km
(Mohajjel  &  Fergusson  2000)  (Fig. 1).  It  separated  from  the
Arabian platform during the Late Triassic to the Early Juras-
sic. Mesozoic rocks are dominant in this zone and Paleozoic
rocks generally are common in the southeastern part (Berberi-
an  1995).  The  SSZ  is  characterized  by  metamorphosed  and
complexly  deformed  rocks  associated  with  abundant  de-
formed and undeformed plutons, as well as widespread Meso-
zoic volcanics. These magmatic rocks, including the Malayer
intrusive complex, generally have calc-alkaline affinities (e.g.
Ahmadi-Khalaji et al. 2007; Azizi & Jahangiri 2008; Ahadne-
jad et al. 2008a; Ghalamghash et al. 2009).

Malayer intrusive complex

The Middle—Jurassic Malayer intrusive complex is an elon-

gated batholith in the northern part of the SSZ. It is composed
of  granite,  granodiorite,  diorite  and  some  small  monzonitic
and gabbroic bodies (Fig. 1). It is 35 km in length and 10 km
in  width  and  located  in  the  southwest  part  of  Malayer  city,
Western  Iran.  The  Malayer  complex  underwent  deformation

in  the  high-strain  shear  zone  and  gained  a  NW—SE  direction
parallel  to  the  SSZ.  Major  structural  features  include  thrust
faults, strike-slip faults, and a variety of cleavages and folia-
tions. Variable composition of rocks indicated different source
rocks and hybridization of mafic and felsic magmas. Analysis
of  selected  samples  using  ICP  showed  that  they  have  SiO

(wt. %) content from 46.82 (gabbro) to 77.35 (alkali-granite).
They are highly peraluminous to metaluminous with high-K
calc-alkaline affinity (Ahadnejad et al. 2008a).

The field, petrography, geochemistry, geochronology and

isotopic data imply that the granitoids are the hybrid products
of partial mixing between basic and granitic melts, generating
hybrid phases, such as the tonalitic rocks. During ascent and
emplacement of magma, mixing was followed by assimilation
of metasedimentary country rocks. The assimilation and con-
tamination  are  shown  by  metasedimentary  enclaves  and  an-
dalusite  xenocrysts  occurrences  in  the  granitoids  especially
next to the contacts. This feature has been detected in the Ma-
layer  pluton,  where  initial

Sr/

Sr ratios increase from an

average of 0.7085 in the marginal Q-diorites and the 0.7087
in granodiorites, to ca. 0.7011 in the syenogranites from the
central part of pluton (Ahadnejad et al. 2011). Furthermore,
co-existence  of  antagonistic  mineral  assemblages  (e.g.
allanite + titanite + monazite + hornblende + muscovite + …)  in-
dicate  that  primary  magma  originating  from  the  mid  to  lower
crust  contaminated  by  supracrustal  materials  and  the  overall
composition is significantly affected by this process. The scat-
tered pattern of elements may have been caused by this process.

Fig. 1. Schematic map of Malayer area and its location (star) in the Sanandaj-Sirjan Zone, Western
Iran. The sample number and positions are shown as well.

U-Pb zircon ages (Middle—

Jurassic)  has  been  obtained
from all rock types of complex
(Ahadnejad  et  al.  2011)  indi-
cate  that  emplacement  of  this
pluton  was  performed  during
the  subduction  of  Neotethys
under Central Iran in an active
continental  margin  tectonic
setting.  In  the  Pearce  et  al.
(1984)

discrimination

dia-

grams  the  studied  rocks  plot
mostly within the field of vol-
canic  arc  granites  (VAG)
(Fig. 2a  and  b).  Most  of  the
data  are  plotted  in  the  peralu-
minous  field  in  the  A/NK—A/
CNK  diagram  (Shand  1943).
Despite  relatively  high  A/
CNK values, the granitic suite
displays  some  affinities  with
I-type  granitoids,  and  some
samples  contain  hornblende
and  allanite.  The  distribution
of  samples  in  the  diagrams
FeO

/MgO versus 10000 Ga/

Al (Fig. 2d) and Nb versus
10000 Ga/Al (Fig. 2e) pro-
posed by Whalen et al. (1987)
do not suggest A-type charac-
ter for Malayer granitoids.

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Petrographic studies show that the texture of the granitoids

is mainly granular to porphyroid (Fig. 3a). They have mafic
microgranular and metasedimentary enclaves

which occur as

angular to ellipsoidal shapes ranging from several cm to 0.6
meter in size. The mafic microgranular enclaves are dioritic
in composition. The rock-forming minerals are mainly com-
posed  of  K-feldspar  (orthose  and  microcline),  plagioclase,
quartz, biotite and minor hornblende and muscovite. The ac-
cessory minerals are garnet, tourmaline, andalusite, cordier-
ite,  allanite,  titanite,  zircon  and  apatite.  The  andalusite  is
mostly  observed  in  the  contact  of  granitoids  and  metasedi-
mentary country rocks. It has reaction rims containing aggre-
gates  of  quartz,  andalusite,  muscovite,  and  biotite  with
symplectitic  relationships  which  imply  its  disequilibrium
with  the  melt.  Clarke  et  al.  (2005)  indicate  that  these  dise-
quilibrated  andalusites  in  the  granitoid  rocks  cannot  have  a
magmatic  origin  and  are  considered  to  be  xenocrystic  de-
rived  from  local  peraluminous  country  rocks.  They  may  be
released from disaggregating, contact-metamorphosed meta-
pelites into a silicate melt and, in general, such xenocrystic
grains would be out of chemical equilibrium with that melt.
Feldspars are euhedral to subhedral and frequently show ex-
solution  lamellas  of  albite  (microperthite)  (Fig. 3b).  Plagio-
clase  minerals  show  polysynthetic  twinning  and  zoning.
They experienced mechanical crash and their crash zones are

filled by quartz and alkali feldspar. Quartz shows undulatory
extinction. Myrmekites can frequently be observed around
feldspars. The secondary minerals formed by alteration are
muscovite, clinozoisite, sericite, clay minerals, chlorite, cal-
cite and Fe-oxides.

The dioritic unit is located in the southeastern part of the

complex  and  is  accompanied  by  subordinate  gabbroic  and
quartz  monzodioritic  rocks.  It  is  lenticular  in  shape,  medi-
um- to coarse-grained, dark-coloured and mainly consists of
plagioclase, amphibole, biotite, K-feldspar, and minor quartz
(Fig. 3c). Accessory minerals are apatite, zircon, epidote and
opaque  minerals.  Andalusite  minerals  are  occasionally  seen
as xenocrysts. Apatite occurs as euhedral prismatic and acic-
ular shapes resulting from rapid cooling of minor mafic com-
ponents added to intermediate or felsic magma chambers.

A subordinate gabbroic unit is located in the corner of the

diorite.  It  is  dark-coloured,  medium-  to  coarse-grained  and
its  main  constituents  are  plagioclase,  amphibole,  olivine,
augite,  and  minor  biotite  and  alkali  feldspar  (Fig. 3d).  The
apatite, epidote, and opaque minerals are accessory minerals.

Hydrothermal alterations in the Malayer granitic rocks are

moderate and mainly occur on the corners of bodies, local
shear zones and fractures. The major observed types of alter-
ations are: sericitization, silicification, chloritization, oxida-
tion and tourmalinization. Sericitization is the most

Fig. 2. a – Ta versus Yb diagram and b – Nb versus Y (after Pearce et al. 1984) for studied rocks showing a volcanic arc setting. c – Most
of the data are plotted in the peraluminous field in A/NK—A/CNK diagram (Shand 1943). d – FeO

/MgO versus 10000Ga/Al and e – Nb ver-

sus 10000Ga/Al (Whalen et al. 1987) do not show A-type feature for Malayer granitoids.

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GEOLOGICA CARPATHICA, 2011, 62, 2, 171—180

widespread alteration in the rocks. It is common in feldspars
(plagioclase and alkali-feldspar) and sericitized grains reflect
the zoning. This alteration implies low pH (acidic) conditions
of  the  mineralized  fluid  (Faulkner  1992).  Chloritization  is
generally  observed  along  with  sericitization  due  to  alteration
of mainly mafic minerals (biotite and amphibole). Amphibole
and  plagioclase  minerals  have  also  been  altered  into  calcite.
The replacement of the feldspar by epidote and sericite and the
hornblende and biotite by chlorite is common and characterit-
ic of the hydrothermal alteration associated with contact meta-
morphism.

Some small veins and veinlets of quartz and also small

quartz crystals are formed due to silicification and rocks are
normally silicified along faults and fractures. Tourmaliniza-
tion in the magmatic rocks occurs as small patches and vein-
lets  and  is  mainly  found  in  contact  with  country  rocks.
Despite  lack  of  mineral  chemistry,  the  field  observations
demonstrate  that  tourmaline  has  probably  originated  from
hydrothermal  fluids.  On  the  field  scale,  the  brown  surfaces
of  the  granitic  hills  demonstrate  chloritization  and  the  iron
oxide varnish of rocks. Locally, these rocks have been deep-
ly weathered and intensely arenized to form soils which are

used as wheat farms and almond-tree gardens and have a
sharp boundary with non-granitic country rocks.

To quantify alteration, the Ishikawa et al. (1976) alteration

index (AI)=100*(K

O+MgO)/(K

O+MgO+Na

O+CaO) was

measured for all the granitic samples (Table 2). The key reac-
tions measured by the index involve the breakdown of sodic
plagioclase and replacement by sericite and chlorite:

3NaAlSi

+2H

= K Al

(OH)

+6SiO

+3Na

Albite Sericite Quartz

2KAl

(OH)

+3H

SiO

+9Fe

+ 6Mg

+ 21H

O =

Sericite

= 3Mg

(OH)

+2K

+28H

Chlorite

Reaction (1) involves a loss of Na

O (and CaO) and a gain

of K

O, whereas reaction (2) involves a loss of K

O and gains

in FeO and MgO, on the basis of constant Al

. Samples

Nos. 116 and 161 display expectedly high AI values (91 %
and 92 %, respectively) and are located in the altered field of
Wilt (1995) but sample No. 106 has a low value (ca. 41 %)

Fig. 3. Photomicrographs of Malayer igneous rocks. a – The porphyroid texture of granodiorite (XPL). b – Exsolution lamellas of albite
(microperthite) in the alkali-feldspar. c – Diorite minerals (XPL). d – The olivine has been largely replaced by symplectic intergrowths
in the gabbroic rocks (XPL). (Mineral abbreviations from Kretz 1983).

(1)

(2)

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THE AMMONIUM IN THE MALAYER IGNEOUS AND METAMORPHIC ROCKS (WESTERN IRAN)

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which  plots  in  the  fresh  field  (Fig. 4).  The  samples  116  and
161  are  from  syenogranite  and  monzogranite,  respectively.
The  alteration  of  these  samples  consists  of  sericitization  of
plagioclase,  chloritization  of  biotite  and  secondary  musco-
vitization of the K-feldspar phenocrysts. The high K content
of  these  samples,  therefore,  could  be  ascribed  to  sericitized
plagioclase and secondary muscovite. The occurrence of an-
dalusite caused the high Al-content in sample No. 161. Sam-
ple  No. 106  is  from  granodiorite  and  shows  alteration  of
K-feldspar  and  biotite  to  albite,  muscovite,  chlorite  and  epi-
dote.  This  is  consistent  with  sodic-calcic  alteration  which  is
supported  by  a  chemical  analysis  that  displays  high  contents
of Na

O and CaO (Table 2). Despite decreases of ammonium

due to alteration of K-feldspar to sodic plagioclase (i.e. albite-
oligoclase)  (Honma  &  Itihara  1981)  and/or  epidote,  musco-
vitization of K-feldspar and chloritization of biotite probably
caused  substantial  enrichment  of  this  sample  from  ammoni-
um. This feature is in good agreement with  Hall’s (1993a) ar-
gument  about  the  high  ammonium  content  of  chloritized  bi-
otite.  Furthermore,  Honma  &  Itihara  (1981)  showed  that  the
muscovite  contains  an  average  of  ~ 40 %  of  N  concentration
in a rock. With respect to high Na

O, FeO, MgO and CaO and

low K

O content of 106 it is concluded that the reaction (2) is

responsible for alteration in this sample which is supported by
petrographic  observations  of  chloritization.  In  addition,  a
characteristic  feature  of  this  sample  is  high  Cl  concentration
which could provide a potential for ammonium concentration
via  ammonium-chlorite  bearing  inorganic  compounds?  (e.g.
NH

ClO, NH

ClO

, etc.). However, minerals such as amphib-

oles, biotite and apatite could contain chlorine. On the other
hand, sample No. 56 from fresh samples shows a high AI val-
ue (ca. 81 %). Concerning petrography it seems that it is in the
incipient step of alteration via reaction (1) as shown by low
Na

O and fairly high K

O and H

O. The MgO content of this

sample is high due to occurrence of augite (clinopyroxene).

The occurrences of tonalite, granodiorite and diorite as hy-

brid rocks, existing of disequilibrated andalusites, garnet, al-
lanite, acicular apatite, titanite and cordierite in the rocks, and
mafic microgranular and metasedimentary enclaves implied
that they were probably produced by interaction of mafic and

Fig. 4. Alteration diagram proposed by Wilt (1995) (weight percent
of SiO

versus Alteration Index = (MgO + K

O)/(Na

O + K

O + CaO-

+ MgO) * 100) shows the altered and fresh samples fields.

felsic rocks accompanied by assimilation into metasedimenta-
ry rocks. This is supported by high values of

Sr/

(0.70797 to 0.7108) which are measured for all plutonic rock
types at Isotope Geochemistry and Mineral Resources, ETH-
Zurich,  Switzerland,  using  the  ID-TIMS  technique.  In  addi-
tion, the high- and low-field anisotropy of magnetic suscepti-
bility (AMS) and paleomagnetic analysis that were performed
by the first author at the Paleomagnetic Laboratory of the In-
stitute  of  Geophysics  at  ETH,  show  that  the  igneous  rocks
have  low  magnetic  susceptibility  (4—706  SI)  and  belong  to
the ilmenite-series of Ishihara (1977). The value of 3 10

—3

unit (equivalent to 100 10

—6

emu/g unit of Ishihara 1979) is

usually taken as the boundary dividing the magnatite- and il-
menite-series granitoids. The high field analyses (HFA) on the
39 among 90 drilled cores from throughout the pluton shows
that all the samples are composed of dominant paramagnetic
components except for 3 samples (8 %) which show a ferro-
magnetic character (Ahadnejad et al. unpubl. data). Finally,
the biotite geochemistry of the rocks (Ahadnejad et al. 2008b)
displays a reduced magma fugacity (10

—15

to 10

—10

bar) for a

crystallizing temperature of 700 °C at QFM buffer.

Metasedimentary rocks

The regional metamorphic rocks are slate, phyllite, and

schist.  Quartz,  muscovite,  biotite  and  K-feldspar  are  the  main
components of the rocks. The micas arranged into preferred ori-
entations  and  caused  slaty  cleavages  in  slates  (Fig. 5a).  The
slates  and  phyllites  show  granular  and  lepidoblastic  to  lepido-
granoblastic  textures,  respectively.  Andalusite  and  garnet  are
the  major  porphyroblasts  in  the  schists  (Fig. 5b).  These  rocks
are  composed  of  biotite,  muscovite,  quartz  and  feldspars.  The
schistosity of the rocks is oriented in the NW-SE direction. Ac-
cording to the diagram FeO

O—SiO

/Al

(Fig. 6) suggest-

ed by (Herron 1988), different metamorphic rock varieties of
Malayer (slate, phyllite and schist) correspond to clays (Fig. 6),
and have relatively uniform chemical compositions which may
indicate differences from similar sedimentary rocks.

Sampling and analytical methods

Two groups of samples were analysed in this study. One

group  consists  of  17  samples  from  various  igneous  rocks
ranging from granite to gabbro including 3 altered samples.
The modal analysis results  indicate that they are syenogra-
nitic to gabbro (Table 1 and Fig. 7). The other group consists
of 9 samples from regional metamorphic rocks (slate, phyl-
lite  and  schist).  Samples  are  represented  all  rock  types  that
occurred  in  the  Mlayer  area  except  for  hornfels.  The  mean
amount of material collected was about 4 kg per sample. We
crushed the samples and analysed them in the AMDEL labo-
ratories, Australia for major elements and in the Geological
Survey of Iran (GSI) for ammonium.

The most popular technique for the determination of ammo-

nium in geological samples is the colorimetric method based
on the formation of indophenol blue and we used this method
to measure the concentration of ammonium in the rocks at the
Geological Survey of Iran (GSI). This method consists of sam-

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AHADNEJAD, HIRT, VALIZADEH and JABBARI BOKANI

GEOLOGICA CARPATHICA

GEOLOGICA CARPATHICA, 2011, 62, 2, 171—180

ple digestion in cold HF for 7 days, followed by separation of
ammonia by distillation from alkaline solution, and a colori-
metric finish using the indophenol blue method. A detailed
description of the method is given by (Hall 1993b).

Results and discussion

In this reaserch we reported average ammonium contents

of 580 for slate, 515 for phyllite, 242 for andalusite schist,

Table 1: Modal analyses of Malayer igneous rocks.

Name

38 44 54 56 61 68 77 80 87 91 149 150 186 192

106*

116*

161*

Rock

type

MG Gd MG Gd SG MG Mz Gd To SG SG Di Gd Gb Gd SG MG

Quartz

21 24 23 17 22 17 4 24 25 26 39 8 19 3 23 43 18

Alkali Feldspar

30 16 28 19 41 30 38 25 3 38 31 3 10 3 23 34 34

Plagioclase

25 43 26 43 18 29 44 30 57 19 16 51 56 50 40 12 27

Biotite

16 7 15 16 11 16 9 11 6 6 8 12 7 10 6 6 14

Muscovite

0 1 0 0 4 0 0 1 1 3 2 0 1 0 1 0 0

Amphibole

3 5 4 1 0 3 0 5 2 4 0 13 5 13 4 0 0

Pyroxene

0 0 0 0 0 0 0 0 2 0 0 8 0 12 0 0 0

Opaque

3 2 2 3 1 3 3 2 3 1 1 4 1 4 2 1 3

Accessory*

2 2 2 1 3 2 2 2 1 3 3 1 1 5 1 4 4

Total

100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100

*Accessory minerals include: Apatite, Zircon, Allanite, Monazite, Tourmaline, Sphene, Andalusite, … .

39 for granitoids, 20 for monzonite, 17 for diorite, 10 for
gabbro in the Malayer rocks.

The ammonium content of the Malayer igneous rocks sys-

tematically increases from basic to more felsic rocks as fol-
lows: 10, 17, 20, and 39 as averages for gabbro, diorite,
monzonite, and granitoids respectively (Table 2). Among gra-

Fig. 5. a – Arrangement of platy minerals forms slaty cleavage in
the direction NW-SE (XPL). b – Andalusite minerals in the
metasedimentary rocks of Malayer.

Fig. 6. Classification of metasedimentary rocks from the Malayer
based on Herron’s (1988) diagram.

Fig. 7. Classification of granitic rocks in the QAP diagram, according
to their actual (modal) mineral constituents (after Streckeisen 1976).

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Table 2:

Whole

rock

geochemistry

and

ammonium

content

Malayer

igneou

rocks

(SG

Syenogranite;

Monzogranite;

Granodiorit

Monzo

ite;

Tonalite;

Diorite;

Gabbro)

(mineral

abbreviations

from

Kretz

1983).

nitic  rocks,  the  muscovite-  and  biotite-bearing  syenogranites
have the highest values of ammonium (49 and 46 ppm). This
is  in  accordance  with  this  fact  that  ammonium  is  an  isomor-
phous  substitute  for  potassium  in  the  rock-forming  minerals
(K-feldspar, muscovite and biotite) via diagenetic recrystalli-
zation,  metamorphic  reactions  or  crystal  fractionation.  The
strongly peraluminous and potassic granitoids contain higher
concentrations  of  ammonium  than  metaluminous  granitoids
and it decreases toward monzonite, diorite and gabbro. This is
probably attributed to a general decrease in alkali earth metal
concentrations from felsic to mafic rocks.

Although some scatters were observed in the diagram,

there is nearly good correlation between SiO

and NH

(Fig. 8a). Furthermore, the diagram of K

O vs. NH

exibits a

strong positive correlation, however, in the altered samples
their correlation is weakly negative (Fig. 8b). Figure 8 ap-
proves a good correlation between NH

and some major ele-

ments (e.g. SiO

and K

O). At least in the fresh samples,

positive correlation between NH

and SiO

implies that NH

could be a good petrological tool for evaluating differentia-
tion of a rock suit. In other words, more evolved rocks have
been enriched in ammonium which is consistent with high
K-feldspar contents of these rocks. This feature is supported
by positive correlation between NH

and K

O as well. These

results document the role of ammonium for balancing potas-
sium in the K-bearing minerals. The ammonium has nega-
tive correlation with FeO

and CaO (not shown), but this

changes  to  positive  in  the  altered  samples.  In  general,  the
Malayer  samples  demonstrate  a  magmatic  trend  of  increas-
ing  CaO + FeO

+ MgO with decreasing NH

(Fig. 9). This

feature may either indicate differentiation by fractional crys-

Fig. 8. The positive correlations of a – SiO

vs. NH

, and b – K

vs. NH

ock

O N

O Fe

O C

l+M

c+B

mp+A

.90

3.8

1.85

0.13

182

2.1

l+K

+Bt+

p+Z

.60

3.2

3.04

0.16

285

0.91

fs+

.30

3.7

3.82

0.13

0.2

568

1.9

Qtz

l+A

+Bt+P

x (A

.90

4.3

2.01

0.14

8.6

247

2.6

fs+

Tur

.10

3.6

0.73

0.46

0.2

+Kf

.80

3.9

2.37

0.20

215

3.2

+Bt+Q

.30

3.2

5.15

0.19

408

0.5

l+K

c+B

.40

3.4

2.32

0.16

317

1.6

Kfs

+Am

.10

2.6

1.83

0.11

322

1.0

+Kf

.60

3.8

0.62

0.32

0.6

149 S

fs+

+Bt

.20

3.1

2.05

149 S

0.09

4.7

322

1.0

150 Di

p+Bt+Q

x+Z

.20

2.4

3.08

150 Di

0.12

216

1.2

186 G

Qtz

l+K

+Bt+

p+A

nd+A

.50

3.8

3.31

186 G

0.11

380

1.9

192 G

p+P

Bt+O

.40

1.0

.70

6.23

192 G

0.06

271

1.4

?
106

l++K

p+Bi

s+E

.40

2.6

4.32

?
106

0.14

103

0.8

?
116

fs+

+Se

.40

6.8

0.52

?
116

0.02

125

0.4

?
161

+Kf

60 65

.70

10.6

0.82

?
161

0.27

156

0.8

I D

AI%

Tot

99 8

.63

0.0

07 7

.57

22 6

.80

81 5

.64

0.1

93 9

.27

26 7

.13

99 5

.87

31 6

.57

05 7

.73

06 9

.39

38 8

.35

23 5

.01

03 7

.38

58 2

.85

92 6

.76

55 8

.07

49 8

.41

0.7

178

AHADNEJAD, HIRT, VALIZADEH and JABBARI BOKANI

GEOLOGICA CARPATHICA

GEOLOGICA CARPATHICA, 2011, 62, 2, 171—180

Fig. 9. The relationship between FeO

+ CaO + MgO and NH

con-

tents in the fresh and altered igneous rocks. Note the magmatic and
alteration trends (symbols are as Fig. 8).

Fig. 10. The diagram of alkali ratios (Na

O/K

O) versus ammoni-

um content of Malayer plutonic rocks show enrichment of altered
samples (symbols are as Fig. 8).

tallization or production by partial melting of metasedimen-
tary rocks. On the other hand, altered samples show an alter-
ation trend of increasing CaO + FeO

+ MgO with increasing

(Fig. 9).

The alkali ratios (Na

O/K

O) against ammonium contents

diagram shows distinctive ammonium enrichment for altered
samples (Fig. 10). As seen in the diagram, by decreasing val-
ue of Na

O/K

O, which implies increasing of alteration (Hall

1999),  the  ammonium  contents  are  raised.  The  variability
and scattering of data can be explained by assimilation pro-
cesses  that  involved  additional  ammonium  from  country
rocks.  This  is  supported  by  abundant  metasedimentary  en-
claves as assimilation traces in igneous rocks. However, no
clear correlation between rock type and NH

content is evi-

dent from this diagram.

Because Rb is incorporated in K minerals and Sr in Ca min-

erals,  during  fractional  crystallization,  Sr  tends  to  become
concentrated  in  plagioclase,  leaving  Rb  in  the  liquid  phase.
Hence, the Rb/Sr ratio in residual magma may increase over
time,  resulting  in  rocks  with  increasing  Rb/Sr  ratios  with  in-
creasing differentiation. Typically, Rb/Sr increases in the or-
der  plagioclase,  hornblende,  K-feldspar,  biotite,  muscovite.
The Rb/Sr ratios of the studied igneous rocks increased from
mafic to felsic rocks. The ammonium contents correlate posi-
tively  with  Rb/Sr  ratios  (Fig. 11)  which  indicate  increasing
ammonium concentration in more differentiated rocks.

Generally, the wide range of ammonium concentration in

the granitoids prevent us classifying them into I- and S-type
based on their ammonium content (e.g. Hall 1999; Kohút &
Pieczka 2003). The S-type granitic rocks, which originate
from molten sedimentary rocks, may have different values of
the NH

due to inhomogeneous sources and overlap with

I-types. However, Tainosho & Itihara (1988) show that NH

contents of biotites from S-type granitic rocks are higher
than those for I-type granitic rocks. In the Malayer grani-

Fig. 11. NH

vs. Rb/Sr diagram for Malayer igneous rocks. Note

the high concentration of ammonium in the most evolved rocks
(symbols are as Fig. 8).

toids, because most of the NH

data overlap, it is difficult to

distinguish I- and S-types granitoids.

The samples of 106, 116, and 161 were selected from al-

tered ones for assessing alteration effects on the ammonium
concentration in granitoids. The results show that the ammo-
nium content is high in these rocks (Table 2).

Hall et al. (1991) assumed that in the altered granites, the

hydrothermal  solutions  have  introduced  additional  ammoni-
um from an external source to the granitic plutons. This source
could  be  decay  of  nitrogeneous  organic  compounds  of  sedi-
ments  or  soil  that  converted  to  ammonia.  Subsequently,  the
ammonia immediately converted to the ammonium ion by so-
lution  in  groundwater  and  can  then  be  incorporated  into  sili-
cate minerals and preserved indefinitely. This is a significant
petrological  characteristic  of  ammonium  that  enables  geolo-
gists to distinguish initial hydrothermal alteration in granites.

179

THE AMMONIUM IN THE MALAYER IGNEOUS AND METAMORPHIC ROCKS (WESTERN IRAN)

GEOLOGICA CARPATHICA

GEOLOGICA CARPATHICA, 2011, 62, 2, 171—180

The 2 samples from slates, 4 samples from phyllites and 3

samples from schists of the Malayer metasedimentary country
rocks were analysed. As shown in Table 3, ammonium is pro-
gressively  depleted  from  slate  with  580 ppm,  to  the  phyllite
with  515 ppm  and  andalusite  schist  with  242 ppm.  Bebout  &
Fogel (1992) suggested that during regional and contact meta-
morphism,  the  ammonium  contents  decrease  with  increasing
temperature. This is fully confirmed by our research, where the
ammonium contents decrease from low-grade metamorphic to
schists.  This  feature  may  reflect  a  loss  of  nitrogen  by  break-
down of NH

-bearing minerals during thermal decomposition,

devolatilization, or cation exchange (Hallam & Eugster 1976).

Conclusions

(i) The current paper presents the first ammonium analysis

for Iranian rocks in the Malayer area. The results show that
ammonium contents increase from mafic (gabbro) to felsic
(granite) igneous rocks. This is probably caused by increas-
ing of potassic minerals in felsic types. Due to good correla-
tion between K and ammonium, it is concluded that at least
in granitoids the main carrier of NH

is biotite and musco-

vite. In the case of mafic types, feldspars could be suitable
hosts for ammonium.

(ii) The altered granitoids are highly enriched in ammoni-

um (with an average of 61 ppm) compared with those fresh
samples (39 ppm) which suggests that the solutions feed
rocks for ammonium from external sources.

(iii) There is a significant negative correlation between

and mafic elements (CaO + FeO

+ MgO). This implies a

magmatic trend for ammonium concentration in igneous
rocks and documents that ammonium concentration increas-
es during differentiation of a rock suit.

(iv) It is difficult to ascribe the Malayer granitic rocks to I- or

S-type granitoids, on the basis of ammonium contents.

(v) The altered samples show the opposite trend of in-

creasing NH

with increasing mafic elements.

(vi) The metasedimentary rocks have high concentration of

ammonium which may imply nitrogen rich source materials
(clays). The micas are the NH

carrier in metamorphic rocks.

(vii) Progress in metamorphism caused a decreasing of

ammonium contents in metamorphic rocks by thermal de-
composition and devolatilization.

Acknowledgments: The authors would like to thank Dr. Mi-
lan Kohút and Igor Petrík for their constructive comments.

Table 3: Major elements and ammonium content of Malayer metamorphic rocks.

Sample Rock Mineralogy

SiO

O Na

O FeO

MgO CaO TiO

MnO P

LOI Total

200 Slate

Qtz+Kfs+Ms+Grt

584 68.31 17.46 4.12 0.14 7.08 1.08 0.46 1.03 0.11 0.09 0.25

100.13

201 Slate

Qtz+Kfs+Ms+Grt

577 65.74 19.23 4.35 0.17 6.74 0.97 0.39 0.87 0.06 0.13 0.91

99.56

209 Phyllite

Qtz+Ms+Bt+Kfs+Grt 518 67.12 16.38 3.15 0.10 7.09 0.97 0.23 0.94 0.14 0.14 3.41

99.67

211 Phyllite

Qtz+Ms+Bt+Grt

511 63.28 20.84 4.21 0.09 6.47 0.86 0.19 1.02 0.09 0.15 2.67

99.87

232 Phyllite

Qtz+Ms+Bt+Grt+Kfs 514 65.10 19.40 3.77 0.12 7.32 0.94 0.17 1.01 0.09 0.14 1.92

99.98

237 Phyllite

Qtz+Ms+Bt+Grt+Kfs++Ser

519 67.41 17.52 3.94 0.09 7.10 0.91 0.22 0.93 0.11 0.11 1.46

99.8

241 Schist

Qtz+Ms+Bt+And+Grt 258 64.74 21.23 3.31 0.11 6.81 0.93 0.19 1.01 0.12 0.14 1.07

99.66

250 Schist

Qtz+Ms+Bt+And+Sil+Grt

231 65.17 18.71 2.95 0.86 7.04 0.79 0.21 0.89 0.91 0.12 2.09

99.74

256 Schist

Qtz+Ms+Bt+And+Grt+Crd

239 63.82 21.19 3.11 0.93 6.75 0.92 0.24 1.03 0.86 0.16 0.84

99.85

This research was financially supported by the Iran National
Science Foundation, Grant No. 86103/32.

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