GeolCarp_Vol66_No4_257

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Introduction

The  textures  and  chemical  compositions  of  magmatic  rocks
provide important information about magmatic processes. The
type of texture in igneous rocks depends on the rate of magma
cooling and crystal nucleation, growth rates and residence time
(Winkler 1949; Vernon 2004). Most of the routine textural in-
vestigation  on  igneous  rocks  deals  with  the  description  of
crystal shapes, the relationship of components with each other
and the relative size of crystals, all of which are often reported
as qualitative data (Seasman 2000; Higgins 2000; Innocenti et
al.  2013).  To  investigate  three-dimensional  state  of  grains,
Randolph & Larson (1971) proposed the Crystal Size Distri-
bution (CSD) theory for synthetic solids, which was later de-
veloped  by  Marsh  (1988)  and  applied  to  magmatic  systems
and the resultant igneous rocks. By using CSD theory, valuable
information on the ratio and density  of nucleation, crystalliza-
tion  degree,  growth  rate  and  magma  cooling  process  can  be
achieved. Some noteworthy studies on the application of CSD
to  terrestrial  rocks  such  as  crystal  shapes  and  defining  new
models to simulate solidification  of magma were done in the
1990s (e.g. Marsh 1988; Armienti et al. 1991; Cashman 1993;
Higgins 1994; Sahagian & Proussevitch 1998). In most of CSD

Crystal size and shape distribution systematics of plagioclase

and the determination of crystal residence times in the

micromonzogabbros of Qisir Dagh, SE of Sabalan volcano

(NW Iran)

HAMED POURKHORSANDI

1,2!

, HASSAN MIRNEJAD

1,3

, DAVOUD RAIESI

and JAMSHID HASSANZADEH

Department of Geology, Faculty of Sciences, University of Tehran, Tehran 14155—64155, Iran; hmirnejad@ut.ac.ir; davood.raeisi@ut.ac.ir

CEREGE UM34, CNRS, Aix-Marseille University, 13545 Aix-en-Provence, France;

pourkhorsandi@cerege.fr

Department of Geology and Environmental Earth Sciences, Miami University, Ohio 45056, USA

Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA; jamshid@caltech.edu

(Manuscript received October 2, 2014; accepted in revised form June 23, 2015)

Abstract: The Qisir Dagh igneous complex occurs as a combination of volcanic and intrusive rocks to the south-east of the
Sabalan volcano, north-western Iran. Micromonzogabbroic rocks in the region consist of plagioclase, alkaline feldspar and
clinopyroxene  as  the  major  mineral  phases  and  orthopyroxene,  olivine,  apatite  and  opaque  minerals  as  the  accessory
minerals. Microgranular and microporphyritic textures are well developed in these rocks. Considering the importance of
plagioclase in reconstructing magma cooling processes, the size and shape distribution and chemical composition of this
mineral were investigated. Based on microscopic studies, it is shown that the 2-dimensional size average of plagioclase in
the micromonzogabbros is 538 micrometers and its 3-dimensional shape varies between tabular to prolate. Crystal size
distribution diagrams point to the presence of at least two populations of plagioclase, indicating the occurrence of magma
mixing and/or fractional crystallization during magma cooling. The chemical composition of plagioclase shows a wide
variation in abundances of Anorthite-Albite-Orthoclase (An = 0.31—64.58, Ab = 29.26—72.13, Or = 0.9—66.97),  suggesting
a  complex  process  during  the  crystal  growth.  This  is  also  supported  by  the  formation  of  antiperthite  lamellae,  which
formed as the result of alkali feldspar exsolution in plagioclase. The calculated residence time of magma in Qisir Dagh,
based on 3D crystal size distribution data, and using growth rate G = 10

—10

mm/s, varies between 457 and 685 years, which

indicates a shallow depth (near surface) magma crystallization and subvolcanic nature of the studied samples.

Key words: Iran, Sabalan, Qisir Dagh, subvolcanic rocks, magma mixing, crystal size distribution.

studies on igneous rocks, measurements have been focused on
plagioclase, because it is the most frequent mineral in igneous
rocks, is stable in a wide spectrum of magmatic conditions, and
can record physico-chemical fluctuations within a magma
chamber (Gagnevin et al. 2007; Ruprecht & Wörner 2014).

Cenozoic magmatic activities in NW Iran are closely

linked  to  the  collision  between  the  Afro-Arabian  and  Eur-
asian  plates  (Berberian  &  King  1981).  The  Eastern  Azer-
baijan region of Iran is part of a vast igneous province that is
situated between two inland seas, the Caspian Sea and Black
Sea (Innocenti et al. 1982; Dilek et al. 2010). The Qisir Dagh
igneous  suit,  located  between  Sarab  and  Nir  cities  in  NW
Iran,  is  mainly  composed  of  basaltic  and  andesitic  volcanic
rocks. Didon & Gemain (1976) reported a “dioritic” igneous
mass in Qisir Dagh. However, there are no adequate data on
the occurrence, formation and the relationship of these rocks
with the other igneous rocks in the region.

The aim of this study is to understand the crystallization ki-

netics of magma by means of physical and chemical observa-
tions and to use them to infer the crystallization processes of the
studied rocks. The dynamics and history of magma cooling in
Qisir Dagh, including the solidification mechanism of the mag-
ma chamber and the emplacement level (i.e. volcanic versus

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of the Afro-Arabian plate and the associated subduction be-
neath  the  southern  margin  of  Eurasia  in  the  Late  Mesozoic
and Early Cenozoic had a major role in the evolution of con-
tinental crust in Iran (Stöcklin 1968; Takin 1972; McQuarrie
et al. 2003; Allen et al. 2011). These tectonic events led to the
development  of  Urumieh-Dokhtar  Magmatic  Arc  (UDMA),
Sanandaj-Sirjan  Metamorphic  Zone  (SSMZ)  and  Folded
Zagros  Zone  (FZZ)  in  Iran  (Ruttner  &  Stöcklin  1967;
Pazirandeh 1973).

Convergent motion between the Afro-Arabian and Eurasian

plates triggered intense magmatic activity in the Cenozoic that
is particularly distinguished in NW Iran, Anatolian plateau of
Turkey, Armenia, Georgia and Azerbaijan. A large igneous
province that occupies an area of about 200,000 km

and hosts

large stratovolcanoes in NW Iran and Eastern Turkey (e.g. Sa-
balan, Sahand and Ararat), formed as a result of this activity
(Fig. 1) (Alberti et al. 1980; Yilmaz et al. 1990; Trifonov et
al. 2012; Kheirkhah & Mirnejad 2014). Some researchers sug-
gest that there is a petrogenetic relationship between the Ceno-
zoic magmatism of this igneous province and those of Alborz
and UDMA in Iran (Alberti et al. 1976; Innocenti et al. 1982;
Jahangiri 2007; Azizi & Moinevaziri 2009; Ahmadzadeh et al.
2010; Allen et al. 2011; Dabiri et al. 2011; Jamali et al. 2012).

Sabalan (4811 m a.s.l.) [Sabalân/Savalan] a Plio-Quaternary

stratovolcano  located  in  the  eastern  Azerbaijan  region  of
Iran, is composed of high-K, calc-alkaline andesitic rocks that
currently  shows  hydrothermal  activities  (Alberti  et  al.  1975;
Didon & Gemain 1976; Dostal & Zerbi 1978; Ghalamghash et
al. 2013). The southern lowlands of Sabalan consist of basal-
tic and andesitic rocks of Cenozoic age, which have reached
the  surface  through  temporary  volcanic  fissures.  Contrary  to
these, Qisir Dagh, a central volcanic structure, is a product of
much more durable magmatic activity. The geological map of
the region is shown in Fig. 2 (Amini 1987). The Miocene vol-
canic rocks of Qisir Dagh are overlain by volcanoclastic sedi-
ments  of  the  pre-Sabalan  stage  (Didon  &  Gemain  1976;

Alberti  et  al.  1976;  Amini  1987).  Didon  &  Gemain  (1976)
suggest that Qisir Dagh is an eroded stratovolcano and the de-
pression which divides it into eastern and western parts is the
eroded remnant of a caldera (Fig. 3a,b). In the northeast of the
depression, which is the focus of this study, igneous rocks oc-
cur  as  dyke  swarms  and  dome-like  structures  (Fig. 3c).  Fine
grained texture and lack of volcanic structures in these rocks
suggest  a  probable  subvolcanic  origin.  Based  on  changes  in
colour, structure and weathering degree, four different variet-
ies  of  micromonzodiorite-monzogabbro  are  distinguished  in
the field (Fig. 4a).

Pourkhorsandi (2014) reports a narrow range of SiO

con-

tent (53.01—56.52 %, average 54.66 %) for these rocks. Con-
sidering the classification scheme of Le Bas et al. (1986), the
studied rocks from Qisir Dagh belong to the monzodiorite-
monzogabbro class of igneous rocks (Fig. 5a). These rocks are
silica saturated/oversaturated and show high-K and shosho-
nitic affinities (Fig. 5b). The Ce/Pb ratio of the studied rock
varies between 1.6 and 3.1 (Pourkhorsandi 2014). Compared
to the ratio of ~ 25 for magmas originating from the mantle
(Rollinson 1993), this is very low and is similar to the ratio
of the upper crustal rocks for which the average is 3.5 (Rollin-
son 1993).

For evaluating the CSD in the Qisir Dagh igneous rocks,

we selected micromonzogabbro samples (Fig. 4b), because
the rocks of this group are more widespread and least altered
compared to other igneous rocks in the region.

Methodology

Fifty rock samples were collected from igneous units in Qisir

Dagh and 30 specimens were selected for petrography and
rock classification. Among these, five thin sections from mic-
romonzogabbros were prepared for quantitative optical micros-
copy. Selected samples had a proper distance from each other

Fig. 3. a – 3D and topographic view of Qisir Dagh (Data source: Google Earth), b – geological profile of Qisir Dagh and the position of
the studied rocks (diorite in this map) (after Didon & Gemain 1976), c – panoramic view of the studied rocks of Qisir Dagh, showing mi-
cromonzogabbros as the cliffs.

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and  were  the  least  altered.  For  two-dimensional  size  mea-
surements,  more  than  95 %  of  the  area  of  a  thin  section,  in
X25 magnification (crossed polars) and with and without the
presence of a gypsum plate, was photographed. The gypsum
plate  increases  colour  contrast  between  crystals,  sharpens
mineral outlines and facilitates size measurements. Lamellar
twinning  and  crystal  shape  of  the  plagioclase  crystals  are
some of the criteria which were used to differentiate plagio-
clase from alkali feldspar, while comparing the section under
“normal”  cross  polarized  light  with  the  one  with  gypsum
plate  inserted.  The  subvolcanic  nature  of  the  studied  rocks
makes the process of crystal outline distinguishing and mea-
suring  difficult.  To  prevent  any  false  results,  crystals  with
unclear  outline  were  omitted  which  decreases  the  number
of  the  crystals  to  measure.  However,  the  overall  number
of  the  measured  crystals  is  more  than  those  reported  from
elsewhere (e.g. Diaz Azpiroz & Fernandez 2003). To estimate
the frequency of different size groups, the length and width
of  2266  plagioclase  grains  were  manually  and  visually
measured in the longest perpendicular directions using JMicro-
Vision  software  platform.  Extraction  of  3D  shape  and  size
distribution  data  from  raw  2D  data  was  done  using  the

CSDSlice5 spreadsheet (Morgan & Jerram 2006) and CSD
Corrections 1.4.0 software (Higgins 2000, 2002).

Chemical compositions of feldspars (both plagioclase and

alkali feldspar) were determined with JEOL JXA-8200 elec-
tron  microprobe  at  the  California  Institute  of  Technology,
using  focused  electron  beam  ( ~ 1 micrometer  in  diameter),
an  accelerating  voltage  of  15 kV  and  a  beam  current  of
25 nA. The data were reduced using the CITZAF algorithm
(Armstrong 1988).

Petrography

The studied samples are classified as micromonzogabbros,

based on modal mineralogy and texture. The micromonzo-
gabbros in Qisir Dagh consist of plagioclase (65—75 %),

Fig. 5. The position of igneous rocks from Qisir Dagh. a –
Na

O + K

O versus SiO

diagram (Cox et al. 1979), b – K

O vs.

SiO

diagram (Peccerillo & Taylor 1976).

Fig. 4. a – Different varieties of micromonzodiorite and micro-
monzogabbro occur in Qisir Dagh, shown as dashed and dash-dotted
line, respectively, b – The hand specimen image of micromonzo-
gabbros that was used for CSD study.

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Point#

216 217 218 219 220 221

SiO

wt. %

65.80 63.31 52.64 62.47 63.40 65.54

TiO

0.10 0.16 0.14 0.12 0.12 0.13

19.27 21.55 29.16 22.23 22.44 19.18

FeO

0.34 0.41 0.44 0.44 0.39 0.24

MgO

0.00 0.00 0.02 0.00 0.00 0.00

CaO

0.70 3.36

12.18 4.16 3.93 0.78

4.69 7.27 4.43 7.58 7.20 3.27

9.58

3.43

0.38

2.46

3.06

11.41

0.01 0.02 0.00 0.03 0.00 0.00

MnO

0.00 0.01 0.01 0.00 0.00 0.02

Oxide totals

100.50 99.53 99.40 99.48

100.55

100.57

Ab mol %

41.24 63.85 38.82 65.91 63.24 29.16

3.38 16.32 58.97 20.02 19.08 3.86

55.38 19.83 2.21 14.06 17.68 66.97

Table 1: Microprobe analyses of feldspar in Qisir Dagh samples.

Fig. 7. a – BSI image showing the analysed points in feldspars,
showing the changes in BSE image color with changing the composi-
tion of feldspar, b – Ternary orthoclase-albite-anorthite (Or-Ab-An)
diagram showing the position of analysed feldspars from Qisir
Dagh micromonzogabbros.

alkali  feldspar  (10—15 %),  olivine  (5—10 %),  opaque  miner-
als (3 %), clinopyroxene (5 %), orthopyroxene (1 %) and ap-
atite.  Microgranular,  intergranular  and  in  some  cases
microporphyritic  textures  also  developed  in  these  samples
(Fig. 6a,b).

Plagioclase is euhedral to subhedral and occurs as pheno-

cryst (average 0.2—0.8 mm) and microphenocrysts (<0.1 mm).
In  some  plagioclase  grains,  exsolution  textures  occur  as  ir-
regular dendritic veins of sodic and potassic plagioclase in-
side calcic varieties of the mineral (Fig. 6f). Subhedral alkali
feldspar occurs as phenocryst (0.1—0.7 mm) and is mainly al-
tered to sericite. Subhedral and anhedral crystals of clinopy-
roxene occur as phenocrysts ( ~ 0.5 mm) and as inclusions in
plagioclase. Possible instability of clinopyroxene in the pres-
ence of magma has led to the development of corrosion tex-
tures.  Alteration  of  clinopyroxene  to  serpentine  and  opaque
minerals  is  evident  from  the  co-presence  of  these  minerals.
Fig. 6a—d  shows  the  opaque  and  clinopyroxene  crystals,
mostly in the vicinity of each other. Orthopyroxene is euhe-
dral  to  subhedral  and  exhibits  smaller  sizes  relative  to  cli-
nopyroxene.  Olivine  ( ~ 0.2—0.7 mm)  occurs  as  euhedral  to
anhedral  gains  and  is  commonly  altered  to  iddingsite  and
serpentine.  Most  of  the  apatite  crystals  in  the  studied  rocks
show poikilitic textures, as most are lineated in pyroxene and
plagioclase (Fig. 6e).

Mineral chemistry

The microprobe analyses of plagioclase and the calculated

cation  numbers  are  presented  in  Table 1.  Fig. 7a  shows  the
backscattered electron (BSE) image of a feldspar and Fig. 7b
depicts  the  range  of  chemical  variations  of  this  mineral  in
an  Or-Ab-An  ternary  diagram.  The  brighter  patches  reflect
areas  characterized  by  higher  Ca/Na  or  K/Na  ratios.  Feld-
spar ranges in compositions An0.31 to 64.58. In the studied
samples,  plagioclase  normally  lacks  chemical  zoning  and
shows  complex  irregular  patchy  textures.  The  K  content  of
plagioclase in the studied samples is higher than that of ordi-
nary  plagioclase  in  igneous  rocks  reported  by  Deer  et  al.
(1992).  High  amounts  of  K  can  be  considered  indicative  of
a complex process that led to the development of exsolution
lamellas (e.g. perthite, antipertithe and cryptoperthite) in pla-
gioclase.

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Theory of CSD

CSD deals with the size distribution of crystal populations

as a function of the number of crystals in a measured volume
and the amount of crystals within a series of defined size in-
tervals  (Marsh  1988).  Stereology  techniques  that  produce
CSD data consist of a body of methods for the exploration of
three-dimensional  space,  when  only  two-dimensional  sec-
tions  through  solid  bodies  or  projections  on  a  surface  are
available (Underwood 1973).

Mathematically, the crystal population density (n) is de-

fined as n(L) = dN(L)/dL, where N(L) is the cumulative num-
ber of crystals per unit volume with long axes equal to or
lower than L and n is the number of crystals per unit volume
in a given size class. Crystal population density is best repre-
sented by a plot of ln(n) versus crystal length (L). The histo-
grams of volume fraction (number of crystals for each size)
vs. size is another method for representing the CSD data
(Higgins 2000; Jerram et al. 2009).

Cashman & Marsh (1988) showed that the shape of the

CSD curves could be related to various magmatic processes
such as crystal fractionation and accumulation, magma mix-
ing  and  coarsening.  In  this  study,  the  program  CSDcorrec-
tions 1.4.0. was used to produce CSD curves, which convert
2-dimensional  intersectional  data  (length  and  width)  to  true
3-dimensional crystal size distributions by incorporating cor-
rections  for  the  intersection  probability  and  cut-section  ef-
fects. Utilization of this program also requires an estimation
of the sample fabric, grain roundness and 3D crystal shape.
Based on petrography of five thin sections, a roundness fac-

Sample

Number of measured crystals

Average length size (µm)

QSD-10b

929

420

QSD-12

418

608

QSD-14

423

569

QSD-17

162

644

QSD-23

334

684

Table 2: Two-dimensional parameters of plagioclase crystals.

Fig. 8. Frequency versus length (a) and width (b) histograms of the measured pla-
gioclase grains. Variation in length sizes is more than widths.

time to growth rate, we chose the value of 10

—10

mm/s used by

Garrido et al. (2001) and Cheng & Zeng (2013) to calculate
sub-volcanic gabbroic magmatic bodies that are not too dif-
ferent from our studied rocks.

CSD results

Distribution of 2D length and width size (intersection di-

mensions) data of the 2266 measured grains is presented in
Table 2 and shown as a frequency versus size histogram in
Fig. 8. The two-dimensional size average of plagioclase in

tor of 0.2 is chosen for Qisir Dagh samples. The
3D shapes of the plagioclase crystals from Qisir
Dagh  were  estimated  using  the  CSDSlice 5
spreadsheet  (Morgan  &  Jerram  2006),  which
compares  the  distribution  of  2D  size  measure-
ments to  a database of shape curves for random
sections through 703 different crystal shapes and
determines a best fit 3D crystal habit based on re-
gression calculations and fitting to the database.
Crystal shapes are defined in terms of “aspect ra-
tio”  that  is  the  ratio  of  short:  intermediate:  long
(S: I: L) dimensions.

As proposed by Marsh (1988), average crystal

residence time is calculated by employing the
equation:

Tr = (—1/G

×m)/31536000

where Tr is the residence time (year), G is the

growth rate of crystals (mm/s), m is the slope of
the  CSD  curve  of  the  long-linear  data  and
3156000  is  a  coefficient  to  convert  seconds  to
years.  Choosing  the  appropriate  growth  rate  is
important  for  calculating  the  residence  time.
Cashman (1993) has considered a growth rate of
10

—9

mm/s and 10

—10

mm/s for 3 and 300 years of

cooling time, respectively. This author also con-
siders a residence time of less than 1000 years for
low-depth magmatic bodies, which are similar in
composition to the Qisir Dagh rocks. Based on
the strong dependence of calculated residence

Table 3: Calculated crystal shape parameters. S, I and L represent
short axis, intermediate axis and long axis, respectively. S/I (short
axis/intermediate axis) and I/L (intermediate axis/long axis).

Sample

Aspect ratio

S/I

I/L

Crystal shape

QSD-10b

1:1.4:2.4 0.71

0.58 Prolate

QSD-12

1:1.5:1.9 0.67

0.79 Tabular

QSD-14

1:1.4:2.4 0.71

0.52 Prolate

QSD-17

1:1.7:2.2 0.59

0.77 Tabular

QSD-23

1:1.7:2.5 0.59

0.68 Tabular

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Fig. 9. Frequency versus 2D sa/la (short axis/long axis) histograms of the measured samples from Qisir Dagh (red lines) together with best-
fit shape output by CSDSlice (dashed lines) and the proposed aspect ratios. The numbers on the right corner represent the aspect ratios as
represented by short axis: intermediate axis: long axis of the crystal.

the  micromonzogabbros  of  Qisir  Dagh  is  538  micrometers.
Plagioclase crystals with lengths of less than one millimeter
are  much  more  abundant  than  those  having  larger  sizes.  As
can  be  seen  in  Fig. 8,  plagioclase  crystals  show  unequal
shapes and their lengths exhibit more variation in sizes than
widths.  However,  we  considered  the  measured  plagioclase
lengths to be real and not the results of clustering of crystals
although  this  cannot  be  excluded  in  the  largest  size  bins
( > 1500 µm,  Fig. 8a),  the  other  reason  for  this  bin  could  be
the presence of xenocrystals. As a result of the fact that larger
crystals show higher degrees of K-metasomatism, this reason

Fig. 10. The position of average aspect ratio of plagioclase in igne-
ous rocks on I/L versus S/I diagram (Zingg 1935). Plagioclase from
Qisir Dagh (filled cricles) plot in prolate and tabular 3D shapes (ex-
cluding one sample that resides between tabular and equant). S, I
and L represent small, intermediate and long axis, respectively.

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sounds likely. Calculated crystal shape data are presented in
Table 3. I/L and S/I factors were calculated from the aspect ra-
tio data. Fig. 9 shows the population shape curves of normal-
ized frequency vs. short axis/long axis ratio together with the
best-fit shape output by CSDSlice 5 for the studied samples
from Qisir Dagh rocks. Dotted lines are the data of some stan-
dard crystals from the data source of CSDSlice 5 and the red
lines are our data. Dotted lines show the most similar crystal
shapes to our data and have been considered to be similar to
our sample crystal factors.

The I/L versus S/I diagram of Zingg (1935) is used here to

present the data and to evaluate the 3D shapes. As can be seen

Fig. 11. CSD diagrams of the studied plagioclase crystals from Qisir Dagh. Note the non-linear trend of the diagrams. Dotted lines are the
regression lines calculated based on the frequency of all crystal sizes. Horizontal axis unit is millimeter.

in Fig. 10, the majority of the plagioclase crystals are either
prolate or tabular in 3D shape, and one sample (QSD-17) is in-
termediate between tabular and equant shapes.

Higgins (2006) states that there is a possible relationship

between the shape of a crystal and its size. To evaluate this
theory, we compared the lengths of studied plagioclase
grains in 2D with those of the 3D shapes (Table 3). Contrary
to Hastie et al. (2013), the average length of prolate plagio-
clase crystals in the studied rocks is less than the amount of
those in tabular crystals.

The CSD curves for plagioclase from Qisir Dagh are pre-

sented in Fig. 11. All of the CSD curves, except the sample

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Table 4: Calculated residence time for micromonzogabbros of Qisir
Dagh based on quantitative textural investigation.

Sample

Residence time

(years)

Regression line

slope

Regression

intercept

QSD-10b

685 –2.16 2.20

QSD-12

621 –1.96 1.48

QSD-14

457

–1.44

0.91

QSD-17

463 –1.46 0.51

QSD-23

460 –1.45 0.71

QSD-10b,  are  concaved  up  and  do  not  follow  a  classic
straight  line  trend.  The  proportion  of  larger  crystals  in  the
studied rocks is low and their sizes are so variable compared
to  smaller  crystals,  which  may  contribute  to  the  relatively
big error of the big segment of CSD curves. The selected re-
gression lines are the ones responsible for the most frequent
crystal size intervals (in this case small crystals) that are de-
picted in CSD curves as the long and straight line in the mid-
dle of the curve. These regression lines are drawn based on
all  the  size  intervals  and  since  the  relatively  small  crystals
are more frequent, these lines mostly follow the segments of
CSD curves which are related to these crystals. Taking a sim-
ple  assumption  of  plagioclase  steady-state  magma  chamber
model and continuous growth rate of 10

—10

mm/s for gabbroic

rocks of dyke structures (Garrido et al. 2001; Cheng & Zeng
2013), the crystal residence times of plagioclase from Qisir
Dagh are calculated to be less than 1000 years (Table 4).

Discussion

In almost all of the CSD studies, the interpretations dealing

with  the  cooling  conditions  of  magma  have  been  done  on
CSD  curves.  Straight  CSD  curves  form  in  a  simple  magma
cooling process while non-straight CSD curves indicate occur-
rence of physico-chemical variations (caused by magma mix-
ing, crystal differentiation, etc.) through magma solidification
(e.g. Cashman & Marsh 1988; Higgins 1996, 2000). Compari-
son  of  CSD  curves  of  the  mircomonzogabbros  from  Qisir
Dagh (Fig. 11) with the standard CSD models of Vinet & Hig-
gins (2010) (Fig. 12) suggests the similarity of CSD curves of
this study to the curve presented in Fig. 12e. As mentioned be-
fore,  CSD  curves  of  the  studied  samples  show  notable  slope
changes and are concave up in shape. Concave up CSD curves
are  considered  to  be  evidence  of  hybrid  crystal  size  popula-

Fig. 12. Schematic examples of processes that can influence the shape of the CSD (after Higgins 2006, modified by Vinet & Higgins 2010).
a – increase in undercooling (or saturation), b – increase in residence time or growth rate, c – accumulation and fractionation of crys-
tals, d – coarsening, e – magma mixing (or mixing of two crystal populations).

tions, formed due to magma mixing, crystal differentiation
and crystal coarsening (alone or in conjunction) inside magma
chamber and/or during magma ascent (Salisbury et al. 2008;
Yu et al. 2012; Van der Zwan et al. 2013; Ngonge et al. 2013).
Since microscopic studies show no evidence of textural coars-
ening in the studied rocks, it is likely that magma mixing and/
or crystal differentiation played important roles in the forma-
tion of plagioclase crystals in Qisir Dagh.

It has been shown by Kouchi et al. (1986) and Higgins

(2006)  that  3D  shapes  of  crystals  have  a  close  relationship
with the conditions of magma crystallization. These authors
mention  that  crystallization  of  magma  in  a  dynamic  system
and  with  a  high  variation  in  chemical  potential  produces
long and thin crystals. Unequant shape of the studied plagio-
clase crystals in our study suggests a similar physico-chemi-
cal condition.

In addition to quantitative petrographic data, the chemistry

of the plagioclase crystals is in accordance with a non-simple
magma crystallization trend. Occurrence of exsolution tex-
tures in plagioclase as well as their microscopic and chemi-
cal attributes point to the influence of chemical and physical
processes on the formation of the micromonzogabbros. Tem-
perature, pressure and the amount of K are the most impor-
tant factors responsible for the development of exsolution
textures (particularly antiperthite) in plagioclase. With an in-
crease in temperature and change in the position of O atoms,
large K

ions can easily replace the Na

ions and on cooling

form  exsolution  textures.  Alongside  the  effect  of  tempera-
ture,  potassic  metasomatism  is  considered  important  in
forming  an  exsolution  texture  (Sen  1959;  Bown  &  Gay
1971; Parsons & Brown 1983). Chemical data of the studied
plagioclase  suggest  the  effects  of  such  activities  (tempera-
ture increasing and potassic metasomatism) through crystal-
lization of the rocks that are also noticeable in CSD curves.

Based on CSD calculations, and using the growth rate

G = 10

—10

mm/s, model residence times of the studied rocks

are  less  than  1000  (457  to  685)  years  (Table 4).  The  ob-
served variations in the residence times could be developed
in  a  thermally  heterogeneous  magma  chamber.  Short  resi-
dence  times  of  this  range  compared  to  those  of  coarse
grained igneous rocks are believed to be indicative of a low-
depth  and  subvolcanic  magmatic  system  (Cashman  1993)
that  has  formed  the  microcrystalline  rocks  of  Qisir  Dagh.
Low  residence  times  justify  the  microcrystalline  texture  of
the studied rocks and are indicative of formation of the mi-

267

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GEOL

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THICA, 2015, 66, 4, 257—268

cromonzogabbros from Qisir Dagh in a subvolcanic mag-
matic system (Vernon 2004; Gill 2010).

Conclusions

Results of CSD and mineral chemistry data of mi-

cromonzogabbros from Qisir Dagh suggest that:

" Based on measurements of 2266 crystals, the average

length size of plagioclase is 538 micrometers which points to
the microcrystalline texture of the host rock and its forma-
tion in a subvolcanic magmatic system;

" Plagioclase crystals show unequal shapes in intersection

dimensions. The size variation of crystal lengths is also more
common than the widths which indicates the crystals are un-
equant;

" The majority of the plagioclase crystals are either prolate

or tabular in 3D shape. The average length of prolate plagio-
clase crystals is less than that in tabular crystals;

" Non-straight and concave up shapes of CSD curves indi-

cate the presence of at least two crystal populations formed
as the result of magma mixing and/or crystal differentiation.
The process of magma mixing is also supported by the min-
eral chemistry of plagioclase and the formation of exsolution
textures in plagioclase grains;

" Calculated model residence times for plagioclase crys-

tals in Qisir Daghi are less than 457—685 years, using
G = 10

—10

mm/s, indicating rapid cooling in a subvolcanic

magmatic system with thermal heterogeneity.

Acknowledgments: The authors would like to thank D.J.
Morgan and D.A. Jerram for providing CSDSlice spreadsheet
and also H. Altafi, S. Nezamdoost and B. Arefi for their assis-
tance with the field work. Dr. I. Petrík is thanked for editorial
handing of the paper and for providing us with valuable scien-
tific comments. Reviews and comments by Prof. H. Yu and an
anonymous reviewer are very much appreciated.

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