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ORIGINAL ARTICLE
Geochemistry and crystal shape, size and spatial distribution
in arc-related gabbro, Urmia, NW Iran
Monir Modjarrad
1
Received: 24 May 2022 / Revised: 13 July 2022 / Accepted: 15 July 2022
ÓThe Author(s), under exclusive licence to Science Press and Institute of Geochemistry, CAS and Springer-Verlag GmbH Germany, part of
Springer Nature 2022
Abstract The mafic intrusive Moskin and Parvaneh
regions in the northwest and south of Urmia were inves-
tigated for their geochemistry and petro-physical properties
such as size, shape, and spatial distribution of their crystals.
This type of study can be a powerful tool for analyzing the
history of magma crystallization, along with geochemistry
examinations. The gabbroic calc-alkaline rocks, with the
Early Cenozoic is the relative age, consist of coarse pla-
gioclase and amphibole with rare clinopyroxene and oli-
vine in the Sanandaj–Sirjan zone at sedimentary-structural
subdivisions of Iran. The depletion of HFSEs and enrich-
ment of LILEs as arc-related rocks, as well as lower crust
contamination (Nb/Th and K/Ce) symptoms, show that
these gabbros formed at the continental arc setting from an
enriched source. Considering the geographical position and
relative age of the gabbros, they possibly outcropped dur-
ing the Neo-Tethys subduction processes after the enrich-
ment by considerably lower crust effects. The amphibole
and plagioclase crystals with considerable abundance show
linear crystal size distribution (CSD) patterns with a slope
of (-1/Gt)-3to-4 for amphiboles and plagioclase
except for Parvaneh plagioclase which shows -16. The
annealing (Ostwald ripening) caused the convex CSD on
the small grains part at Parvaneh gabbro. Therefore, the
ratio of nucleation to growth rates (J/G) for these minerals
is 5–6 times, only for Parvaneh plagioclase is ten times
which numerous nuclei were made by a significant degree
of overstepping in the rocks. There is no magma mixing
evidence because of the linear (one population of crystals)
CSD patterns. The shape of amphibole crystals in gabbros
is bladed (with S/I = 0.4–0.5 and I/L = 0.1–0.3) and bladed
to tabular for plagioclase (S/I = 0.4 and I/L = 0.6–0.7). The
spatial distribution patterns (SDPs) are calculated from a
big R-value based on the nearest neighbor’s theory for
amphibole and plagioclase crystals, confirming clustered
SDP for all of them. Also, the dihedral angle measurements
(between amphibole and plagioclase grains) display that
solid-state equilibrium is prevailing in the rocks. In this
study, we tried to establish a relationship between geo-
chemistry and petro-physical results and investigate the
evolution of igneous rocks more accurately and all-inclu-
sive. We should not neglect the importance of quantifying
the texture of rocks.
Keywords Gabbro Lower crust CSD SDP DA
Urmia-Iran
1 Introduction
The Urmia intrusive suit is located within the NW part of
the Sanandaj–Sirjan structural zone of Iran, a NW–SE zone
which belongs to the 1500 km long Zagros orogenic sys-
tem, between the Urmia–Dokhtar magmatic arc and the
folded Zagros thrust belt (Alavi 1994) (Fig. 1). The Zagros
orogenic system, the Iranian part of the Tethys orogenic
belt. It was formed by the subduction of Neo-Tethys
oceanic crust and the subsequent continental collision
between Gondwana and Eurasia from Mesozoic to Tertiary
time (Berberian and King 1981; Ricou 1994). The Sanan-
daj–Sirjan zone is considered metamorphic rocks which are
associated with several plutons and widespread Mesozoic
&Monir Modjarrad
m.modjarrad@urmia.ac.ir
1
Faculty of Science, Department of Geology, Urmia
University, Urmia, Iran
123
Acta Geochim
https://doi.org/10.1007/s11631-022-00557-8
volcanism (Mohajjel et al. 2003a,b). Intrusions are well
developed from northwest (Urmia) to southeast (Golpieh-
gan) extending beside ca. 800 km (Fig. 1).
Understanding the formation and transfer of crustal
melts is essential for concluding continental crust frac-
tionation methods (McMillan et al. 2003). In this study, we
focus on the hornblende-gabbros around Urmia to deter-
mine its tectonic setting and crystallization processes of it.
In many cases, arc cumulate gabbros are physically and
chemically related to the gabbro-diorite (basalt-andesite)
fractionation process and have evidence of crystal segre-
gation that plays a vital role in their petrogenesis (Beard
1986). Some studies demonstrated that most arc-related
calk-alkaline rocks typically formed in places where stress
is eliminated during the post-collision phase (Fan et al.
2003; Guo et al. 2001).
Nonetheless, hornblende-bearing gabbroic rocks (in the
form of xenolith or intrusion) are less common in sub-
duction-related magmatic collections and indicate frac-
tional magmatic processes in arc magma (Heliker 1995;
Hickey-Vargas et al. 1995). The main phase in the sub-
duction regions gabbroic rocks, hornblende, maybe as
primary phase crystallized from hydrous mafic magma
(Beard and Borgia 1989) or a reaction product of primary
minerals such as olivine, pyroxene, and plagioclase with
the water-saturated phase in the interaction between the
melt/hydrous fluid (Costa et al. 2002).
The texture of a rock provides valuable information
about the physical processes involved in the rock forma-
tion, parameters such as size (Moazzen and Modjarrad
2005; Modjarrad 2015; Modjarrad and Sheykhbaglou
2016), shape, and spatial distribution of crystal patterns
(CSD and SDP) and Dihedral angle (DA) measurements
considered in this regard need measured. Of course, work
Fig. 1 a,bLocation of Northwest of Iran (study area) in large scale geodynamic map of the region in relation to the Neo-Tethys subduction
between Arabian plate and Central Iran micro-continents. cThe Zagros orogenic belt position in Iran (Alavi 1994). dThe Urmia–Golpaiehgan
Plutonic Belt (in black) in the Sanandaj–Sirjan zone and location of dated samples. The abbreviations CP, Mu, As, Br, Al and Am refer to name
of plutons Chir-Mokin (this study), Muteh, Astaneh, Boroujerd, Alvand and Almoqolaq respectively, and numbers gives the results of age
determination. The red box (study area) is illustrated in the Fig. 2
c
Fig. 2 Simplified geological maps of the study areas using 1:100,000
geological map of Serow (Moskin), north of Urmia (Sabziei et al.
2004), and 1:100,000 geological map of the Oshnavieh (Parvaneh),
south of Urmia (Naghizadeh 2004) with structural zone determination
of Ghasemi and Talbot (2006) at the middle part
Acta Geochim
123
Acta Geochim
123
on SDP and DA is much more limited, and how to measure
or interpret it is still unclear. We must combine such data
with geochemical results to draw perfect conclusions about
rock generation. This type of study is time-consuming but
could be a powerful tool for analyzing the history of
magma crystallization along with geochemistry
examinations.
2 Geological setting
The intrusive complex of Urmia in the north-west of the
Sanandaj–Sirjan zone (with the Mesozoic-tertiary age
igneous belt within the metamorphic rocks) and the north
and south of the Urmia city (Fig. 1) is a part of the Neo-
Tethys ocean suture zone (Sengo
¨r1990; Mohajjel 1997;
Mohajjel and Fergusson 2000).
The Urmia igneous suite has been replaced among the
destructive-carbonate units of the Upper Permian and the
Upper Triassic- Lower Jurassic such as Durud, Ruteh, and
Naiband formations. Also, the Upper Precambrian and
Paleozoic units of Kahar, Soltanieh, Barut, and Lalon have
been cut off by the intrusion of this complex (Ghalamghash
et al. 2009a). In the southwest of the area ultramafic rocks
are dependent on the ophiolitic complex with the upper
Cretaceous age, has exposed, whose boundaries with the
intrusive units are fault zone.
The Urmia intrusive complex has an expansion of about
700 km
2
and has emerged from 9 to 10 outcrops with stock
dimensions that according to age characteristics and whole-
rock composition can be classified into three rock types’
diorite-gabbro, granite, and alkali syenite-granite (Ghala-
mghash et al. 2009a).
According to the field evidence and age measurement by
the K/Ar method on separated amphibole, feldspar, and
mica, the Urmia intrusive suite occurred in the Late Cre-
taceous time in two stages (Fig. 1). The emplacement of
diorite-gabbro and granites occurred simultaneously at
100–92 Ma, and the alkaline rock types in the time of
86–80 Ma (Ghalamghash et al. 2009b). It seems that the
diorite-gabbro magmatic event (100–92 Ma) occurred
during subduction, whereas the second one (at ca. 80 Ma)
probably arose through the collision between the Arabian
margin and the Sanandaj–Sirjan zone (Ghalamghash et al.
2009b).
The studied gabbroic rocks are in the two regions of
Moskin, with longitudes of 44°5501600 to 45°000and lati-
tudes of 37°450to 37°4601200 and the Parvaneh region, with
Fig. 3 a,bField photos from the massive gabbros outcrop. cIntrusion of the light colored (plagioclase-rich) dioritic dike into the Parvaneh
gabbro. dSome dark colored melano-gabbro enclaves were shown at the gabbros with more hornblende
Acta Geochim
123
longitudes of 45°0500100 to 45°1201000 and latitudes of
37°0403000 to 37°0601500 located at the 1:100,000 geological
map of Serow (Sabziei et al. 2004) and Oshnaveih
(Naghizadeh 2004). Moskin and Parvaneh gabbros are
located in the sedimentary-structural sub-divisions (Gha-
semi and Talbot 2006) in the Sanandaj–Sirjan zone of Iran
(Fig. 2).
Based on the mentioned maps and field studies, the
outcrops in these areas are frequently hornblende-gabbro to
gabbro-norite (Fig. 3a, b), and metamorphic and sedi-
mentary rocks are also observed around the studied
intrusions.
A dioritic dike in the Parvaneh pluton has cut gabbroic
rocks (Fig. 3c), which is shown in the classification dia-
grams. The presence of dark-colored enclaves of fine-
grained melano-gabbro is considerable (Fig. 3d). The
studied gabbros are massive with coarse amphibole and
plagioclase, visible at a mesoscopic scale. Regarding the
color grade, these rocks were observed with various med-
ium to dark colors. In addition, the petrographic and
petrofabric studies of metamorphic rocks around the Par-
vaneh region have formed at the lower greenschist facies to
the lower amphibolite facies. These rocks are mainly
composed of hornblende schists, amphibolites, and grani-
toid mylonites. The investigation reveals that metamorphic
rocks are formed at high temperature-low pressure regimes,
and it seems that the low-pressure metamorphism of the
region is related to arc tectonic settings (Borzoie and
Pourmafi 2001).
3 Methods
The present study focused on two separate areas in the
north and south of Urmia using field and laboratory eval-
uations. Eight fresh samples were selected from each
region and were sent to the ALS Minerals Laboratory in
Ireland for chemical analysis (Tables 1,2). The major
oxide elements were analyzed by the X-ray fluorescence
(XRF) method. Other trace elements were also evaluated
by the inductively coupled plasma mass spectrometry
(ICP-MS). For measurement of the petro-physical param-
eters, we use 2D outcrop images of the rocks, draw crystal
outline sketches for classification of images, then analyze
the result with image processing software, Digitizer for
measuring width, length, area, and roundness of crystals.
CSD Corrections (after Higgins 2000) was used to calcu-
late the CSD patterns in 3D, slope, and intercept values of
the curve. For other cases such as shape, SDP, and DA, we
directly measure the factors on the sample or photograph
with a ruler and percolator concerning scale and checked
the measurements by ASAP software.
4 Petrography
According to microscopic thin sections, petrography pla-
gioclase, and amphibole are abundant minerals and rare
pyroxene and olivine observed in the gabbroic rocks of the
Moskin and Parvaneh regions (Fig. 4). Moreover, minor
minerals included titanite, apatite, biotite, and opaque.
Chlorite, epidote, and clay minerals also considered alter-
ation product minerals in small amounts. Additionally, the
predominant textures observed in gabbroic rocks encom-
passed intergranular, poikilitic, ophytic, granular, and
corona. The olivine is scattered in small crystals in the
rock, usually is an-hedral, and has a corona texture due to
the reaction rim around olivine crystals (Fig. 4a).
The plagioclases were subhedral to anhedral and had
albitic twinning visible in thin sections (Fig. 4d, f, h). In
some cases, plagioclases had chemical zoning, indicating a
change in the magma composition during crystal growth or
overgrowth on primary cores. The amphiboles are sub- to
an-hedral in the studied samples, and most of them have
simple twinning.
5 Geochemistry
The silica content of Parvaneh and Moskin specimens is
45–52 wt% except for a diorite dike specimen in Parvaneh
rocks (Tables 1,2). In addition, Al
2
O
3
percentage changes
from 15 to 19 wt%. These rocks have high MgO and CaO
while small amounts of P
2
O
5
, TiO
2,
and K
2
O.
In the Zr/Ti diagram against Nb/Y, one of the samples
(dioritic dike) is in the andesite field, and the remaining
samples are basalt (Fig. 5a).
We use two magma series determination, and it is clear
that the samples of the studied areas have a calc-alkaline
affinity (Fig. 5b, c).
It revealed that as MgO increases, Na
2
O, K
2
O, P
2
O
5
,
and Al
2
O
3
decrease while TiO
2
, CaO, and Fe
2
O
3
display an
increase. An increase in MgO, P
2
O
5
decreases, indicates
that minerals such as apatite and possibly accessory min-
erals including monazite may have entered the crystal. The
ascending trend of CaO demonstrates that it is present in
minerals such as pyroxene, calcium plagioclase, and
amphibole, which are abundant in the studied rocks. The
emplacement of TiO
2
in the titanite and ilmenite led to its
positive trend and shows the nearly high depth of magma
generation. The Nb, Th, Zr, V, La, Ce, Ba, Sr, and Rb
against MgO, have a negative trend except for vanadium
due to the presence of vanadium in the pyroxene, amphi-
bole, and biotite that the studied gabbroic rocks are rich of
them. The studied gabbros are non-cumulate arc-related
types (Fig. 5d).
Acta Geochim
123
Table 1 Whole rock XRF (for
major oxide elements) and ICP-
MS analysis (for trace elements)
results of the Parvaneh region
gabbro, south of Urmia
Sample P.1b P.2a P.2b P.2c P.3b P.3d P.4a P.4b
SiO
2
52.5 48.1 49.3 62.6 48.4 49.4 49.6 49.9
Al
2
O
3
18.05 19.8 18.7 16.55 11.7 14.3 19.25 19.65
Fe
2
O
3
8.72 11.05 9.16 6.73 9.57 8.86 8.67 8.86
MnO 0.15 0.21 0.14 0.09 0.17 0.14 0.13 0.13
MgO 3.68 3.77 4.9 2.25 10.5 9.38 4.23 4.17
CaO 7.76 9.93 9.47 5.13 10.6 10.9 8.61 8.93
Na
2
O 3.61 3.7 3.45 3.29 2.01 2.36 3.64 3.66
K
2
O 1.17 0.51 0.88 1.74 0.57 0.7 1.17 1.13
TiO
2
1.07 1.25 1.72 0.83 0.99 1.33 1.25 1.17
P
2
O
5
0.32 0.46 0.37 0.14 0.1 0.13 0.31 0.31
SrO 0.05 0.06 0.05 0.02 0.03 0.03 0.05 0.05
BaO 0.02 0.01 0.02 0.03 0.01 0.01 0.02 0.02
Total 99.44 100.02 99.87 101.47 99.41 99.74 99.72 100.42
Ba 196 101.5 219 276 90.8 123 193 170.5
Rb 37.5 9.7 18.5 53.6 13.1 19.1 38.9 34.8
Sr 429 461 431 202 189.5 266 436 438
Y 36 47.2 43.1 33.6 23.7 26.2 35.8 35.1
Zr 178 190 214 227 79 98 112 82
Nb 9.2 6.6 31.4 9.5 11.6 11.8 7.9 7.4
Th 2.2 1.23 2.72 5.38 1.63 2.16 0.92 0.94
Pb 66 32 8272
Ga 20.5 23.2 21.6 19.3 13.4 15.1 22 22.1
Cu 37 43 57 22 81 74 62 56
Ni 18 1 36 10 151 128 31 30
V 191 182 246 128 235 321 225 215
Hf 4.4 4.5 5.5 5.7 2.5 2.7 2.9 2.3
Cs 1.7 0.71 0.44 1.87 0.38 0.55 1.71 1.52
Ta 0.4 \0.1 1.1 0.4 0.3 0.3 0.1 0.1
Co 24 21 26 14 63 43 25 23
U 0.64 0.65 0.79 1.18 0.38 0.65 0.36 0.73
W 11 12 2212
Cr 10 10 30 10 210 170 20 20
La 17.9 21.5 25.3 19.5 11.1 13.2 16.2 15
Ce 37.8 45.8 54.3 37.6 23 24.5 36.1 33.7
Pr 4.77 5.64 6.56 4.35 2.85 2.89 4.52 4.22
Nd 24.3 28.3 33 21 13.7 14.4 22.7 21.9
Sm 5.39 7.21 7.16 4.85 3.19 3.94 5.58 4.91
Eu 1.52 2.1 1.92 1.43 1.18 1.16 1.72 1.68
Gd 5.98 8.07 7.49 5.5 3.7 4.22 5.98 5.6
Tb 1.03 1.35 1.28 0.97 0.65 0.69 0.98 0.93
Dy 6.32 8.3 7.98 5.79 4.23 4.53 6.05 5.82
Ho 1.27 1.7 1.44 1.24 0.86 0.86 1.24 1.17
Er 3.89 5.21 4.64 3.79 2.81 2.68 3.95 3.67
Tm 0.61 0.88 0.71 0.55 0.35 0.36 0.54 0.56
Yb 3.89 4.53 4.36 3.52 2.46 2.38 3.41 3.28
Lu 0.6 0.81 0.67 0.59 0.37 0.36 0.58 0.51
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123
Table 2 Whole rock XRF (for
major oxide elements) and ICP-
MS analysis (for trace elements)
results of the Moskin region,
northwest of Urmia
Sample M9 M7 M2 M4 M7 M10 M12 M15
SiO
2
51.93 47.44 49.69 50.8 45.87 47.49 45.2 45.99
TiO
2
1.21 0.9 0.54 0.78 3.95 2.6 1.25 0.61
Al
2
O
3
15.14 14.9 15.48 14.73 13 13.95 14.15 15.64
Fe
2
O
3
10 10.72 9.46 8.95 15.98 13.87 13.43 9.18
MnO 0.15 0.19 0.17 0.16 0.2 0.28 0.2 0.16
MgO 6.45 9.18 7.45 8.52 5.64 6.77 8.19 12.02
CaO 8.27 11.03 10.89 11.15 9.79 9.56 10.91 12.17
Na
2
O 3.41 2.68 3.34 2.72 3.73 3.49 3.16 1.83
K
2
O 1.2 0.78 0.69 0.58 0.54 0.45 0.45 0.1
P
2
O
5
0.31 0.11 0.15 0.18 0.19 0.3 0.26 0.12
Total 99.81 99.79 99.78 99.89 99.79 99.82 99.63 99.74
Ba 1491 187 456 324 243 305 172 51
Rb 23.37 16.54 12.19 14.63 10.04 8.28 9.67 3.13
Sr 252 171 329 190 421 336 222 120
Y 25.4 16.65 26.83 21.08 11.59 18.86 19.32 10.95
Zr 15.36 12.39 62.57 11.88 17.23 12.83 17.85 28.45
Nb 7.93 1.26 5.85 2.27 8.32 14.18 1.8 0.74
Th 1.02 \0.01 0.28 0.84 2.14 0.37 0.16 \0.01
Pb 1.49 3.02 \0.01 \0.01 0.24 \0.01 \0.01 \0.01
Ga 19.72 15.49 17.23 15.12 18.57 16.13 15.19 12.23
Cu 9.79 26.85 9.84 19.71 35.28 36.94 12.46 53.79
Ni 21 110 91 49 12 69 76 166
V 2.7 2.2 1.7 2.2 4 1.7 2.2 1.7
Hf 0.65 0.59 2.01 0.62 0.91 0.73 0.83 0.74
Cs 0.3 0.25 0.27 0.21 0.1 \0.01 0.2 0.12
Ta 0.32 \0.001 0.3 \0.01 0.56 1.26 \0.01 \0.01
Co 36.08 44.86 31.37 37.53 41.11 38.34 39.16 49.94
U 0.16 \0.01 \0.01 0.22 0.3 \0.01 \0.01 \0.01
W 1 0.67 1.33 0.4 0.91 1.8 2.64 0.88
Cr 89 303 231 426 49 183 160 529
La 24.13 1.4 8.28 5.86 8.74 5.63 3.45 1.31
Ce 61.71 4.07 18.31 12.76 16.16 12.97 8.09 3.1
Pr 7.95 1.02 3.52 2.42 2.64 2.57 1.7 0.72
Nd 35.46 7.32 21.93 15.08 15.09 16.36 11.12 4.75
Sm 7.58 2.04 4.82 3.31 2.77 3.55 2.64 1.24
Eu 2.34 0.76 1.98 1.03 1.25 1.81 1.24 0.51
Gd 6.56 2.41 4.93 3.52 2.8 3.82 3.12 1.56
Tb 1.16 0.57 1.06 0.78 0.53 0.78 0.7 0.36
Dy 6.08 3.62 6.32 4.68 2.72 4.3 4.04 2.2
Ho 1.29 0.83 1.35 1.05 0.56 0.93 0.91 0.5
Er 3.13 2.1 3.29 2.63 1.32 2.31 2.34 1.29
Tm 0.68 0.47 0.71 0.59 0.26 0.49 0.51 0.29
Yb 3.39 2.38 3.44 2.92 1.26 2.43 2.57 1.46
Lu 0.39 0.24 0.34 0.29 0.13 0.25 0.25 0.14
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123
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123
6 Discussion
We illustrate the rare earth elements pattern of 16 samples
normalized to chondrite (Fig. 5e). As shown, the fractional
pattern (enrichment from LREE to HREE) is much more
noticeable in the case of Parvaneh gabbro, while the
samples of the Moskin region represent a flat and occa-
sionally even convex pattern in the middle part. In addition
to partial melting, LREE enrichment can also result from
the influence of crustal matters.
Generally, rare earth elements show a nearly flat trend
from La to Lu for Moskin gabbro rocks. Moreover, the
values of La
n
/Sm
n
,La
n
/Yb
n
, and Sm
n
/Yb
n
ratios demon-
strate that LREEs have a higher amount compared to
MREE and HREE. Additionally, observing the positive
anomaly in the Eu (Fig. 5e) indicates the dominant phase
of plagioclase in the rocks. Similarly, positive anomalies
for MREEs in Moskin gabbro can be attributed to the
presence of the hornblende because this mineral is a good
preservative for MREEs (Rollinson 1995). In addition,
small amounts of HREEs in Moskin gabbro can be due to
the presence of the garnet in the residual source and non-
participation of this mineral in the molten production
process.
The spidergram for gabbros in both regions shows the
high values of HFSEs and LILEs in these rocks (Fig. 5f),
which are the characteristics of arc-related rocks (Orsoev
et al. 2015), respectively. The concentration of LILEs
elements is a function of the fluid phase or crustal con-
tamination (Borisova et al. 2001; Harris et al. 1986;
Kamber et al. 2002). If the concentration of HFSEs is
controlled by the composition of the source rock and
crystalline/molten processes that occur during rock
bFig. 4 Microphotographs of the studied gabbroic rocks. aand
bPyroxene and olivine crystals in the Moskin region, PPL and XPL.
cindicates high relief titanite which found in the Moskin rocks.
dlarge plagioclase crystal with albitic twinning and the presence of
pyroxene, amphibole and opaque minerals from Parvaneh rocks. eand
fcoarse plagioclase, PPL and XPL. glarge amphibole crystal with
Manbach twinning. hplagioclase crystal with intergranular texture
Fig. 5 a Classification diagram of the studied igneous rocks based on the trace elements (Winchester and Floyd 1977), most of the samples are in
the gabbro (basalt) field. bDetermination of magmatic series for the studied rocks, SiO
2
versus Na
2
O?K
2
O series (Ewart 1982). cAl
2
O
3
versus
FeO
T
(Abdel-Rahman 1994). All the samples fit at the calc-alkaline series field. dAFM discrimination ternary diagram for the Urmia gabbroic
rocks. Fields of cumulate (light gray) and non-cumulate rocks (dark grey) are from Beard (1986). All of the samples lot in the non-cumulate field.
eThe chondorit normalized REE pattern (McDonough et al. 1992) of the Moskin and Parvaneh gabbros. fThe spidergram normalized to the
primitive mantle (McDonough et al. 1992) for the gabbros
Acta Geochim
123
Acta Geochim
123
formation (Stolper and Newman 1994). The HFSEs are
relatively immobile in hydrous fluids (Tatsumi 1989;
Keppler 1996). Therefore, the enrichment of these ele-
ments in mantle wedges is a sign of the mixing of the
molten oceanic crust with the original magma (Plank and
Langmuir 1992). These elements are low in the Moskin
gabbro but have a slightly high content in the Parvaneh
gabbro.
Furthermore, the presence of negative Ta and Nb ele-
ments (HFSE) in all specimens probably indicates the
involvement of subduction factors in the enrichment of
mantle melt, which is related to the active margin of the
continent (Aldanmaz et al. 2000).
6.1 Tecteno-magmatic setting
We use several discrimination diagrams to determine the
position of the Moskin and Parvaneh (Fig. 6a–e). All of
them show the volcanic arc relative to the subduction
position for the gabbros.
The trace element ratios such as Sm/Yb against La/Sm,
as well as the Sm/Yb to Sm (Aldanmaz et al. 2000; Zhao
and Zhou 2007) display that data related to the Parvaneh
and Moskin gabbros are generated from the spinel lher-
zolite to spinel-garnet lherzolite with a melting rate of
about 10%–30% (Fig. 6f). The La/Sm ratio also considered
a suitable criterion for measuring the enrichment of the
source area of LREE (Koglin et al. 2007). This factor is
more significant than the Moskin for the Parvaneh gabbro.
As well as Nb versus Zr (as low-mobility incompatible
elements) is used to distinguish between the enrichment
and depletion of the origin (Jung et al. 2006), and the rocks
are confirmed from an enriched type (Fig. 6g).
Some trace element ratios like K/Ce, Rb/La, and Nb/Th
show that the lower continental crust has been well
involved in forming the studied gabbroic magmas. The
contamination of the arc magma with crustal components
has also occurred in areas such as the gabbroic intrusions of
Astara (Salavati and Ashori 2016).
6.2 Geodynamic of the mafic intrusions
The subduction of Neo-Tethys below the Central Iran
microplate (Fig. 7) is from the southwest to the northeast at
an angle of about 55°(McClay et al. 2004; Heidari et al.
2016). It is because of compressional phases that the
Arabian Peninsula exerts on Iran’s microplate after the
closure of Neo-Tethys (Berberian and King 1981; Mohajjel
et al. 2003a,b; Elmas and Yilmaz 2003; McClay et al.
2004; Molinaro et al. 2005). The effect of such a function is
to develop faults along with the right-hand slip. Such faults
are abundant in northwestern Iran, especially in West
Azerbaijan (around Urmia). Intrusive mafic rocks formed
at the depth with the nature associated with the arc have
found the possibility of surface emergence in such an
environment and when the stress caused by the collision is
over.
6.3 Petro-physical studies
6.3.1 Crystal size distribution (CSD) patterns
Analysis and interpretation of CSD and other physical
parameters of the crystal population enable us to distin-
guish between different origins and different growth his-
tories. This ultimately enhances our understanding of
magmatic processes. Composite-based physical measure-
ments can show different stages of development the reveal
the time scale for diffusion under crystallization conditions.
One of the most common aspects of quantitative textural
studies is the distribution of grain and crystal dimensions.
bFig. 6 a,bThe tectonic setting discrimination diagrams after
Vermeesch (2006), all the samples are located in the IAB region,
cafter Muller et al. (1992), dTh
N
against Nb
N
diagram (Saccaini,
2015) which are normalized to the primitive mantle (McDonough
et al. 1992) and eis the discrimination diagram of the Th/Yb and Nb/
Yb (Pearce 2008). All the studied gabbroic samples are within the
continental arc position. fDiagram for source determination of the
gabbros parental magma (Regelous et al. 2003) which show the spinel
peridotitic source materials for them. gDiagram of showing the
quality of melting source of Nb versus Zr (Jung et al. 2006). The
gabbros show enriched source
Fig. 7 The schematic model of
the Neo-Tethys oceanic crust
subduction under the Central
Iran micro-continent after Sudi
Ajirlu et al. (2016). The
continental arc formation along
the Sanandaj–Sirjan structural
zone of Iran as a result of the
second episode of the
subduction is cleared
Acta Geochim
123
Acta Geochim
123
However the first study on the dimensions of igneous rock
crystals was published 120 years ago, but until 30 years
ago, such studies did not receive much attention until the
main article by Cashman and Marsh (1988) addressed this
issue.
For igneous rocks, kinetic issues of nucleation and early
growth must also consider. The driving force of kinetic
processes can be considered under-cooling or super-satu-
ration of the system (Cashman 2020). The resulting crystal
population is altered by mechanical factors such as sorting,
coarsening, Ostwald-ripening, or subsequent mixing (Hig-
gins 2006).
The primary functions that control growth textures are
the nucleation rate, which is a function of time, and the
growth rate, is depending on the time and crystal size. Then
the crystal size distribution over time can be calculated
based on both functions. We sometimes can assume that
the growth rate is self-governing and not controlled by size.
For example, during equilibrium, the growth rate depends
on the size. For a phase in such a system:
n0
VðLÞ¼n0
Vð0ÞeL=Gs
This is equivalent to the length of all crystals with a
linear CSD pattern of size zero to infinity. This distribution
is linear on the population density diagram versus size.
Items such as Ostwald-ripening (Moazzen and Modjarrad
2005), mixing of two magmas (Modjarrad 2015), filter
pressing, and crystal aggregations lead to changes and
bending in the straight-line CSD pattern (Higgins 2006).
In the present study, the ratio between nucleation to the
growth of amphibole and plagioclase crystals in gabbro is
calculated from the intercept value of the CSD pattern
(Figs. 8,9,10,11). This ratio (J/G) is 5–6 times, just for
Parvaneh plagioclase is ten times which on these rocks the
numerous nuclei were formed by a vast degree of over-
stepping the rocks. However, the CSD patterns show a
slope of (-1/Gt)-3to-4 for amphiboles and plagio-
clase except for Parvaneh plagioclase which shows a slope
of -16. Considering the standard proposed growth rate
for crystals in silicate malts, 10
-11
cm/s, a time of at least
bFig. 8 Total picture is formed from sample photograph, illustrated
sketches from amphibole crystals, histogram graphs and CSD pattern,
extracted for Moskin gabbro
Fig. 9 Total picture is formed from sample photograph, illustrated sketches from plagioclase crystals, histogram graphs and CSD pattern,
extracted for Moskin gabbro
Acta Geochim
123
700–800 years was needed generally for the studied crys-
tals were growing (for the slope of pattern equal to -1/
Gt= -4). Also, CSD patterns show that magmatic mix-
ing did not occur in the formation of the gabbros, but
annealing has led to the convexity of the immediate part of
patterns, at the part of the small grains. In many cases
nucleation occurs at a surface, interface, impurity, or other
heterogeneities in the system. When the rate of free energy
change becomes negative, then a crystal can grow. Crystal
growth rates commonly increase with increased under-
cooling. Our study on the gabbro shows that at the initial
stages, the undercooling was high, and the nucleation rate
was more than the growth rates, but the continued under-
cooling decrease caused heterogeneous nucleation and
large crystals formation.
6.3.2 Crystal shapes
To extract the general shape parameters of amphibole and
plagioclase crystals in these rocks, the measured axial
ratios of the enclosed parallelograms of the crystals have
used. Dimensions of the smallest parallelogram enclosed
inside a crystal: three parameters are small diameter (S),
intermediate (I), and long (L), and the axial ratio obtained
from it. For mathematical reasons, it is better to divide the
overall axial ratio into, S/I and I/L. After extracting the
data from the classified images of two-dimensional sec-
tions, two critical parameters, width (w), and length (l)
measured. The simplest way to estimate S:I:L is to use the
parallelogram model. For rectangular grains without ori-
entation (massive), the value of w/l equals S/I.In most
cases, the w/l ratio is used to calculate the CSD pattern in
Fig. 10 Total picture is formed from sample photograph, illustrated sketches from amphibole crystals, histogram graphs and CSD pattern,
extracted for Parvaneh gabbro
Acta Geochim
123
terms of ease. Measuring I/L on the crystal will be more
accurate than all of the above methods. We measured the
w/l ratio of crystals on the 2D sections, which is equal to S/
I, and calculated the I/L from Garrido et al. (2001)
equation:
I/L ¼mean w=lþ1:09 mode w=lðÞ
21:49 mode w=lðÞþ0:056
=0:126
Then we use the Higgins (1996) graph for crystal shape
determination. The shape of amphibole crystals in the
Moskin and Parvaneh gabbros (Fig. 12) are bladed (with S/
I= 0.4–0.5 and I/L = 0.1–0.3) and bladed to tabular for
plagioclase (S/I = 0.4 and I/L = 0.6–0.7).
6.3.3 Spatial distribution patterns (SDPs)
In many rocks, the grains are not always randomly dis-
tributed in space but may be distributed in clusters, layers,
or chains. Such spatial patterns can be defined by the
presence of grains, their size, shape, and orientation, and
their associated minerals.
Analysis of the SDP of rock crystals is a valuable
technique for evaluating the processes involved in rock
formation (Jerram et al. 1996). In this paper, the SDP of
crystals is quantified in two-dimensional sections. The
Fig. 11 Total picture is formed from sample photograph, illustrated sketches from plagioclase crystals, histogram graphs and CSD pattern,
extracted for Parvaneh gabbro
Fig. 12 The crystal shape of amphibole and plagioclase in the
gabbros is measured directly and plot on the mentioned graph of
Higgins (1996)
Acta Geochim
123
distance between the centers of the grains and the nearest
center of the neighbor grain was measured and used to
determine the SDP pattern. This number is then normalized
to a random distribution of points with the same population
density to obtain big Rvalues. This value is then applied to
the sum of the other underlying minerals to obtain SDP. By
considering the random distribution of spherical grains and
then a straight line called RSDL is created in this diagram.
If the grains are plotted above this line, they will be ordered
and if they are plotted below the line, it means the cluster
distribution of crystals in the rock.
The maximum R-value is 2.148 for a full ordered
square/hexagonal cube packing. The minimum of this
value depends on the grain ratio in the sample volume and
is about 1.2 for 30% of the crystal. Changes in grain size
matching may change the value of Rby 0.25. The
mechanical compaction of the integrated framework of the
grains will lead to stronger packing and increase the value
of R. If this compression continues, it may decrease Rand
form a cluster SDP in the direction perpendicular to the
main stress. The overgrowth changes the location of the
center of grains and increases Rwhile decreasing porosity
at the X-axis of the diagram.
As long as the temperature and fluid composition are
constant, the position of the new core will be random. If a
region of liquid or melt is depleted of the constituents of
the crystal (for example, due to the crystallization of an
adjacent crystal), it may cause the stopping of nucleation.
This leads to the production of large-scale distribution
patterns due to geochemical self-organization.
Nucleation can also occur in current phases, which is
called heterogeneous nucleation. The position of the new
crystal nucleus will depend on the location of the host
crystal. One of the easiest ways to describe the SDP is to
use the average distance of the nearest neighbor, which can
be calculated from the centers of the grains.
The position of the grain centers examined when other
textural parameters are known. The easiest way to deter-
mine the list of Npositions of the crystalline center is to
find the distance to the nearest neighbor r(Jerram et al.
2003). The average distance to the nearest neighbor is:
rA¼1=NðÞ
Xr
The mean distance of the nearest neighboring for points
with random scattering at the same population density
(number of crystals per unit area), NA, is:
rE¼1=2ffiffiffiffiffiffi
NA
p
The ratio of these two parameters is equal to the big R
equal to the position distribution of the grains:
R¼rA=rE
In principle, for distributing random distribution, the big
Ris equal to one. This number will be less than one for
cluster and more than one for ordered distribution. We
measured the big R-values for amphiboles and plagioclase
in the Moskin and Parvaneh gabbros (Fig. 13). All crystals
of the samples plot on the clustered field. Crystal clusters
are made simply in magmas. If the crystals nucleate
heterogeneously any change in the position of crystals in
the magma system created different shapes of crystal
(Schwindinger 1999), and the crystals re-mobilization form
clusters. The formation of mineral clusters in the magmatic
productions suggests that the clumps form before the
emplacement of the intrusion (Jerram et al. 2003). Another
one is that they formed during the rise and the
emplacement.
Suspicions in the mush zones at the top of a magma
body (Philpotts and Dickson 2002) might produce crystal
aggregations, which re-arrange individual crystal clusters
to the bottom of the magma chamber (Jerram et al. 2003).
6.3.4 Dihedral angles (DAs)
If a substance is completely solid, four crystals will be in
contact in the corners of each crystal. Also, three crystals
will have a common border along the edges of the crystal.
On a plane perpendicular to the edges, the angle between
the crystal boundaries is called the Dihedral Angle (DA).
DA shows the surface energy difference between phases, at
equilibrium. If the three isotropic phases collide at the
edges, then:
Fig. 13 The graph of big R against porosity (sum of other phases) for
amphibole and plagioclase grains in the gabbros. The SDP of the
crystals in rocks is clustered. The diagram after Jerram et al. (2003)
Acta Geochim
123
c1
sin a1¼c2
sin a2¼c3
sin a3
The face of fractions is the surface energy between the
phases along the grain boundary and the denominator angle
of the same DA between the crystals on the opposing side.
To measure this angle, large grains can be cut in specific
directions, and the inner parts of the grain can be examined.
In this paper, the DAs of the amphiboles in the slab
measured directly by the protractor (Fig. 14). Then it was
implemented in the relevant diagram (Holness 2005), and it
found that the equilibrium in the solid-state was dominant
for the crystallization of these rocks because the angles
between the crystal surfaces were often about 120 degrees.
7 Conclusion
In general, the mafic rocks of the Parvaneh and Moskin
regions near Urmia were studied. The most prevalent
minerals in these rocks are reported as plagioclase,
amphibole, (?/-) olivine, and pyroxene (in Moskin rocks).
The nature of these rocks is calc-alkaline gabbro. In
addition, the pattern of rare earth elements for the studied
rocks showed enrichment in LREEs (a flat trend for
Moskin rocks) compared to HREEs. Furthermore, the
geochemical evidence of trace and rare elements for
Moskin and Parvaneh gabbro rocks indicated the origin of
the spinel-garnet lherzolitic mantle source for them. It
reported that rocks belong to a continental arc environment
given, and the magma has powerfully recorded the lower
crust characters as it passes through it. The gabbroic rocks
of the study area have been created by the subduction of the
Neo-Tethys oceanic crust below the Sanandaj–Sirjan area
(Central Iran microplate) at the active continental margin
area.
The CSD pattern of the gabbro’s amphibole and pla-
gioclase also are calculated, and it is clear that there is no
magma mixing occurred at the gabbros magma generation.
Also, the nucleation was faster than the growth rate (J/G
was 5–6 times and exceptionally ten times) at the Moskin
and Parvaneh gabbros which determined the initial over-
stepping degree (undercooling rate) was high and a sig-
nificant number of nuclei per unit volume for these rocks.
The shape of amphibole crystals in gabbros are bladed and
bladed to tabular for plagioclase based on the aspect ratio
(S:I:L) direct measurements on 2D sections. The Spatial
Fig. 14 Dihedral angle
measurement for Moskin
gabbro. The diagram after
Holness (2005)
Acta Geochim
123
Distribution Patterns (SDPs) are calculated from a big R-
value based on the nearest neighbor’s theory for amphibole
and plagioclase crystals, recognized clustered SDP for all
of them. Clusters would improve by reactivation of crystal
bodies that solidified at the chamber’s walls and roof or by
crashing during resolving.
Also, the dihedral angle (DA) measurements on the
outcrop images (between amphibole and plagioclase
grains) display that solid-state equilibrium is dominant at
the Moskin gabbro.
Acknowledgements The author thanks the editors and reviewers of
the Journal. The author received financial support from the Natural
Science Foundation of Urmia University.
Declarations
Conflict of interest The author declares that there is no conflict of
interest.
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