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GeochemistryandpetrologyofbasalticlavasfromNE-Qorveh,Kurdistanprovince,WesternIran
RESEARCH·JANUARY2016
DOI:10.13140/RG.2.1.1899.6248
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Introduction
In the Kurdistan (Qorveh-Takab-Bijar) region of western Iran, subduction of the Neotethyan oceanic crust and sub-sequent collision between the Arabian and Iranian plates was followed by widespread Neogene and Quaternary volcanic activity. This volcanism is manifested widely in a zone about 120 km in length, also known as Hamedan-Tabriz volcanic zone (Azizi & MoinevAziri 2009). Geo-logically, this region is part of the Turkish-Iranian plateau and located on the northern axis of the Urumieh-Dokhtar magmatic arc (Fig. 1a).
Western Iran can be divided into three major tectonic zones: the Zagros fold and thrust belt, the plutonic-meta-morphic Sanandaj-Sirjan zone and the Urumieh-Dokhtar magmatic arc (e.g. Stöcklin & nAbAvi 1973, AlAvi 1994, GhASeMi & tAlbot 2005, torkiAn et al. 2008, torkiAn 2011, torkiAn & FurMAn 2015). As a result of Mesozoic and Early-Cenozoic subduction of the Neotethys below the stable Central Iran block, a convergent plate system developed associated with back arc extension and forma-tion of the Urumieh-Dokhtar arc (Fig. 1a). The arc rocks consist of various lithological units including mafic to
felsic intrusive rocks, basaltic to trachytic lava flows and pyroclastic deposits (berberiAn & berberiAn 1981). Sub-sequent collision between the Arabian and Iranian plates resulted in folding and thrusting of previously deposited Carboniferous to Miocene sedimentary sequences and the formation of the Alborz and Zagros mountain ranges (e.g. Şengör & nAtAlin 1996, GhASeMi & tAlbot 2005).
The NE-Qorveh basaltic rocks, in the following re-ferred to as NEQB, represent a Quaternary volcanic zone which formed in the aftermath of this collision (Allen et al. 2013). The NEQB defines one of the active or recently active centres of volcanism within the Turkish-Iranian plateau (Şengör & kidd 1979, kheirkhAh & MirnejAd 2014) (Fig. 1b). According to richArdS et al. (2006) (cited in Azizi et al. 2014) the Qorveh volcanoes were active in the Quaternary.
The NEQB rocks are located between 47° 46′ – 47° 59′ E and 35° 18′ – 35° 30′ N, c. 80 km NW of Hame-dan (Fig. 1c). This paper focuses on basaltic rocks from this region. Hitherto, Quaternary volcanic rocks in NW Iran were investigated by boccAletti et al. (1976), MoinevAziri & AMinSobhAni (1988), MAlekootyAn et al. (2007), kheirkhAh et al. (2009), rAzAvi & SAyyAreh (2010), dA
Geochemistry and petrology of basaltic lavas from NE-Qorveh, Kurdistan province, Western Iran
A. Torkian, N. Salehi, W. Siebel
With 12 figures and 4 tables
Abstract: Quaternary volcanic rocks at NE-Qorveh, Kurdistan province, western Iran, formed in a post-collisional continental arc setting. This paper focuses on the basaltic rocks of this area. These rocks display microlitic to porphyritic textures with clinopyrox-ene and olivine as phenocrysts and plagioclase, pyroxene, amphibole and biotite as microcrystals. Pyroxenes are either normally zoned or show growth zones with sieve textures. The basalts are alkaline in composition with variable amounts (5 –14 %) of norma-tive nepheline. They display positive correlations between MgO and CaO, FeO, TiO2, Ni and Cr and negative correlation between MgO and SiO2 and these trends can be accounted for by various degrees of fractional crystallization. Geochemical characteristics such as enrichments in large ion lithophile elements (Ba, Sr) and light rare earth elements (e.g., La, Ce), negative Nb and Ta anoma-lies, and positive Pb, Ba, Th anomalies suggest that the lavas were derived from an undepleted or a re-enriched mantle source. High ratios of (Th/Yb)N, (Tb/Yb)N and (Dy/Yb)N and Nb/La < 1 and low ratios of Zr/Nb and Y/Nb show that the melts were generated from a garnet peridotite source by low degree of partial melting.
Key words: alkali-basalt, intra-plate volcanism, Iran, NE-Qorveh, subduction
© 2016 E. Schweizerbart’sche Verlagsbuchhandlung, Stuttgart, Germany www.schweizerbart.deDOI: 10.1127/njma/2016/0296 0077-7757/2016/0296 $ 4.50
N. Jb. Miner. Abh. (J. Min. Geochem.) 193/1 (2016), 95–112 ArticleStuttgart, January 2016E
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97Geochemistry and petrology of basaltic lavas from NE-Qorveh, Kurdistan province
biri et al. (2011), torkiAn et al. (2013), kord (2013), Allen et al. (2013) and Azizi et al. (2014). boccAletti et al. (1976), MoinevAziri & AMinSobhAni (1988) and rAzAvi & SAyyAreh (2010) interpreted the basaltic rocks of the NE-Qorveh area to originate from an OIB-type mantle source in an intracontinental tectonic setting. boccAletti et al. (1976) suggested that the mafic and felsic volcanic rocks derived from two different magma sources. According to MAlecootyAn et al. (2007) the volcanic activity resulted from intraplate-extension but this extension was restrict-ed in space and time. These authors also suggested that crustal contamination played an important role in modify-ing the melt composition. Quaternary basaltic rocks from Qorveh-Bijar and Takab show a wide range in geochemi-cal and isotopic composition (Allen et al. 2013). These authors argue that the rocks stem from at least two dif-ferent mantle sources and that crustal contamination was not a significant process governing the chemical modifi-cation. Azizi et al. (2014) characterized the mafic rocks as high-Nb basalts (HNB) that originated from subducted material of Neo-Tethyan oceanic crust in a post-colli-sional orogenic stage. These authors emphasized the role of melt contribution from the oceanic slab during basalt formation. Therefore, the genesis of the volcanic rocks is still problematic. The principal aims of this paper are to employ petrographic, mineral and whole-rock geochemi-cal data of the NEQB in order to illuminate their compo-sitional features and this will allow us to shed further light on the petrogenesis of the basaltic rocks.
Field relationships of the volcanic rocks
The early products of volcanism in the NE-Qorveh area include pyroclastic rocks being composed of black or dark brown scoria, lapilli tuffs and bombs and lava flows which were formed during the Quaternary. These lavas mainly consist of basalts, basanites, trachytes, trachy-andesites, latite-andesites and phonolites. They overlie the Miocene to Pliocene argillaceous limestones (Figs. 1, 2a). In the study area basaltic rocks are more abundant than intermediate and acid volcanic rocks. In the field, basaltic rocks, however, is generally used for all mafic volcanic rocks such as basalts, basanites and andesites. The felsic volcanic rocks are found in close association with the basic rocks although there is a compositional gap between the two lava groups.
The basalts are black to dark grey, mesocratic to mel-anocratic with colour-index between 60 and 78 %. They are vesicular in hand specimen and, in the field, form low-profile scoria cones and lava plateaus. They fre-quently contain gneissic (Fig. 2b) and ultramafic xeno-liths interpreted as fragments of basement rocks entrained in the lava during eruption. In many cases the enclaves still preserve the textural features of basement gneisses. Ultramafic enclaves include some clinopyroxenites with glomeroporphyric texture.
Fig. 1. a) Main geological zones of Iran; b) Distribution of Quaternary volcanic belts in northwestern Iran based on Azizi & MoinevAziri (2009), SCV = Sanandaj Cretaceous volcanic belt, HTV = Hamedan-Tabriz volcanic belt, SBV = Songor-Baneh volcanic belt; c) Simplified geological map of the volcanic centers (modified from hoSSeini 1997).
Fig. 2. a) Contact between basaltic rocks and argillaceous limestone at Nader Shah; b) gneissic xenoliths and a fusiform bomb from cinder cone in Ghezelge Kand.
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98 A. Torkian et al.
Tabl
e 1. S
elec
ted
mic
ropr
obe
anal
yses
of m
agm
atic
clin
opyr
oxen
e an
d pe
rcen
tage
of w
olla
ston
ite (W
o –
CaS
iO3)
, ens
tatit
e (E
n –
MgS
iO3)
and
ferr
osili
te (F
s – F
eSiO
3) e
nd-m
embe
rs.
Num
ber o
f cat
ions
cal
cula
ted
on th
e ba
sis o
f 6 o
xyge
n at
oms.
Gha
le P
aria
n M
ount
ain
Ilan
luG
heze
lge
Kan
dG
hare
Bol
agh
36-M
68-M
79-M
67-D
73-D
74-D
75-D
65-N
48-N
37-U
78-U
60-I
S64
-IS
40-G
H42
-GH
43-G
H54
-GH
55-G
h49
-Gh
No.
of
anal
yses
197
86
108
68
715
78
96
154
46
8
SiO
249
.31
48.1
250
.69
47.5
948
.32
48.2
248
.98
50.2
451
.60
50.7
750
.60
51.6
050
.08
50.2
551
.41
50.2
448
.61
50.1
646
.00
TiO
21.
612.
071.
241.
942.
042.
161.
881.
210.
831.
421.
550.
911.
321.
390.
710.
991.
611.
553.
09A
l 2O3
5.07
5.21
3.41
5.60
5.71
5.82
5.09
4.15
4.21
3.68
3.80
2.65
3.62
4.80
2.95
3.90
5. 4
84.
887.
60Fe
O5.
944.
605.
806.
056.
886.
566.
275.
654.
465.
566.
234.
735.
055.
916.
677.
706.
776.
117.
91C
r 2O
30.
230.
190.
150.
410.
080.
210.
250.
290.
710.
290.
090.
430.
630.
100.
050.
000.
010.
040.
00M
nO0.
090.
090.
100.
080.
100.
100.
090.
090.
080.
090.
100.
110.
120.
100.
150.
200.
120.
100.
11M
gO14
.61
14.5
715
.85
14.8
714
.19
14.1
514
.63
15.4
116
.38
15.6
515
.22
16.7
016
.30
15.1
615
.17
14.4
814
.79
15.1
012
.47
CaO
22.7
022
.76
22.4
922
.38
22.5
622
.70
23.2
523
.13
22.3
223
.21
23.0
822
.90
22.9
221
.73
21.7
519
.49
22.2
022
.11
22.4
5N
a 2O
0.45
0.45
0.39
0.48
0.49
0.51
0.44
0.41
0.61
0.39
0.42
0.35
0.30
0.74
0.79
1.14
0.65
0.72
0.58
K2O
0.01
0.01
0.01
0.01
0.01
0.05
0.01
0.01
0.01
0.00
0.01
0.00
0.01
0.00
0.01
0.81
0.04
0.01
0.02
Tot
al99
.84
99.5
910
0.12
99.3
910
0.45
100.
4710
0.83
100.
8810
1.13
100.
9810
1.90
100.
3810
0.34
100.
1999
.66
99.3
010
0.26
100.
7610
0.49
Si1.
811.
871.
851.
761.
770.
061.
781.
821.
861.
841.
841.
821.
851.
841.
891.
851.
781.
821.
70T
i0.
040.
050.
030.
050.
050.
050.
050.
040.
020.
030.
040.
030.
020.
040.
020.
020.
040.
040.
08A
lIV0.
140.
160.
100.
160.
160.
170.
160.
120.
110.
110.
110.
100.
190.
120.
080.
110.
170.
130.
21A
lVI
0.07
0.06
0.03
0.05
0.07
0.08
0.05
0.04
0.05
0.04
0.04
0.04
0.09
0.80
0.04
0.05
0.06
0.07
0.11
Fe3+
0.09
0.12
0.09
0.13
0.12
0.11
0.12
0.10
0.06
0.09
0.09
0.07
0.06
0.09
0.10
0.16
0.01
0.06
0.13
Fe2+
0.08
0.07
0.08
0.06
0.09
0.08
0.08
0.07
0.02
0.07
0.09
0.07
0.09
0.09
0.10
0.07
0.04
0.08
0.10
Mg
0.80
0.80
0.86
0.81
0.77
0.78
0.79
0.83
0.88
0.84
0.82
0.89
0.60
0.83
0.83
0.81
0.80
0.81
0.68
Ca
0.89
0.89
0.83
0.88
0.89
0.89
0.91
0.90
0.86
0.90
0.90
0.84
0.92
0.85
0.85
0.77
0.87
0.82
0.90
Na
0.03
0.03
0.02
0.03
0.03
0.03
0.03
0.02
0.04
0.02
0.02
0.04
0.03
0.04
0.05
0.08
0.04
0.05
0.04
Mg#
8185
8583
8179
7981
8387
8381
8685
8280
7780
81W
o%50
.15
50.3
948
.19
50.4
950
.74
50.1
251
.40
49.8
947
.56
49.5
549
.43
48.4
253
.11
48.2
047
.90
46.2
850
.53
48.9
853
.14
En%
44.9
845
.28
47.2
346
.63
44.1
644
.15
44.7
246
.17
48.5
346
.17
45.3
148
.81
40.4
246
.79
46.1
946
.93
46.8
246
.36
40.4
4Fs
%4.
974.
335.
582.
882.
884.
733.
813.
943.
914.
285.
262.
706.
435.
015.
914.
092.
564.
566.
42
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99Geochemistry and petrology of basaltic lavas from NE-Qorveh, Kurdistan province
Analytical methods
Whole-rock geochemical analyses of 21 representa-tive fresh basaltic samples were performed at the Geo-analytical Laboratory of Washington State University, USA. Gneissic and ultramafic xenoliths were carefully removed prior to grinding. Whole-rock powders were an-alysed by X-ray fluorescence spectrometry (XRF) and In-ductively Coupled Plasma-Mass Spectrometry (ICP-MS). Petrographic polished thin-sections were prepared at the Department of Geology, Bu-Ali Sina University, Iran. Pyroxene, olivine and plagioclase crystals were analysed in the Electron Microprobe Laboratory, School of Envi-ronment, Washington State University (USA). Chemical microanalysis of minerals was performed by wavelength-dispersive spectrometry using 15 kV acceleration volt-age, 10 –15 nA beam current and 2 μm spot size using a Cameca SX50 electron microprobe. A selection of end-member mineral compositions is reported in Tables 1– 3. Data processing was done using the program Minpet (ver-sion 2.02; richArd 1995).
Results
Petrography
Constituent minerals of the studied basaltic rocks are py-roxene, plagioclase, olivine, hornblende, biotite, Ti-Fe oxides and mesostasis material (Fig. 3). The grain size of the basaltic rocks is slightly variable, and textures are glomero-microlitic-porphyric. Pyroxene and olivine oc-cur as euhedral and subhedral phenocrysts in a glassy or microcrystalline matrix (Figs. 3a–c). Often, the ground-mass defines a weak flow texture. Pyroxenes occur as zoned crystals that sometimes show sieve texture, result-ing from corrosion and partial resorption of the crystal (Fig. 3e). Such disequilibrium textures can result from magma mixing, phenocryst recycling, increasing volatile content in the magma or from volatile loss during decom-pression (Shelley 1993). Plagioclase occurs as zoned laths. Euhedral to subhedral hornblende crystals display brown colour and are 0.2 –1.5 mm in length (Fig. 3f). Quartz xenocrysts show embayed texture (Fig. 3d). Such disequilibrium textures suggest that the quartz crystals were entrapped from basement rocks and were not com-pletely consumed by the basaltic melt.
Mineral chemistry
PyroxenePyroxenes from all basaltic rocks are classified as Ca-Mg-Fe clinopyroxene in the J-Q [Q = Ca+Mg+Fe2+, J = 2Na] Ta
ble
2. S
elec
ted
mic
ropr
obe
anal
yses
of o
livin
e an
d ca
lcul
ated
fors
terit
e (F
o –
Mg 2
SiO
4), f
ayal
ite (F
a –
Fe2S
iO4)
and
teph
roite
(Te
– M
n 2Si
O4)
com
pone
nts.
Num
ber o
f cat
ions
cal
cu-
late
d on
the
basi
s of 4
oxy
gen
atom
s.
Gha
le P
aria
n M
ount
ain
Ghe
zelg
eK
and
Nad
er S
hah
Gha
re B
olag
hG
hale
Par
ian
Mou
ntai
n
99-N
100-
M10
4-M
105-
M12
1-U
107-
GH
113-
GH
125-
N12
6-G
101-
G87
-D88
-D89
-D92
-M93
-U94
-U96
-D98
-MSi
O2
39.4
739
.85
40.0
239
.63
40.0
339
.91
40.3
039
.90
38.8
039
.26
40.0
436
.20
39.7
239
.70
40.0
439
.80
39.7
040
.15
FeO
14.2
614
.16
13.1
51.
8712
.44
12.5
112
.41
11.7
118
.14
14.8
013
.39
14.3
813
.29
12.8
613
.12
12.7
113
.56
12.6
3M
nO0.
230.
200.
170.
150.
160.
160.
160.
150.
310.
200.
170.
200.
180.
160.
170.
160.
190.
16M
gO44
.43
45.2
046
.07
45.2
446
.23
46.1
646
.46
47.1
841
.71
43.9
446
.00
38.2
845
.60
45.5
646
.49
46.4
545
.16
46.3
4C
aO0.
260.
200.
190.
140.
140.
190.
210.
100.
200.
170.
190.
310.
190.
170.
180.
170.
210.
18C
r 2O
30.
030.
020.
030.
010.
040.
040.
030.
020.
020.
010.
060.
040.
030.
030.
030.
030.
040.
04N
iO0.
200.
200.
210.
230.
320.
280.
280.
370.
120.
180.
230.
180.
240.
280.
220.
250.
190.
24T
e%0.
250.
210.
180.
160.
160.
170.
170.
340.
330.
220.
180.
240.
190.
170.
180.
170.
200.
17Fo
%84
.23
84.6
485
.82
85.0
387
.52
86.4
386
.58
87.3
879
.89
83.7
385
.59
82.2
085
.56
85.9
786
.01
86.3
585
.17
86.3
8Fa
%15
.41
14.8
813
.74
14.6
212
.91
13.1
412
.97
12.1
719
.49
15.8
213
.98
17.2
813
.99
13.6
113
.61
13.2
514
.35
13.2
1
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100 A. Torkian et al.
diagram (Fig. 4a) (Schweitzer et al. 1974) and as diopside (Wo46.28 – 53.14 En40.42 – 48.81Fs2.56 – 6.43) in the Ca-Mg-Fe tri-angular classification diagram (Fig. 4b). The mineral has low TiO2 (mostly 0.71– 3.09 wt.%), Cr2O3 (< 0.71 wt.%) and Al2O3 (2.65 –7.60 wt.%) contents and high to inter-mediate Mg numbers ranging from 77 to 87 (MoriMoto et al. 1988). The chemical composition of pyroxene is given in Table 1. Some clinopyroxene phenocrysts of the volcanic rocks display complex zoning (Fig. 5). Experi-mental studies (e.g. thoMpSon et al. 1978) have shown that the Al content in Ca-rich clinopyroxene increases with increasing crystallization pressure and temperature. The sodium content of all studied pyroxene samples is low (c. 0.5 wt.%). In the AlIV – AlVI (a.p.f.u.) diagram the pyroxenes plot in the sector between 2 and 4 and in the Na+AlIV – (AlIV +2Cr+Ti) (a.p.f.u.) diagram they plot above Fe3+= 0, i.e. in the range of clinopyroxene compo-sitions that formed at medium to low pressure and high oxygen fugacity (Figs. 6a, b).
Olivine
Olivine phenocrysts are generally magnesium rich. They often occur as rounded grains, and in some cases they are corroded (Fig. 7). Their compositions range from Fo87.52 – 79.89 to Fa19.49 – 12.17. Olivine crystals with the highest forsterite contents (Fo87.38 and Fa12.17) occur in the basalts of the Nader Shah extrusion (Table 2).
Plagioclase
Plagioclase from the NEQB occurs as microlites in the groundmass with acicular shape. They range in composi-tion from An18.21 to An61.03, from Ab35.40 to Ab77.34 and from Or0.36 to Or9.75. In basalts from Ghezelge Kand the anor-thite content of plagioclase ranges from An18.21 to An30.11 and in Ghale Parian Mountain area from An28.30 to An61.03 (Table 3).
Whole rock geochemistry
Major, trace and rare earth element (REE) abundanc-es of the NEQB are given in Table 4. In the diagram Zr/TiO2 × 0.0001–Nb/Y (Zr in ppm and TiO2 in wt.%; Floyd & wincheSter 1977) the volcanic rocks clas-sify as alkali basalts (Fig. 8). The CIPW normative mineralogy calculation yields 5 –14 % nepheline. Fur-thermore, the rocks are characterized by low abun-dances of SiO2 (44.52 – 51.63 wt.%), high contents of FeOtot (5.87 – 9.83 wt.%), MgO (6.30 –13.87 wt.%), CaO (9.15 –13.97 wt.%), Al2O3 (9.29 –14.45 wt.%) and TiO2 (1.36 – 2.31 wt.%) and Mg numbers [mol. Mg# = 100× Mg /(Mg+Fe2+)] range from Mg# = 63 to 79 (Table 4).
Fenner-type variation diagrams (Figs. 9a–h) display positive trends between some major elements (such as TiO2 and FeOtot; Figs. 9b, g) and Mg, whereas Si (Fig. 9a) has a negative correlation with Mg. Cr and Ni also de-crease with decreasing MgO (Table 4 and Figs. 9d, f).
Table 3. Selected microprobe analyses of plagioclase and percentage of anorthite (An – CaAl2Si2O8), albite (Ab – NaAlSi3O8) and ortho-clase (Or – KAlSi3O3) end-members. Number of cations calculated on the basis of 8 oxygens.
Ghezelche Kand Ghale Parian Mountain GhareBolagh
15-GH 16-GH 17-GH 18-GH 20-U 25-M 26-M 27-M 28-M 30-D 31-M 19-GSiO2 62.64 63.32 60.48 61.57 54.38 52.81 55.38 54.47 54.87 52.37 60.71 53.37TiO2 0.01 0.02 0.02 0.01 0.19 0.14 0.14 0.19 0.20 0.18 0.01 0.16Al2O3 24.68 23.75 25.98 23.99 29.65 29.79 28.77 28.85 28.36 30.76 25.24 31.50FeO 0.07 0.07 0.12 0.10 0.65 0.83 0.79 0.72 0.91 0.79 0.10 0.57MnO 0.05 0.03 0.01 0.01 0.04 0.06 0.06 0.02 0.02 0.05 0.05 0.03MgO 0.03 0.03 0.01 0.05 0.06 0.06 0.07 0.05 0.05 0.01 0.01 0.02CaO 4.44 3.47 5.77 4.23 9.29 11.09 11.16 9.26 9.73 11.97 5.45 11.28Na2O 8.36 8.14 7.05 6.74 5.01 4.22 4.30 4.92 4.92 3.82 7.24 4.26K2O 0.06 0.71 0.49 1.50 0.49 0.26 0.24 0.44 0.44 0.53 0.35 0.23An% 22.59 18.21 30.11 23.21 49.07 58.21 58.07 49.51 50.80 61.03 28.30 58.50Ab% 77.03 77.34 66.82 66.92 47.81 40.14 40.41 47.67 46.41 35.40 68.51 40.10Or% 0.36 4.43 3.05 9.75 3.01 1.62 1.55 2.82 2.74 3.22 3.01 1.41
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Fig. 3. Microphotographs of the NEQB (Cpx: Clinopyroxene; Pl: Plagioclase; Qtz: Quartz; Hbl: Hornblende): a) porphyritic texture; b) glomeroporphyric texture due to accumulated pyroxenes; c) plagioclase microlites, clinopyroxene microcrysts and phenocrysts; d) em-bayed quartz crystals; e) sieve texture in clinopyroxene; f) hornblende phenocrysts
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Fig. 4. a) Q-J diagram [Q = Ca+Mg+Fe2+; J = 2Na] (Schweitzer et al. 1974) showing magmatic clinopyroxene phenocrysts from the NEQB. All samples plot into the Quad field [Mg-Fe-Ca pyroxenes]; b) Plot of pyroxene composition into the Wo-En-Fs diagram (MoriMoto et al. 1988); the pyroxenes exhibit diopsidic composition.
Fig. 5. Back-scattered electron images of the NEQB rocks. Arrow indicates the direction of EMPA analysis from the complexly zoned clinopyroxene.
Fig. 6. a) Al IV -AlVI (a.p.f.u.) and b) Na+AlIV – (AlIV+2Cr+Ti) (a.p.f.u.) diagrams for magmatic pyroxenes from the NEQB indicating that they were generated at medium to low pressure conditions and high oxygen fugacity.
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Fenner-type element variation diagrams also show that there is little variation in K2O and other mobile elements such as Rb (Fig. 9h). TiO2, K2O (Fig. 9b, c) and Ba, Sr, Pb and Zr (Table 4) contents of Illanlu and Ghezelcheh-kand samples are different from the Ghale Parian Mountain and Ghareh-Bolagh samples.
Chondrite-normalized REE distribution patterns for the NEQB (Fig. 10a) are parallel to each other and charac-terized by steep and enriched light REE (LREE) and rela-tively flat heavy REE (HREE) profiles [(La/Sm)N = 5, (Gd/Yb)N = 5.5] and negligible Eu anomalies. From this it is evident that the rocks are derived from similar magma source(s) and from parting melting of a garnet peridotite source. The high La/Yb and Sr/Y (56 to 126) ratios can be explained by residual garnet in the mantle source. The absence of a negative Eu anomaly suggests that the rock composition was not strongly modified by fractional crys-tallization processes, an observation that has already been pointed out by Azizi et al. (2014) for other volcanic rocks of the Kurdistan volcanic province. For most mafic alka-line rocks of the NEQB, the pressure of melting was prob-ably above the stability range of feldspars (~10 kbar) and no Eu anomaly is expected to form during partial melting of a feldspar free mantle source and this seems to be con-firmed by our data.
In primitive mantle normalized multi-element varia-tion diagrams (Fig. 10b with normalization values adopted from Sun & McdonouGh 1989), the rocks exhibit positive anomalies in Pb and negative anomalies in Rb, Nb and Ta.
Nb/U ratios vary between 8 and 28. The NEQB have dis-tinctive troughs at Nb and Ta characteristic of subduction-related magmatic rock (wilSon 1988).
Discussion
Nature of the mantle source
The composition of olivine crystals from the NEQB is similar to those of mantle-derived magmas (Fo > 80). Some clinopyroxene phenocrysts display normal zonation (Mg depletion and Fe enrichment towards their rims) as expected for closed system evolution of a basaltic mag-ma. The NEQB rocks have high Mg-numbers between 63 and 79 similar to basaltic magmas derived from high-Mg parental melts (coleMAn & McGuire 1988). Ni contents are much higher compared to MORB (average 148 ppm), and the large spread in Ni concentration (142 – 287 ppm) is probably due to some extent of magmatic differentiation.
Oceanic basalts (i.e., ocean island basalts, OIB, and mid ocean ridge basalts, MORB) typically show negative Pb anomalies and positive Nb–Ta anomalies in primi-tive mantle-normalized trace element diagrams (hoFMAnn 1997). In contrast, the NEQB rocks exhibit strongly negative Nb–Ta anomalies and positive Pb anomalies (Fig. 10b), indicating that they were not derived from nor-mal MORB- or OIB-type mantle sources. In addition, the Ce/Pb (avg. 16) and Nb/U (avg. 16.38) ratios of the NEQB are significantly lower than in OIBs (c. 25) and MORBs
Fig. 7. Microphotograph showing corroded texture in olivine crystal of the NEQ basaltic rock.
Fig. 8. Diagram proposed by wincheSter & Floyd (1975) for clas-sification of volcanic rocks (Zr in ppm and TiO2 in wt.%). NEQB samples plot in the alkali basalt field.
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Table 4. Representative major (wt.%), trace and rare earth elements (ppm) composition of NEQB.
Ghale Parian MountainU1 U2 U3 D1 D2 D3 M1 M2 M3 N1 N2
SiO2 45.63 45.29 47.23 44.52 45.68 46.17 45.06 48.11 44.98 44.90 44.81TiO2 2.12 2.17 2.02 2.22 2.31 2.05 1.93 2.02 2.25 2.02 2.13Al2O3 13.75 13.93 14.10 14.03 14.42 13.97 13.94 14.08 13.59 13.56 13.59FeO* 9.12 9.24 9.25 9.24 9.83 9.01 8.56 9.01 8.99 8.94 8.84MnO 0.14 0.14 0.10 0.14 0.15 0.14 0.14 0.14 0.13 0.14 0.25MgO 9.32 9.17 9.16 9.43 9.79 9.34 8.34 8.61 9.24 9.02 9.21CaO 10.16 10.20 9.96 10.17 9.33 9.92 10.90 9.84 11.17 10.58 10.52Na2O 4.63 4.16 4.56 3.95 5.32 4.47 3.43 5.01 4.67 3.77 3.65K2O 1.82 1.56 2.06 2.81 1.01 2.85 2.37 1.63 1.65 2.75 2.63P2O5 1.38 1.41 1.37 0.87 0.91 1.36 1.17 1.19 1.37 1.45 1.45L.O.I 0.96 0.91 0.49 0.69 0.82 1.06 1.73 0.75 0.59 0.58 0.63Sum 99.03 98.18 100.30 98.07 99.57 100.34 97.57 100.39 98.63 97.71 97.71Ni 188 179 200 180 192 198 164 169 186 188 200Cr 306 298 287 262 281 295 244 252 296 277 283Sc 16.9 17.5 16.6 16.6 18.0 16.7 16.6 16.7 16.6 15.9 16.5V 177 179 171 188 192 172 159 166 173 169 171Ba 1102 1162 1087 765 773 1010 1040 946 1140 1052 1098Rb 24 19 37 48 54 53 48 23 22 53 51Sr 1566 1656 1623 1164 1229 1650 1622 1625 1548 1690 1721Zr 261 265 258 247 255 247 232 246 257 248 248Cs 1.41 1.41 1.50 0.92 0.95 1.38 1.25 1.27 1.72 1.33 1.32Nb 50.3 50.7 47.9 45.3 47.9 47.4 44.2 46.3 50.5 48.4 47.5Ga 19.8 20.1 20.5 19.4 20.6 18.2 19.0 20.6 19.6 19.1 18.5Cu 36.8 37.2 45.6 43.4 39.9 51.3 37.7 42.9 44.9 40.2 37.7Hf 5.56 5.60 5.44 5.21 5.38 5.34 4.99 5.01 5.51 5.27 5.20Ta 2.77 2.82 2.53 2.77 2.91 2.53 2.49 2.49 2.76 2.53 2.51Zn 116 114 117 98 102 115 107 114 113 111 111Pb 11.8 11.9 12.1 6.3 9.1 12.5 12.4 12.4 11.4 13.4 13.5Th 15.5 16.4 14.6 10.6 10.2 14.9 13.7 14.8 15.1 14.8 15.7U 2.9 2.7 2.8 1.6 3.1 2.6 3.7 3.9 3.5 3.6 5.1Y 24.0 24.7 25.4 20.7 20.8 23.7 23.1 25.6 23.3 23.7 23.3La 105 109 100 69 72 103 101 99 103 108 106Ce 193 201 188 128 135 195 188 184 193 205 201Pr 22 22 21 14 15 22 21 20 21 23 22Nd 76 79 76 52 54 79 73 72 76 80 79Sm 12.25 12.73 12.18 8.99 9.37 12.42 11.73 11.67 11.93 12.72 12.51Eu 3.31 3.41 3.27 2.65 2.78 3.34 3.22 3.13 3.20 3.41 3.29Gd 8.56 8.79 8.55 6.81 7.09 8.54 8.10 8.18 8.49 8.65 8.52Tb 1.12 1.15 1.10 0.93 0.96 1.11 1.07 1.08 1.11 1.13 1.08Dy 5.61 5.79 5.52 4.86 4.99 5.57 5.44 5.5 5.49 5.54 5.44Ho 0.95 0.97 0.95 0.84 0.85 0.94 0.92 0.93 0.94 0.92 0.90Er 2.21 2.25 2.18 1.92 1.95 2.21 2.14 2.17 2.20 2.16 2.08Tm 0.29 0.29 0.28 0.24 0.25 0.28 0.28 0.28 0.28 0.28 0.27Yb 1.65 1.62 1.60 1.36 1.41 1.59 1.58 1.56 1.60 1.59 1.52Lu 0.23 0.23 0.23 0.20 0.20 0.23 0.23 0.22 0.22 0.22 0.23Mg# 65 64 65 65 64 65 63 63 65 64 65(La/Yb)N 42 43 46 34 34 43 43 43 43 47 46(Dy/Yb)N 2.2 2.2 2.3 2.2 2.3 2.2 2.2 2.2 2.2 2.3 2.2Nb/U 25 26 17 20 24 16 16 21 28 15 16(Nb/Nb*) 0.52 0.43 0.42 0.6 0.7 0.55 0.44 0.5 0.52 0.52 0.41La/Nb 2.09 2.15 2.09 1.52 1.50 2.17 2.29 2.14 2.04 2.23 2.23Nb/La 0.48 0.47 0.48 0.66 0.67 0.46 0.44 0.47 0.49 0.45 0.45Ba/La 10.5 10.7 10.9 11.1 10.7 9.8 10.3 9.5 11.1 9.7 10.3Zr/Nb 5.2 5.2 5.4 5.5 5.3 5.2 5.2 5.3 5.1 5.1 5.2Y/Nb 0.47 0.49 0.53 0.45 0.43 0.50 0.52 0.55 0.46 0.49 0.49Nb/Y 2.09 2.05 1.88 2.18 2.30 2.00 1.91 1.80 2.16 2.04 2.03Rb/Y 1.00 0.77 1.45 2.31 2.60 2.23 2.07 0.89 0.94 2.23 2.18
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Table 4. Continued
Illanlu Ghezelcheh-kand Ghareh-BolaghI1 I2 GH1 GH2 GH3 GH4 GH5 G1 G2
SiO2 45.97 46.11 46.18 46.79 47.93 51.63 49.74 45.04 45.94TiO2 1.97 1.81 1.45 1.82 1.55 1.36 1.57 2.28 2.30Al2O3 9.29 12.75 12.24 12.89 12.94 13.26 13.28 14.28 14.45FeO* 6.76 8.27 6.21 6.98 6.61 5.87 6.66 9.64 9.68MnO 0.09 0.13 0.12 0.14 0.11 0.11 0.11 0.15 0.14MgO 13.87 10.51 6.99 7.47 7.43 6.30 7.33 9.45 9.38CaO 13.97 10.55 12.8 10.83 10.88 9.15 9.52 9.94 9.63Na2O 1.90 4.19 3.89 4.49 4.62 4.87 4.70 4.66 4.54K2O 3.28 3.10 3.18 3.35 3.25 3.08 3.45 1.46 0.96P2O5 1.17 1.35 1.51 1.90 1.65 1.35 1.51 0.97 0.89L.O.I 0.32 1.50 2.32 1.95 1.03 1.96 1.20 0.89 0.67Sum 98.59 100.27 98.60 98.61 98.00 98.94 99.07 98.76 98.58Ni 271 287 160 169 169 142 176 187 193Cr 373 388 188 176 204 164 232 258 262Sc 17 17.6 12.6 13.1 12.6 11.7 12.9 18 18.9V 158 167 120 142 127 115 134 173 197Ba 1460 1537 1603 1668 1658 1613 1697 788 797Rb 45 49 60 61 58 55 69 19 29Sr 2772 2642 2272 2291 2285 2032 2062 1310 1283Zr 259 272 300 331 312 299 329 256 255Cs 1.12 1.15 1.64 1.75 1.43 1.73 1.87 1.85 1.00Y 22 22.2 20.3 23.2 22.6 20.5 23.1 21.3 22.0Nb 38.2 40.2 42.1 46.6 43.5 40.0 44.7 46.9 46.8Ga 15 19.0 18.1 20.4 19.0 18.3 20.2 20.0 20.6Cu 64 65.2 62.5 64.5 53.2 46.1 57.0 46.1 50.1Zn 105 108 105 121 108 102 113 102 99Hf 5.76 5.94 6.38 6.93 6.57 6.42 7.00 5.34 5.36Ta 2.15 2.23 2.31 2.48 2.31 2.11 2.44 2.84 2.85Pb 18 20.9 26.4 16.9 18.3 22.0 22.2 7.9 7.4Th 16 16.5 24.2 25.2 24.0 24.0 25.2 11.4 10.6U 5 3.9 6.4 5.5 5.7 3.8 3.7 3.1 1.0La 130 134 139 160 146 131 144 75 70Ce 258 271 267 292 276 250 274 144 133Pr 30 31 30 33 32 28 31 16 15Nd 111 114 109 119 113 102 111 57 53Sm 16.32 16.48 16.39 17.86 16.86 15.3 16.51 9.78 9.13Eu 4.24 4.24 4.22 4.55 4.32 3.86 4.14 2.82 2.75Gd 9.69 10.06 9.88 10.81 10.41 9.43 10.00 7.25 7.00Tb 1.15 1.18 1.17 1.24 1.17 1.08 1.16 0.97 0.97Dy 5.38 5.55 5.32 5.68 5.39 4.90 5.35 5.02 4.98Ho 0.88 0.90 0.84 0.93 0.85 0.79 0.89 0.86 0.85Er 1.96 2.05 1.90 2.05 1.94 1.82 2.05 1.99 1.98Tm 0.25 0.25 0.24 0.25 0.24 0.23 0.26 0.25 0.25Yb 1.41 1.47 1.40 1.48 1.40 1.32 1.52 1.43 1.43Lu 0.20 0.20 0.21 0.21 0.20 0.19 0.22 0.20 0.20Mg# 79 69 67 66 67 66 66 64 63(La/Yb)N 62 62 67 71 64 73 67 36 35(Dy/Yb)N 2.4 2.4 2.4 2.5 2.2 2.4 2.4 2.2 2.2Nb/U 13 12 9 10 10 8 9 20 12(Nb/Nb*) 0.31 0.33 0.34 0.30 0.31 0.25 0.31 0.60 0.61La/Nb 4.18 6.04 6.84 6.90 6.46 6.39 6.23 3.52 3.18Nb/La 0.29 0.30 0.30 0.29 0.29 0.30 0.31 0.62 0.66Ba/La 11.23 11.47 11.53 10.42 11.35 12.31 11.78 10.50 11.38Zr/Nb 6.78 6.76 7.12 7.10 7.17 7.47 7.36 5.45 5.44Y/Nb 0.60 0.55 0.48 0.49 0.52 0.51 0.51 0.45 0.47Nb/Y 1.73 1.81 2.07 2.00 1.92 1.95 1.93 2.20 2.12Rb/Y 2.04 2.20 2.96 2.63 2.57 2.69 2.99 0.89 1.31
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Fig. 9. Fenner-type variation diagrams of selected major (wt.%) and trace elements (ppm) versus MgO (wt.%). Symbols are as in Fig. 8.
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(c. 47) (hoFMAnn 1997). The NEQB have also higher La/Nb (avg. 2.4) and La/Ta (avg. 44.46) ratios compared to OIBs with average La/Nb = 0.77 and La/Ta = 14 (hoFMAnn et al. 1986, Sun & McdonouGh 1989).
The chemical characteristics of the NEQB rocks are indicative of the involvement of an enriched source. Such signatures could be generated by interaction of a mafic primitive melt and a melt derived from the lithospheric mantle or the continental crust. In case of mantle material this could be an enriched mantle or the mantle above a subduction zone previously enriched by subduction zone fluids. The parallel REE patterns are in line with low de-grees of partial melting in the presence of residual garnet.
This interpretation is confirmed by high (Sr/Yb)N ratios (56 to 119), high Zr contents (331 ppm ˃ Zr ˃ 232 ppm), and by fractionated heavy REE (HREE). Melting in the presence of residual garnet is also indicated in the Ce/Sm versus Sm/Yb diagram (cobAn 2007) (Fig. 11a).
All NEQB rocks have high Th/Yb and low Nb/La ra-tios (Table 4). These geochemical features suggest that the melts likely originated from a subcontinental lithospheric mantle (SCLM) source. The asthenospheric mantle is de-pleted in high field-strength elements (HFSE), such as Nb and Ta, and has Nb/La > 1 (brAdShAw & SMith 1994). Although the basalts are characterized by HFSE deple-tion they have Nb/La ratios ˂ 1, which rules out derivation
Fig. 10. (a) REE patterns normalized to chondrites (tAylor & MclennAn 1985) and (b) multi element diagram normalized to primitive mantle values (Sun & McdonouGh 1989), (c) Ta/Hf–Th/Hf diagram (wAnG et al. 2001): I. Divergent plate margin MORB; II. Convergent plate margin basalts; III. Oceanic within-plate basalts; IV. Continental within plate basalts (IV1: Intra continental rift+continental margin rift tholeiites; IV2: Intra continental rift alkali basalts; IV3: Continental extensional zone/initial rift basalts); V. Mantle plume basalts, (d) discrimination diagram by AGrAwAl et al. (2008); DF1 = – 0.5558Log(La/Th) – 1.4260Log(Sm/Th)+2.2935Log(Yb/Th) – 0.6890 Log(Nb/Th)+4.1422); DF2 = – 0.9207Log(La/Th)+3.6520Log(Sm/Th) – 1.9866Log(Yb/Th) +1.0574 Log(Nb/Th) – 4.4283). Symbols are as in Fig. 8.
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from an asthenosphere mantle source. Subduction related magmas are characterized by enrichment in large ion lith-ophile elements (LILE, e.g., Rb, Ba, Sr), Pb and LREE relative to the HFSE (Nb, Ta, Zr, Hf) and HREE. More-over, those magmas are depleted in Nb and Ta, and en-riched in Pb compared to primitive mantle composition (e.g., peArce 1982, keppler 1996, hAwkeSworth et al. 1997, elburG et al. 2002, turner et al. 2003) and these geochemical characteristics are also found in the NEQB (Figs. 9a, b).
Fluids derived from dehydration of subducted conti-nental sediments and/or melts derived from partial melt-ing of the subducted continental lithosphere can cause enrichment of the mantle wedge in LIL elements and other fluid-mobile trace elements (e.g., U, Pb) above a
subducted slab (e.g., elburG et al. 2002, Guo et al. 2006, Guo & wilSon 2012). On the other hand, because of their low solubility, HFSE such as Nb, Ta, Zr and Hf are left behind after fluid or melt extraction in the subducted slab. Therefore melts with La/Ta ratios > 30 can be considered to have been derived from the SCLM which was previ-ously enriched or re-fertilized by slab-derived fluids and melts (e.g., thoMpSon & MorriSon 1982). Partial melts of subducted sediments, however, are characterized by rela-tively high Th, LILE and LREE contents (Guo & wilSon 2012). As these authors pointed out, partial melts of subducted sediments have lower Ba/Th, U/Th and Sr/Th ratios than those from other mantle sources. woodheAd et al. (2001) demonstrated that ratios of fluid-mobile trace elements to fluid-immobile trace elements, such as the
Fig. 11. (a) Ce/Sm vs. Sm/Yb diagram (cobAn 2007) showing the presence of garnet in the mantle source for the basaltic rocks; (b) Ba/La vs. Th/Nd diagram. NEQB samples follow the slab-derived fluid melt array; (c) Ba/La vs. Nb/La diagram portraying gap between the samples from Illanlu and Ghezeche-Kand and Ghale Parian Mountain and Ghareh-Bolagh; (d) Zr/Nb vs. Y/Nb diagram demonstrating within-plate enrichments rather than subduction zone enrichment. Symbols are as in Fig. 8.
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Ba/La and Th/Nd, may effectively reflect the importance of subduction fluids and/or melts in the mantle source of subduction zone magmas. For the NEQB the Ba/La vs. Th/Nd diagram confirms the involvement of a subduction-modified mantle source during their generation (Fig. 11b).
Although Illanlu and Ghezelcheh-kand samples have similar Th/Nd and Ba/La than samples from Ghale Par-ian Mountain and Ghareh-Bolagh (Fig. 11b), they differ in TiO2, K2O, Ba, Sr, Pb, Zr and LREE contents. As shown in diagrams Zr/Nb–Y/Nb and Ba/La–Nb/La (Fig. 11c–d), there is a gap between the samples from Illanlu and Ghez-elcheh-kand and Ghale Parian Mountain and Ghareh-Bolagh. Furthermore, Ba/Nb and Ba/Ta ratios of the studied samples are variable. For instance, the Ba/Ta and Ba/Nb ratios of Illanlu and Ghezelcheh-kand basalts are 673 –764 and 36 – 40, respectively. High Ba/Ta and Ba/Nb ratios are the most diagnostic characteristics of arc mag-mas (Gill 1981). The variable and high Ba/Ta and Ba/Nb ratios may also be related to various degrees of crustal contamination. The Ghale-Parian Mountain and Ghareh-Bolagh basalts have lower Ba/Ta (266 – 437) and Ba/Nb ratios (16 – 23) (Table 4), demonstrating within-plate en-richments rather than subduction zone enrichment.
blundy et al. (1998) suggested that melts generated from a garnet lherzolite mantle source have (Dy/Yb)N ra-tios ˃1. The high (Dy/Yb)N ratios (˃2) in the NEQB sug-gest that garnet was a residual phase during partial melt-ing. Enrichment in LREE and LILE can also be explained by low degree of partial melting (~0.5 %) of garnet pe-ridotite. Alternatively, these geochemical features can be
ascribed to crustal contamination and/or partial melting of a metasomatised mantle peridotite (reAGAn & Gill 1989, deFAnt et al. 1992, deFAnt & kepezhinSkAS 2001, cAStillo 2012).
Tectonic setting of the NEQB and process of magma formation
It is widely accepted that the late Cenozoic magmatism in the Mediterranean–Iran regions occurred in a post-collisional tectonic setting (e.g., ShAFAii MoGhAdAM et al. 2014) and the same holds true for the volcanic activity in western Iran (Azizi et al. 2014). In the Zr/TiO2 versus Nb/Y diagram (Fig. 8) the NEQB plot in the field of alkali basalt. Geochemical parameters, such as TiO2 concentra-tions around 2 wt.% and (Nb/Y)/(Zr/P2O5 × 10000) ratios ˃ 1 indicate an intra-plate setting for these rocks (wincheSter & Floyd 1975). In the Ta/Hf versus Th/Hf diagram (wAnG et al. 2001) and in the DF1 versus DF2 diagram (AGrAwAl et al. 2008) the NEQB plot in the field of conti-nental rift basalts (Figs. 10c, d). verMA (2009) has pointed out that the Nb value is critically influenced by the tec-tonic setting. (Nb/Nb*)pm values, defined as 2 × (Nbsa)/Nbpm)/[(Basa)/Bapm) + (Lasa)/Lapm)] (sa: concentration of element in sample, pm: concentration in the primitive mantle) in intra-plate basalts vary between 0.25 and 0.70. These values are in the range of the NEQB (avg. 0.46) and confirm that the Quaternary volcanic activity in the NE-Qorveh region took place in a post-collisional continental geodynamic setting. Volcanism was probably driven by
Fig. 12. Sm/Yb vs. La/Sm diagram for the NEQB showing melt curves obtained us-ing the non-modal batch melting equation of ShAw (1970). The NEQB rocks plot closely to the garnet peridotite melting curve with 0.5 % of partial melting of a source with primitive-mantle REE abundance ratios. Melt curves for spinel-peridotite (with mode and melt mode of Ol53+Opx27+Cpx17+Sp3 and Ol– 40 +Opx30+Cpx90+Sp20 and garnet- peri dotite (Ol60+Opx20+Cpx10+Gt10 and Ol10+ Opx18+Cpx30+Gt42 are from johnSon (1998) and keleMen et al. (1993), cited in wAnG et al. (2004) (see Table 3 and references therein). Tick marks and numbers along the curves show the degree of partial melting for a given mantle source. Trace-element partition coefficients are from hAllidAy et al. (1995) and SAlterS (1996). C0 for both garnet peridotite and spinel perido-tite are based on primitive mantle values cited in pAlMe & o’neill (2004). Symbols are as in Fig. 8
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mantle convection processes which, in turn, might have been triggered by a late stage slab steepening or break-off beneath the Sanandaj-Sirjan zone. For this region, peArce et al. (1990) & keSkin (2003, 2007) suggested lithosphere delamination and break-off of the subducting Neotethys oceanic slab. Geophysical evidence for thinning of the northern Sanandaj-Sirjan Zone lithosphere beneath the Iranian plateau is provided by vAn der voo et al. (1999) and MAGGi & prieStley (2005).
The NEQB area is underlain by Precambrian–Paleozo-ic metamorphic and non-metamorphic crustal lithologies. Gneissic xenoliths found in several samples are likely to be fragments of crustal metamorphic rocks incorporated into the basaltic magma during ascent. Despite their high MgO concentrations (6.30 –13.87 wt.%), the occurrence of gneissic xenoliths in the basalts, the presence of quartz xenocrysts (Fig. 3) and sieve-textured clinopyroxene demonstrate that contamination by local crustal rocks has contributed to some extent to the evolution of NEQB.
Apparently, some changes in the SCLM must have triggered the generation of the NEQB. It is likely that the magma generating process was controlled by local pro-cesses like preexisting mantle weaknesses or extensive metasomatism and re-fertilization of the SCLM. These conditions favored low degrees of partial melting of a garnet lherzolite mantle source. The strongly fractionated REE patterns with high (Tb/Yb)N and (Dy/Yb)N ratios (> 2) favour a garnet lherzolite mantle source from which the melts were extracted.
boccAletti et al. (1976) suggested that volcanic ac-tivity in the Qorveh-Takab-Bijar region was triggered by regional tensional tectonics, resulting in a network of fractures whose principal trends are subparallel to the Za-gros Main Fault. Other researchers (reGArd et al. 2006, MAlecootyAn et al. 2007) pointed out, that there is still an extensional tectonic regime prevailing in the study area. On the other hand, Allen et al. (2013) did not find evi-dence for major Cenozoic extension in this zone. In their model an extensional regime developed in the northern Sanandaj-Sirjan zone due to oceanic plate roll back and mantle upwelling after the collision of the Iranian and Arabian plates. Because of Red Sea rifting at the same time, the area was mainly under compression. So, even-tually we can speculate that volcanism in the Kurdistan province was a result of local crustal extension and micro plate movement.
Conclusions
Quaternary basaltic rocks from the NE-Qorveh region, NW Iran, are characterized by the presence of norma-
tive nepheline and alkaline character. The rocks show porphyritic to microlitic-porphyritic texture with olivine, clinopyroxene as phenocrysts and plagioclase as micro-lites, in some cases pyroxene crystals show sieve texture. The presence of gneissic xenoliths and quartz xenocrysts points to some interaction with crustal material. In addi-tion, sieve textured pyroxenes and the existence of differ-ent pyroxene populations is suggestive of magma mixing processes.
Quaternary volcanism in the NE-Qorveh region post-dates the subduction of Neo-Tethyan oceanic lithosphere. The volcanic region is located in an intercontinental zone that was affected by some extension and rifting after the subduction and collision process. Fractionated REEs pat-terns (i.e. high LREE/HREE ratios), depletions in Rb, Nb and Ta and enrichment in Pb) of the NEQB still bear the characteristics of the subduction zone setting. Such geo-chemical features are common in other parts of the NW Iranian volcanic zone (kheirkhAh et al. 2009) and indi-cate that they were generated along a subduction modi-fied plate margin and derived from partial melting of a SCLM. Low Zr/Nb and Y/Nb ratios as well as Nb/La ˂ 1 show that the melts formed from a garnet lherzolite man-tle source by low degree of partial melting.
Acknowledgments
This work was supported by the Bu-Ali Sina University (Iran). We thank Drs. M.C. Rowe and R.M. Conrey, Geo-Analytical Laboratory Washington State University for analytical help. We express our thanks to Prof. Heinz-Günter Stosch for reading the first draft of the manuscript. The authors would like to thank Dr. E. Ṣen and an anony-mous reviewer for their constructive comments leading to important improvements of the manuscript.
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Manuscript received: June 22, 2015; accepted: October 28, 2015.Responsible editor: H.-G. Stosch
Authors’ addresses:A. Torkian*, N. Salehi, Department of Geology, Bu-Ali Sina University, Hamedan, Iran.W. Siebel, Department of Geosciences, Tübingen University, Germany.*Corresponding author: e-mail: [email protected]
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