GeolCarp_Vol68_No3_229

GEOLOGICA CARPATHICA

, JUNE 2017, 68, 3, 229 – 247

doi: 10.1515/geoca-2017-0017

www.geologicacarpathica.com

Geochemical and isotopic evidence for Carboniferous

rifting: mafic dykes in the central Sanandaj-Sirjan zone

(Dorud-Azna, West Iran)

FARZANEH SHAKERARDAKANI

1

, FRANZ NEUBAUER

, MANFRED BERNROIDER

ALBRECHT VON QUADT

, IRENA PEYTCHEVA

, XIAOMING LIU

, JOHANN GENSER

BEHZAD MONFAREDI

and FARIBORZ MASOUDI

Department of Geography and Geology, University of Salzburg, Hellbrunner Str. 34, A-5020 Salzburg, Austria;



farzaneh.shakerardakani@sbg.ac.at

Institute of Isotope Geology and Mineral Resources, ETH Zürich, CH-8092 Zürich, Switzerland

State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Northern Taibai Str. 229, Xi’an 710069, China

Institute of Earth Sciences, University of Graz, Universitätsplatz 2, A-8010 Graz, Austria

Faculty of Earth Sciences, Shahid Beheshti University, 19839-63113, Tehran, Iran

(Manuscript received July 4, 2016; accepted in revised form March 15, 2017)

Abstract: In this paper, we present detailed field observations, chronological, geochemical and Sr–Nd isotopic data and

discuss the petrogenetic aspects of two types of mafic dykes, of alkaline to subalkaline nature. The alkaline mafic dykes

exhibit a cumulate to foliated texture and strike NW–SE, parallel to the main trend of the region. The

Ar/

Ar amphibole

age of 321.32 ± 0.55 Ma from an alkaline mafic dyke is interpreted as an indication of Carboniferous cooling through

ca. 550 °C after intrusion of the dyke into the granitic Galeh-Doz orthogneiss and Amphibolite-Metagabbro units,

the latter with Early Carboniferous amphibolite facies grade metamorphism and containing the Dare-Hedavand

metagabbro with a similar Carboniferous age. The alkaline and subalkaline mafic dykes can be geochemically categorized

into those with light REE-enriched patterns [(La/Yb)

= 8.32– 9.28] and others with a rather flat REE pattern

[(La/Yb)

= 1.16] and with a negative Nb anomaly. Together, the mafic dykes show oceanic island basalt to MORB

geochemical signature, respectively. This is consistent, as well, with the (Tb/Yb)

ratios. The alkaline mafic dykes were

formed within an enriched mantle source at depths of ˃ 90 km, generating a suite of alkaline basalts. In comparison,

the subalkaline mafic dykes were formed within more depleted mantle source at depths of ˂ 90 km. The subalkaline

mafic dyke is characterized by

Sr/

Sr ratio of 0.706 and positive ɛ

(t) value of + 0.77, whereas

Sr/

Sr ratio of 0.708

and ɛ

(t) value of + 1.65 of the alkaline mafic dyke, consistent with the derivation from an enriched mantle source. There

is no evidence that the mafic dykes were affected by significant crustal contamination during emplacement. Because of

the similar age, the generation of magmas of alkaline mafic dykes and of the Dare-Hedavand metagabbro are assumed to

reflect the same process of lithospheric or asthenospheric melting. Carboniferous back-arc rifting is the likely geodynamic

setting of mafic dyke generation and emplacement. In contrast, the subalkaline mafic sill is likely related to the

emplacement of the Jurassic Darijune gabbro.

Keywords: mafic dyke, Sanandaj-Sirjan Zone,

Ar/

Ar dating, whole-rock Sr–Nd isotopes, Carboniferous rift,

Palaeotethys.

Introduction

The emplacement of mafic dyke swarms exhibits short-lived

magmatic events that convey key information on important

temporal and chemical constrains for the evolution of the

lithospheric mantle and the upper crust into which these

basaltic magmas intrude. Mafic dykes are commonly the result

of extensional tectonic regimes in a variety of tectonic settings

(e.g., Halls & Fahrig 1987; Ernst & Buchan 2001, 2002;

Goldberg 2010; Peng et al. 2011; Srivastava 2011; Li et al.

2013; Saccani et al. 2013) and provide valuable information

on processes generating large volumes of mafic magmas.

Furthermore, mafic dyke swarms can provide essential

insights into the nature of mantle sources (e.g., Maurice et al.

2009; Pirajno & Hoatson 2012; Khanna et al. 2013).

In the Sanandaj-Sirjan Zone (SSZ), however, only a few

studies have focused on these rocks (e.g., Saccani et al. 2013;

Sharifi & Sayari 2013; Deevsalar et al. 2014). For instance,

the gabbroic dykes exposed across the Malayer-Boroujerd

plutonic complex close to the study area, located 30 km to the

north of the Dorud region, are investigated by Deevsalar et al.

(2014). The geochemical signature of the dykes from the

Malayer-Boroujerd plutonic complex provides insights for the

formation of syn-collisional arc related and intraplate late to

post-orogenic mafic magmatism, which accords in two main

stages including asthenosphere-derived melt, followed by low

pressure fractionation and accumulation at high crustal levels.

No detailed information on timing of these mafic dykes is

available. Further, in the central SSZ, northeast of Golpaygan,

alkaline and subalkaline mafic dykes of uncertain age indicate

232 SHAKERARDAKANI, NEUBAUER, BERNROIDER, VON QUADT, PEYTCHEVA, LIU, GENSER, MONFAREDI and MASOUDI

GEOLOGICA CARPATHICA

, 2017, 68, 3, 229 – 247

Amphibolite-Metagabbro unit, which also includes the Dera-

Hedavand metagabbro. To the southeast, these units are intru-

ded by the Darijune gabbro (Shakerardakani et al. 2015).

The existence of a Panafrican basement in this part of SSZ has

been proposed based on recent U–Pb dating of the granitic

Galeh-Doz orthogneiss that yielded ages of 608 ±18 Ma

and 588 ± 41 Ma, respectively (Shakerardakani et al. 2015).

The calc- alkaline granitoids of the Panafrican basement were

formed during subduction of the Proto-Tethys and are similar

to those exposed in Central Iran (Shakerardakani et al. 2015).

In addition, the mafic rocks of the Amphibolite-Metagabbro

unit exhibit sub-alkaline to alkaline basaltic compositions, and

have an E-MORB geochemistry (Shakerardakani et al. 2015).

A laser-ablation ICP-MS U–Pb zircon age of 314.6 ± 3.7 Ma

has been reported for the Dare-Hedavand metagabbro from

this complex (Shakerardakani et al. 2015) testifying a Late

Carboniferous age of gabbro formation. Recently, Fergusson

et al. (2016) reported an age of 336 ± 9 Ma for a metagabbro,

which was previously undated and considered to be part of the

isotropic Darijune gabbro (Shakerardakani et al. 2015).

The metamorphic complex with the Panafrican basement of

granitic Galeh-Doz orthogneiss has almost invariably under-

gone a complex history of repeated shearing, folding, transpo-

sition and associated polyphase greenschist- to amphibolite-

facies metamorphism (Shakerardakani et al. 2015).

The southeastern part of the study area is intruded by the

Darijune gabbro, which stretches over about 5 km in a NW–SE

direction. The gabbro body is confined by the presumed Upper

Triassic June complex in the north and by the Upper Jurassic

to Lower Cretaceous andesitic lavas and pyroclastic rocks in

the south. The geochemical and petrographic characteristics of

the Upper Jurassic (U–Pb zircon age: 170.2 ± 3.1 Ma) Darijune

gabbro indicate a cumulate signature. Shakerardakani et al.

(2015) proposed that the cumulate Darijune gabbro was deri-

ved from a source highly depleted in incompatible elements,

which is interpreted as generated in a continental arc setting.

The studied mafic dykes are located mainly in the centre of

the Ediacaran granitic Galeh-Doz orthogneiss. These dykes

are characterized by a thickness varying from 0.2 to 4 meters.

The majority of dykes strike WNW–ESE and dip steeply to

NNE (Fig. 3a). Individual mafic dykes (alkaline dyke; see

below) are mostly subvertical, and typically trend NW–SE

subparallel to the main trend of the Galeh-Doz orthogneiss.

They are mainly foliated with the sharp contact with the host

rock (Fig. 3c, d, e). Some dykes and sill-like exposures occur

around the Shur-Shur and Dare-Hedavand villages in the cen-

tral part of the study area (Fig. 2a). Close to Dare-Hedavand

these dykes (subalkaline dyke; see below) trend ENE–WSW

or NNE–SSW (Fig. 3b). Based on the presence of other mafic

magmatic rocks, several potential relationships could be

envi saged: (1) The mafic dykes could represent a dyke swarm

of the Upper Jurassic Darijune gabbro, (2) the dykes could

relate to the Upper Carboniferous Dare-Hedavand metagabbro

or, (3) they are derived from none of the mentioned major

mafic bodies. Age and geochemical affinities could resolve the

open potential relationships.

Petrography

Based on the petrographic evidence as well as geochemical

signatures, two groups of mafic dykes are identified in the

Dorud-Azna region. The main petrographic features of the

mafic dykes of each rock-type are summarized in Table 1.

The type i (sample L J-111) is a subalkaline mafic dyke (see

below; Table 1) and show a medium-grained cumulitic texture

(Fig. 4a, b). The type ii samples (J-112, J-209, L J-136) are

alkaline mafic dykes and display cumulitic or porphyroclastic

textures (Fig. 4c–f). All samples are affected by variable

degrees of alteration and/or low-grade metamorphism, what

resulted in the partial replacement of primary igneous textures,

which are generally well preserved (Fig. 4a–f).

Plagioclase and alkali feldspar grains are mainly altered

along the rim and among the cleavage planes, although their

textures are preserved in the centre. Chlorite, epidote and

sericite are the main secondary minerals after amphibole and

plagioclase. The chlorite and epidote displaying various tex-

tures of mafic dykes have been metamorphosed at greenschist

facies-grade metamorphic conditions with variable preserva-

tion of primary magmatic texture.

In one sample (L J-111), minor amounts of clinopyroxene

occur at the margins of amphibole (Fig. 4b). Euhedral to sub-

hedral amphibole is the predominant mineral, whereas euhed-

ral plagioclase and alkali feldspar are subordinate (Fig. 4a, c, d).

Chlorite and epidote are the main secondary minerals, although

epidote in the one sample (J-112) can be observed as a mag-

matic mineral (Fig. 4c). Biotite is a fairly common mineral and

occurs as a secondary mineral in some samples. Sphene,

musco vite, Fe-Ti oxides and apatite represent accessories.

Clinopyroxene and the majority of plagioclase grains have

been partially replaced by metamorphic or hydrothermal

mine rals such as epidote, chlorite and sericite. In addition,

some fractures in a few samples are filled by secondary calcite

and epidote (Fig. 4e).

The strongly foliated alkaline mafic dykes are generally

fine- to medium-grained and bear a porphyroblastic texture

(Fig. 4f). The major magmatic mineral phases in the mafic

dykes are euhedral to subhedral pale- to dark green amphibole,

subhedral plagioclase and alkali feldspar, although quartz, and

greenish biotite are recognized in this mafic dyke group.

The accessory minerals consist of Fe-Ti oxides and apatite.

Analytical methods

Electron Microprobe Analytical Technique

Polished thin sections of samples were analysed using a fully

automated JEOL 8600 electron microprobe at the Dept. Geo-

graphy and Geology, University of Salzburg, Austria. Point

analyses were obtained using a 15 kV accelerating voltage and

40 nA beam current. The beam size was set to 5 µm. Natural

and synthetic oxides and silicates were used as standards for

major elements. Structural formulas for all amphiboles were

234 SHAKERARDAKANI, NEUBAUER, BERNROIDER, VON QUADT, PEYTCHEVA, LIU, GENSER, MONFAREDI and MASOUDI

GEOLOGICA CARPATHICA

, 2017, 68, 3, 229 – 247

and age calculations were carried out at the ARGONAUT

Laboratory of the Department of Geography and Geology at

the University of Salzburg. The sample containing primary

amphibole was crushed and sieved following standard proce-

dures. By comparison, the 200–350 µm fraction was separated

under the microscope. The mineral concentrate was wrapped

in Al foil. The sample packet was loaded in 5×5 mm wells on

30 mm Al-disk for irradiation, which was done without

Cd shielding in the LVR-15 reactor at Rez, Czech Republic, at

a thermal neutron fluence of

4.8×10

n/cm

s and a thermal to

fast neutron ratio of

2.1. All ages were calculated using Fish

Canyon sanidine as a flux monitor (28.305 ± 0.036 Ma; Renne

et al. 2010). Errors on ages are 1σ. The decay constants used

are those given in Renne et al. (2010). Corrections for inter-

fering Ar isotopes were done using

Ar/

(Ca)

= 0.000245,

Ar/

(Ca)

= 0.000932,

Ar/

(Ca)

= 0.01211 and

Ar/

(K)

0.0183 and by applying 5 % uncertainty. Isotopic ratios, ages

and errors for individual steps were calculated following sug-

gestions by McDougall & Harrison (1999) and Scaillet (2000).

Definition and calculation of plateau ages was carried out

using ISOPLOT/EX (Ludwig 2003).

Major and trace element geochemistry

Whole-rock major and trace elements for 4 fresh dyke sam-

ples from Dorud-Azna region were determined. Fresh chips of

whole rock samples were powdered using an agate mill, and

preserved in a desiccator for analysis after being dried in

an oven at 105 °C for 2 hours. Major and trace element com-

positions were analysed by means of XRF (Rigaku RIX 2100)

and ICP-MS (PE 6100 DRC), respectively, at the State Key

Laboratory of Continental Dynamics, Northwest University,

China. For major element analysis, 0.5 g sample powders were

mixed with 3.6 g Li

, 0.4 g LiF, 0.3 g NH

and minor

LiBr in a platinum pot, and melted to form a glass disc in

a high frequency melting instrument prior to analysis. For

trace element analysis, sample powders were digested using

HF+HNO

mixture in high-pressure Teflon bombs at 190 °C

for 48 hours. Analyses of USGS and Chinese national rock

standards (BCR-2, GSR-1 and GSR-3) indicate that both ana-

lytical precision and accuracy for major elements are generally

better than 5 %, and for most of the trace elements except for

transition metals are better than 2 % and 10 %, respectively.

Additionally, one sample was randomly selected to be analy-

sed twice to test the accuracy.

Sr–Nd isotope analysis

Details of the analytical techniques of Rb–Sr and Sm–Nd

isotope geochemistry, taking into account their accuracy and

precision can be found in von Quadt et al. (1999). For the iso-

topic analysis,

50 mg of whole-rock powder was dissolved in

HF and HNO

, followed by Pb, Nd and Sr separation by

exchange chromatography techniques on 100 μl TEP columns

with Sr Tru and Ln spec Eichrom resin. Nd was purified on

a Ln spec column (2 ml) using 0.22 n and 0.45 n HCL. Nd and

Sr isotopes were analysed on a ThermoPlus mass spectrometer

by static mode measurements at ETH Zürich. Sr was loaded

with a Ta emitter on Re filaments. The measured

Sr/

ratios were normalized to an

Sr/

Sr value of 8.37521.

The mean

Sr/

Sr value of the NBS 987 standard obtained

during the period of measurements was 0.710252 ± 12 (2 σ,

n=18). Pb was loaded with a silica gel on Re filaments. A mass

fractionation correction of 1.1 ‰ per atomic mass unit was

applied, based on replicate analyses of the NBS 982 reference

material. Nd was loaded with 2 n HCL on Re filaments.

The measured

143

Nd/

144

Nd ratios were normalized to the

146

Nd/

144

Nd value of 0.7219. The mean

143

Nd/

144

Nd value of

the Nd Merck standard obtained during the period of measure-

ments was 0.511730 ± 1 (2 σ, n=12). Age-corrected Nd and Sr

isotope ratios were calculated using Rb, Sr, Sm and Nd con-

centrations determined by LA-ICP-MS.

Mineral chemistry

The mafic dykes are classified into two types based on their

mineral paragenesis: (i) the mafic dyke comprising clino-

pyroxene + amphibole + plagioclase + K-feldspar + epidote +

chlorite + sphene ± opaque minerals + apatite (subalkaline mafic

dyke, sample L J-111) and (ii) the mafic dykes containing

amphibole + plagioclase + K-feldspar + quartz + epidote + sphene

+ chlorite ± biotite ± muscovite + apatite + opaque minerals (alka-

line mafic dykes). In type (i), clinopyroxene is augite (accor-

ding to Morimoto 1988) with relatively high Mg# (Mg# = 64.5)

and FeO (8.84 wt. %), whereas the Al

content (0.37 wt. %)

Sample

no.

Latitude

(°N)

Longitude

(°E)

Rock Type

Mineralogy

Texture

L J-111 33°34ʼ52” 49°11ʼ42” subalkaline mafic dyke

cpx + amp + pl + kfs + ep + chl + ser + spn + opq + ap

cumulitic texture

J-112

33°35ʼ06” 49°11ʼ04” alkaline mafic dyke

amp + pl + kfs + ep + chl + bt + qz + ser + spn + opq + ap

cumulitic texture

J-209

33°35ʼ49” 49°08ʼ39” alkaline mafic dyke

amp + pl + kfs + ep + chl + spn + qz + ms + ser + opq + ap

cumulitic texture

L J-136 33°34ʼ48” 49°12ʼ02ˮ alkaline mafic dyke

amp + pl + kfs + ep + chl + ser + spn + opq + ap

highly foliated and

porphyroblastic texture

Table 1: Location, mineralogy and texture of the investigated mafic dyke samples. Abbreviations: pl — plagioclase, kfs — k-feldspar,

cpx — clinopyroxene, amp — amphibole, bt — biotite, chl — chlorite, ms — muscovite, spn — sphene, ep — epidote, ser — sericite,

qz — quartz, ap — apatite, opq — opaque mineral.

236 SHAKERARDAKANI, NEUBAUER, BERNROIDER, VON QUADT, PEYTCHEVA, LIU, GENSER, MONFAREDI and MASOUDI

GEOLOGICA CARPATHICA

, 2017, 68, 3, 229 – 247

52.81, and they have large K

O contents (0.65–1.41 wt. %).

Ps contents of the epidote grains in the type (i) are within

a narrow range from Ps

to Ps

. The pistacite content in

type (i) is higher (Ps

). Plagioclase in type (ii) is mainly albite

with the albite content ranging from 88 to 96%. In compari-

son, the main composition of plagioclase in type (i) is oligo-

clase (Ab

69–88

) while a minor amount of albite exists (Ab

Alkali feldspar in these rocks is mainly orthoclase (Or

96–97

)

with smaller amounts of anorthoclase and sanidine (Or

15–23

Ar/

Ar dating

Sample J-112, which belongs to type (ii), alkaline dykes

(see below), yields a well preserved magmatic amphibole,

which is nearly not overprinted by subsequent low-grade

metamorphism. The medium-sized amphibole grains together

with biotite and feldspar define the stretching lineation and

foliation in this peculiar sample. Amphibole grains are com-

monly replaced by biotite along the micro-fractures and clea-

vage planes (Fig. 4c). The amphibole grains bear no or only

a few inclusions of other mineral phases (Fig. 4c).

The analytical results are given in Table 3 and are graphically

shown in Fig. 6. The sample shows some small portion of

excess argon in the first step. Steps 4 to 8 (comprising 98.8 %

Ar released) yield a plateau age of 321.32 ± 0.55 Ma.

Isotopic inversion yields an age of 319.3 ± 1.1 Ma and initial

Ar/

Ar ratio of 262 ± 21 implying the presence of some

minor argon loss. We suggest, therefore, that the slightly older

Sample

type

Subalkaline dyke

Alkaline dyke

Sample

No.

L J-111

L J-136

J-209

L J-136

J-209

Mineral Cpx

Amp

Kfs

Amp

SiO2

57.11 45.46 46.07 44.96 46.20

37.76 37.72

59.64 60.54 65.36

64.23 63.65 41.07 40.83 42.32 43.84 43.02

38.61

65.72 30.00

Al2O3

0.37

9.89 9.93 10.52 9.54

23.77 23.86

24.40 23.69 20.93 18.26 18.43 10.81 11.25 9.44 10.63 10.00

22.15 21.16 2.58

MgO

18.61 13.00 12.98 12.43 12.19

0.00 0.02

0.00 0.06 0.01

0.00 0.00

6.42 6.51 7.47

6.97 9.08

0.00

0.00 0.01

Na2O

0.04

1.37 1.46 1.70 1.36

0.01 0.00

7.98 8.49 10.27

0.31 0.29

1.61 1.44 1.56

2.00 1.42

0.00

9.96 0.02

CaO

12.99 11.89 11.83 11.85 11.66

23.47 23.31

6.36 4.70 2.14

0.00 0.09 10.94 10.85 10.79 11.48 11.48 22.52

2.41 27.59

TiO2

0.00

0.78 0.87 0.92 0.82

0.04 0.05

0.00 0.00 0.00

0.00 0.03

0.70 0.58 1.01

2.76 0.87

0.06

0.00 34.28

FeO

8.84 13.39 12.88 13.88 13.61

11.01 11.45

0.06 0.28 0.13

0.10 0.06 24.18 23.92 22.50 17.71 20.26 12.59

0.15 1.98

MnO

0.21

0.25 0.26 0.21 0.25

0.05 0.09

0.00 0.01 0.01

0.00 0.01

0.51 0.54 0.56

0.23 0.30

0.19

0.00 0.04

Cr2O3

0.00

0.11 0.08 0.13 0.06

0.02 0.00

0.00 0.00 0.01

0.00 0.00

0.00 0.04 0.01

0.03 0.00

0.01

0.02 0.02

K2O

0.00

0.59 0.59 0.67 0.70

0.00 0.00

0.11 0.77 0.17 16.55 16.38

1.33 1.41 1.03

0.65 1.06

0.00

0.04 0.00

Total

98.26 96.73 96.95 97.30 96.37

96.13 96.53

98.56 98.54 99.04 99.45 98.94 97.59 97.39 96.68 96.34 97.48 96.13

99.46 96.52

2.08

6.75 6.80 6.66 6.87

6.02 6.00

10.78 10.94 11.60

11.97 11.92

6.44 6.41 6.62

6.69 6.59

3.11

11.60 1.02

0.02

1.73 1.73 1.84 1.67

4.48 4.48

5.20 5.05 4.38

4.01 4.07

2.00 2.08 1.74

1.91 1.81

2.10

4.40 0.10

1.01

2.88 2.85 2.75 2.70

0.00 0.00

0.00 0.02 0.00

0.00 0.00

1.50 1.52 1.74

1.59 2.08

0.00

0.00 0.00

0.00

0.39 0.42 0.49 0.39

0.00 0.00

2.79 2.97 3.54

0.11 0.11

0.49 0.44 0.47

0.59 0.42

0.01

3.41 0.00

0.50

1.89 1.87 1.88 1.86

4.01 3.97

1.23 0.91 0.41

0.00 0.02

1.84 1.82 1.81

1.88 1.88

0.00

0.46 1.00

0.00

0.09 0.10 0.10 0.09

0.00 0.01

0.00 0.00 0.00

0.00 0.00

0.08 0.07 0.12

0.32 0.10

0.00

0.00 0.87

0.27

1.66 1.59 1.72 1.69

1.47 1.52

0.01 0.04 0.02

0.02 0.01

3.17 3.14 2.94

2.26 2.60

0.76

0.02 0.06

0.00

0.03 0.03 0.03 0.03

0.01 0.01

0.00 0.00 0.00

0.00 0.00

0.07 0.07 0.07

0.03 0.04

0.00

0.00 0.00

0.00

0.01 0.01 0.01 0.01

0.00 0.00

0.00 0.00 0.00

0.00 0.00

0.00 0.00 0.00

0.00 0.00

0.00

0.00 0.00

0.00

0.11

0.11 0.13 0.13

0.00 0.00

0.03 0.18 0.04

3.94 3.91

0.27 0.28 0.21

0.13 0.21

0.00

0.01 0.00

Total

98.17 38.55 38.50 38.62 38.46

41.00 41.00

52.04 52.11 51.99 52.05 52.05 38.86 38.84 38.73 38.39 38.72

8.05

51.91 8.06

Mg#

64.5

72.1 70.1 67.8 66.1

39.77 44.78 41.56 52.81 41.23

22.81 23.45

26.62

0.69 0.89 0.92

0.030 0.028

0.880 0.97

0.30 0.10 0.07

0.001 0.000

0.117 0.01

0.01 0.01 0.01 0.969 0.972

0.002 0.03

Table 2: Representative microprobe analyses of Cpx – clinopyroxene, Amp – amphibole, Ep – epidote, Pl – plagioclase, Kfs – K-feldspar,

Mg# = Mg / (Mg+Fe)*100; Ps = Fe

/(Fe

+Al)*100.

Fig. 5. Composition of amphiboles in the subalkaline and alkaline

mafic dykes plotted in the classification diagrams by Leake et al.

(1997).

237

CARBONIFEROUS RIFTING IN THE CENTRAL SANANDAJ-SIRJAN ZONE (IRAN)

GEOLOGICA CARPATHICA

, 2017, 68, 3, 229 – 247

plateau age is geologically significant and represents the

approximate age of cooling through ca. 550 °C after intrusion

of the dyke. As no thermal effects are known, cooling was

likely rapid, and the age of 321.32 ± 0.55 Ma is, therefore,

close to the real age and represents the approximate age of

the intrusion.

Geochemistry

The geochemical results of the four samples of the studied

mafic dykes are listed in Table 4. For comparison, these data

have been merged in some diagrams with the geochemical

data of the Amphibolite-Metagabbro unit and Darijune gabbro

from a previous study (Shakerardakani et al. 2015). Note that

because of the similar Carboniferous age the isotropic

(Darijune) gabbro of the previous study is now merged with

the Dera-Hedavand metagabbro. The immobile elements

during low-temperature alteration and metamorphism are used

for the geochemical features of the mafic rocks. Although,

the relative variation of SiO

, CaO, Al

and Fe

against

immobile elements (e.g., Zr, Y) in a few samples, and also the

light and heavy REE (LREE, HREE) elements show good

step

±σ

Ar*/

± σ

Ar*

age

(Ma)

± 1 σ (Ma)

age

(Ma)

± 1 σ (Ma)

meas.

decay corr

meas.

decay corr

meas.

Sample: J-1

12, amphibole, 200–355 μm, 7 grains, J-V

alue: 0.010764 +/

− 0.000007

2.2859E+02

5.24E+00

1.2708E+03

3.1

1E+01

7.5528E+01

4.19E+00

1.9145E+02

2.26E+01

9.1890E+04

8.65E+01

128.4204

17.2714

26.5

0.1

1566.1

140.8

1577.1

141.5

6.67882

0.1

1367

2.2787E+02

5.63E+00

1.6620E+03

2.1

1E+01

1.1707E+02

5.08E+00

6.2651E+02

3.03E+01

1.2901E+05

1.51E+02

98.86935

5.49173

47.8

0.3

1307.5

51.7

1317.1

52.0

2.659380

0.08869

3.9861E+02

7.59E+00

5.2256E+03

6.74E+01

2.6108E+02

6.75E+00

1.6806E+03

5.24E+01

2.1388E+05

1.83E+02

57.56723

2.24774

44.9

0.8

870.0

27.0

876.8

27.1

3.1

18483

0.07798

1.4318E+02

3.47E+00

7.2780E+04

1.58E+02

4.3167E+03

2.03E+01

1.7337E+04

2.20E+01

3.4337E+05

3.21E+02

17.73671

0.06622

87.7

8.3

315.2

1.1

318.0

1.1

4.214360

0.20806

2.8744E+02

3.87E+00

4.1946E+05

8.39E+02

2.7719E+04

6.91E+01

1.1488E+05

1.44E+02

2.1081E+06

1.54E+03

17.93423

0.02815

96.0

55.2

318.4

0.5

321.2

0.5

3.66374

0.20504

7.2439E+01

4.01E+00

1.5166E+05

3.96E+02

9.0512E+03

3.72E+01

3.8044E+04

8.91E+01

6.9010E+05

8.56E+02

17.93045

0.05712

96.9

18.3

318.4

1.0

321.2

1.0

4.00145

0.19955

2.8930E+01

2.70E+00

3.9272E+04

1.62E+02

2.0750E+03

1.63E+01

8.6876E+03

3.43E+01

1.6099E+05

1.57E+02

17.94778

0.1

1785

94.7

4.2

318.7

1.9

321.5

1.9

4.53962

0.19684

5.4103E+01

3.47E+00

1.0714E+05

2.07E+02

6.5810E+03

3.33E+01

2.6579E+04

5.99E+01

4.8363E+05

3.42E+02

17.95190

0.05758

96.7

12.8

318.7

1.0

321.5

1.0

4.04607

0.20905

4.5643E-01

3.49E+00

1.6985E+03

3.06E+01

1.2142E+02

8.09E+00

2.7010E+02

4.33E+01

1.2930E+04

1.45E+02

48.10751

8.67566

99.0

0.1

752.8

111

758.9

111

6.32561

0.39742

Table 3:

Analytical results of

Ar/

Ar amphibole dating of an

alkaline mafic dyke in Dorud-Azna region.

Fig. 6.

Ar/

Ar release pattern of an amphibole concentrate of

sample J-112, an alkaline mafic dyke.

238 SHAKERARDAKANI, NEUBAUER, BERNROIDER, VON QUADT, PEYTCHEVA, LIU, GENSER, MONFAREDI and MASOUDI

GEOLOGICA CARPATHICA

, 2017, 68, 3, 229 – 247

correlations with immobile elements that indicate they have

not been mobilized during alteration processes. Two different

types of mafic dykes can be distinguished based on geoche-

mical features. The two dyke groups are plotted in the basaltic

field of the total alkalis-silica diagram (figure not shown).

All the mafic dykes are characterized by a narrow range of

SiO

(45.81–51.02 wt. %), K

O (0.91–1.07 wt. %) and Na

(3.10 –3.90 wt. %). Based on Winchester & Floyd’s (1977)

diagram, the type (i) mafic dykes plot into the subalkaline

(sample L J-111), whereas the type (ii) dykes plot into the alka-

line field (Fig. 7). The subalkaline sample of type (i) is charac-

terized by moderate MgO (9.27 wt. %), low contents of TiO

(1.24 wt. %), P

(0.09 wt. %) and total Fe

(10.89 wt. %)

and Mg# (42.33). Compared with this type (i), the alkaline

mafic type (ii) dykes consist of an average lower concentra-

tion of MgO (3.31–5.66) and higher TiO

(2.80 –3.57 wt. %),

(0.41–1.25 wt. %) and total Fe

(12.66–15.29 wt. %)

contents and Mg# (18.40 –24.64) (Table 4). The geochemical

characteristics of mafic dykes are further explored in chon-

drite-normalized rare earth- and incompatible trace multi-

element variation diagrams (Fig. 8a, b). Although all samples

display regularly decreasing normalized patterns from Ba to

Yb in the primitive mantle-normalized diagram, they show

significant differences between the two types of dykes (Fig. 8a).

Unlike alkaline mafic dykes, the subalkaline mafic dyke is

characterized by enrichment in LILE (e.g., Cs, Rb, Ba, Pb and

K) and Nb and Zr negative anomalies, suggesting that the sub-

alkaline dyke has some arc features (Sun & McDonough 1989;

Bezard et al. 2011). Further, no significant Ti anomaly indi-

cates that fractionation of Fe-Ti oxides is not involved in

magma genesis (Briqueu et al. 1984). The alkaline mafic

dykes have higher REE abundances and are enriched in LREE

with (La/Sm)

and (La/Yb)

ratios ranging from 2.21 to 2.46

and 8.32 to 9.28, respectively (Fig. 8b), that are similar to

those of enriched basalts, such as E-MORBs and OIBs.

Although rather low (La/Yb)

and (La/Sm)

ratios (1.16 and

1.00 respectively) in the subalkaline mafic dyke imply a source

more depleted in LREE than E-MORB. No obvious Eu ano-

maly (Eu/Eu* = 0.94–1.06) in both alkaline and subalkaline

mafic dykes indicates that plagioclase was not a major accu-

mulating mineral phase.

Sr-Nd isotopes

Sr and Nd isotopic compositions are presented in Table 5.

Mafic dyke samples accompanied by samples of granitic

Galeh-Doz orthogneiss, Dare-Hedavand metagabbro from the

Amphibolite-Metagabbro unit and Darijune gabbro were

subject to Rb–Sr and Sm–Nd isotope analyses too, for com-

Sample no.

L J-111

J-112

L J-136

J-209

Mafic dyke
Subalkaline

Alkaline

SiO

wt.%

48.91

46.02

51.02

45.81

13.70

12.91

13.45

13.32

10.89

15.29

12.66

14.93

MgO

9.27

5.49

3.31

5.66

CaO

9.53

10.12

7.58

9.52

3.10

3.13

3.90

3.15

1.02

1.01

0.91

1.07

TiO

1.24

3.48

2.80

3.57

0.09

0.47

1.25

0.41

MnO

0.17

0.21

0.19

LOI

2.17

2.10

3.13

2.08

TOTAL

100.09

100.23

100.2

99.71

Mg#

42.33

23.64

18.40

24.64

Li (ppm)

6.63

8.05

13.83

10.09

0.71

2.31

2.85

2.32

36.41

30.76

21.35

31.35

238.8

423.1

220.1

417.1

386.99

61.91

13.55

51.56

40.17

44.15

17.71

49.57

167.46

58.72

7.11

55.50

14.20

84.68

13.01

62.34

113.32

145.70

122.47

119.28

16.66

24.24

25.54

23.58

1.76

1.85

2.28

2.08

29.17

24.46

34.61

30.15

209.2

274.6

364.1

435.9

27.79

28.24

47.78

27.71

34.5

211.9

196.5

202.4

1.47

40.59

40.79

37.37

0.39

0.25

2.69

0.30

198.0

134.8

179.0

228.4

4.72

28.43

42.65

25.20

12.59

64.26

93.13

59.55

1.73

8.31

12.47

7.83

8.89

34.43

53.10

33.05

2.96

7.28

11.34

7.16

1.06

2.40

3.79

2.41

4.00

6.76

10.89

6.71

0.69

0.99

1.57

0.98

4.59

5.52

8.77

5.46

1.00

0.99

1.65

0.99

2.88

2.58

4.33

2.57

0.41

0.35

0.59

0.35

2.74

2.07

3.48

2.04

0.40

0.29

0.50

0.29

1.21

4.85

4.67

4.69

0.10

2.31

2.43

2.19

5.64

7.22

3.73

5.97

0.58

2.49

5.09

2.10

0.60

1.42

1.75

0.69

Table 4: Representative major and trace element analyses of mafic

dykes from the Dorud-Azna region.

Fig. 7. Nb/Y vs. Zr/TiO

diagram for classification of the Dorud-Azna

mafic dykes (Winchester & Floyd 1977). Mafic dykes plot in the sub-

alkaline and alkaline fields.

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parison and further discussion of the origin of mafic dykes.

Isotopic data of all samples are plotted in the

Sr/

(i)

versus

143

Nd/

144

(i)

and

Sr/

(i)

versus ɛ

Nd(t)

diagrams (Fig. 9a, b).

The subalkaline mafic dyke has an initial

Sr/

Sr ratio of

0.706 and ɛ

Nd315

value of + 0.77. The alkaline mafic dyke has

an initial

Sr/

Sr ratio of 0.708 and ɛ

Nd315

value of +1.65,

higher than the subalkaline dyke. The higher

Sr/

Sr value

observed in alkaline mafic dyke is probably the result of melts

with an enriched mantle signature (Fig. 9a, b).

In the Dare-Hedavand metagabbro, initial

Sr/

Sr ratios

calculated at 314 Ma (U–Pb zircon age) vary from 0.704 to

0.706 in the isotropic and cumulate metagabbro, respectively.

However, the isotropic metagabbros (S-105, L J-146) have

higher values of ɛ

Nd314

(+ 1.66 to + 2.58) than cumulate meta-

gabbro (+ 0.88), which is significantly affected by partial

melting/fractionation crystallization processes during genera-

tion of the mafic melt (Fig. 9c).

The low initial

Sr/

Sr ratios and the positive ɛ

Nd170

value

clearly indicate a mantle origin of the Darijune gabbro.

The cumulate Darijune gabbro (T-108) also has a low

Sr/

initial ratio (0.7041) and the highest ɛ

Nd170

(+ 5.43) within all the

investigated mafic samples, indicating a depleted mantle source.

The Galeh-Doz orthogneiss exhibits a wide range of

age-corrected (588 Ma; U–Pb zircon age)

Sr/

Sr initial ratio

from 0.704 to 0.707 (Fig. 9a, b), whereas the age-corrected

Nd(t)

values for these samples define a restricted range from

− 2.36 to − 3.27. In this orthogneiss, Sr

(i)

ratios increase with

increasing SiO

(Fig. 9c), indicating that addition of crustal

materials (i.e., crustal contamination) likely occurred during

magma emplacement (e.g., De Paolo 1981). The presence of

inherited zircons in the granitic orthogneiss (Shakerardakani

et al. 2015) is also indicative of crustal assimilation or magma

hybridization (e.g., Bonin 2004).

Discussion

Pressure and temperature constraints

In order to evaluate the depth of origin of material and the

depth of intrusion of the mafic dykes, temperature and pressure

were calculated using the PET software (Dachs 2004) based

on the hornblende-plagioclase thermometer of Holland &

Blundy (1994) and the aluminum-in-hornblende barometer

(Hammarstrom & Zen 1986; Hollister et al. 1987) respec-

tively.

The estimated equilibration temperature and pressure for the

subalkaline dyke are in the range of 700 ± 25 °C at 5 ± 0.8 kbar.

P–T estimate for the alkaline dyke assemblage range from

720 ± 30 °C at 5 ± 1.5 kbar. The temperature estimates for the

retrograde stage were obtained using the amount of Al

base

in chlorite of Cathelineau (1988). Estimates range from 332 °C

to 187 °C in both subalkaline and alkaline dykes, indicating

greenschist facies and sub-greenschist facies conditions during

metamorphism and alteration.

Sample Age (Ma)

Rb/

Sr/

2 σ

147

Sm/

144

143

Nd/

144

2 σ

(0) e

(t)

G-112

588 ± 41

5.59

75.9

0.2078

0.70714 0.000022 0.70888

11.0

51.4

0.1349

0.512289

0.000009

−6.81 −2.369

L J-112

588 ± 41?

107

161

1.8775

0.70401 0.000055 0.71975

7.84 37.4

0.1319

0.512231

0.000006

−7.94 −3.274

J-112

321 ± 0.55

24.5

275

0.2513

0.70803 0.000054 0.70915

7.28 34.4

0.1331

0.512592

0.000003

−0.90

1.653

L J-111

321? or 170? 29.2

209

0.3935

0.70653 0.000045 0.70829

2.96

8.89

0.2101

0.512705

0.000012

1.31

0.771

T-108

170 ± 3.1

4.16

90.3

0.1301

0.70411

0.000019 0.70442

1.03

2.97

0.2189

0.512941

0.000018

5.91

5.431

L J-146

170 ± 3.1

25.2

452

0.1576

0.70474 0.000032 0.70512

6.66 30.4

0.1379

0.512602

0.000017

−0.70

0.574

S-100

314 ± 3.7

0.55 114

0.0137

0.70500 0.000016 0.70507

2.94 13.0

0.1425

0.512572

0.000032

−1.29

0.886

S-105

314 ± 3.7

6.28 452

0.0392

0.70626 0.000029 0.70644

3.77 13.2

0.1790

0.512734

0.000005

1.87

2.583

Table 5: Sr–Nd isotopic compositions from Galeh-Doz orthogneiss, mafic dyke, Darijune gabbro and Amphibolite-Metagabbro unit.

Fig. 8. Chondrite-normalized REE patterns (Boynton 1984) and

primitive mantle-normalized incompatible element pattern (Sun &

McDonough 1989) for mafic dykes.

240 SHAKERARDAKANI, NEUBAUER, BERNROIDER, VON QUADT, PEYTCHEVA, LIU, GENSER, MONFAREDI and MASOUDI

GEOLOGICA CARPATHICA

, 2017, 68, 3, 229 – 247

Magma generation

The normalized multi-element and REE patterns of the sub-

alkaline and alkaline mafic dykes suggest that the magmas

were derived from a mantle source between MORB and OIB-

like composition, respectively (Fig. 8). However, low Cr and

Ni concentrations in alkaline mafic dykes (13–61 and

7–58 ppm) indicate that these are not primary magmas and

underwent some crystal fractionation most likely of spinel,

olivine and clinopyroxene. The Al

/TiO

against Ti diagram,

which is used as an indicator of differentiation, shows that the

mafic dyke as well other investigated mafic groups are mainly

characterized by the crystallization of plagioclase + clinopy-

roxene in the relatively less evolved rocks followed by the

crystallization of Fe-Ti-oxides in the more evolved rocks

(Fig. 10 a). However, it must be noted that it is sometimes

difficult to differentiate between the effects of fractional crys-

tallization on the compositions of primary magmas and those

of partial melting. A steep trend in the La/Yb vs. La diagram

(Fig. 10 b) suggests that the effects of partial melting and

source composition were more important than fractional crys-

tallization in controlling the compositional variation between

the two groups of mafic dyke (e.g., Jiang et al. 2005). In addi-

tion, the budget of the moderately incompatible elements (for

example, La/Yb, Fig. 10b) in the Dare-Hedavand metagabbro

Fig. 9. a, b — Sr–Nd isotopic composition of the mafic dykes with

representatives of country rocks including the Panafrican granitic

Galeh-Doz orthogneiss, Carboniferous metagabbro and Jurassic Dar-

ijune gabbro (data from Shakerardakani et al., 2015). MORB: Mid-

Ocean Ridge Basalts; OIB: Ocean Island Basalts; EM1: enriched

mantle 1; EM2: enriched mantle 2. c — Plot of Sr

(i)

vs. SiO

. Note that

magmatic rocks of different crystallization ages are plotted together.

Fig. 10. a — Al

/TiO

vs. Ti diagram for mafic dykes from the

Dorud-Azna region. Fractional crystallization (F. C.) vectors of

magnetite (mt), orthopyroxene (opx), olivine (ol), clinopyroxene

(cpx), and plagioclase (pl) are shown. Modified after Saccani et al.

(2013). Arrow shows the estimated fractionation trend. b — La/Sm

vs. La diagram for the mafic dykes. Vector arrows show the effect of

increasing degrees of partial melting (P. M.) and fractionation (F. C.).

241

CARBONIFEROUS RIFTING IN THE CENTRAL SANANDAJ-SIRJAN ZONE (IRAN)

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is largely controlled by partial melting processes rather than

crystal fractionation. Compared with subalkaline magmas, the

magma that produced the alkaline mafic dykes probably

experienced subsequent fractionation after partial melting

(Fig. 10b). These samples reveal a remarkable negative cor-

relation of MgO, Al

and Ni with increasing Zr contents and

positive correlation of LREE, HREE and Nb (Fig. 11). These

patterns are consistent with the fractionation of olivine, pyro-

xene and plagioclase (Ordóñez-Calderón et al. 2011). Further-

more, the positive correlation between TiO

and Zr contents

(Fig. 11) could suggest that Fe-Ti oxides minerals were not

significant mineral phase during fractionation (Ordóñez-

Calderón et al. 2011; Deng et al. 2013).

In order to constrain the geochemical characteristics of the

possible mantle sources for the two different types of mafic

dykes together with previously investigated mafic rocks, some

hygromagmatophile element ratios (e.g., Tb/Yb, La/Sm) have

been used. In the (Tb/Yb)

vs. (La/Sm)

diagram (Fig. 12a),

there are two distinct groups indicating that partial melting

played an important role in the chemical compositions of

investigated mafic rocks.

High Tb/Yb ratios (2.06–2.20) of the alkaline mafic dykes

reflect a deep melting level in the stability field of the garnet-

peridotite mantle likely at a depth of more than 90 km, whereas

the subalkaline mafic dyke has a lower Tb/Yb ratio (1.15)

indicative for melting of shallower spinel-peridotite mantle at

a depth of less than 90 km (Wang et al. 2002; Khudoley et al.

2013). In comparison, Tb/Yb and La/Sm ratios of the source

region of Dare-Hedavand metagabbros range from 1.30 to

2.08 and 1.40 to 2.65, suggesting they were produced through

various degrees of partial melting and/or point to a stronger

enrichment of their mantle source. Moreover, lower La/Sm

ratios in the subalkaline mafic dyke as well as the cumulate

Darijune gabbro than in other mafic groups can be also caused

by melting of a depleted source. In contrast, increased partial

melting or crustal contamination can enhance the La/Sm

ratios. In fact, the magma ascending through continental crust

raises the possibility that crustal contamination may have

Fig. 11. Variation diagrams of Zr versus the major and trace elements for mafic dykes of the Dorud-Azna region. The data compared with the

geochemical data of mafic samples from Amphibolite-Metagabbro unit and Darijune gabbro. Symbols are the same as in Fig. 10.

242 SHAKERARDAKANI, NEUBAUER, BERNROIDER, VON QUADT, PEYTCHEVA, LIU, GENSER, MONFAREDI and MASOUDI

GEOLOGICA CARPATHICA

, 2017, 68, 3, 229 – 247

caused some of the compositional characteristics of the mafic

rocks. In respect of the (Th/Yb)

vs. (Nb/Yb)

diagram,

all distinct groups are in the field of mantle-derived melts

(Fig. 12b), suggesting that chemical influence of the conti-

nental lithosphere component was very limited or absent, as

also supported by the plot in the Nb/La versus Nb/Th diagram

(Fig. 12c). For the subalkaline mafic dyke, the initial

Sr/

ratios is negatively correlated to SiO

, suggesting that crustal

contamination plays a minor role in its genesis (Fig. 9c) (e.g.,

Lin et al. 1990). Therefore, the differences in La/Sm ratios can

refer to a diffe ring degree of partial melting and/or magma

source. It is assumed that alkaline mafic dyke magmas were

produced from a garnet-bearing enriched mantle source (high

Tb/Yb ratio) from the deeper source, while the magmas of

subalkaline mafic dykes were generated in the shallow and

depleted mantle source (low Tb/Yb ratio).

Tectonic implications

The E-MORB and OIB-type affinities of the mafic dykes

are typically found in tectonic settings including oceanic-

island/seamounts, continental-rifts and some flood-basalt

provinces (e.g., Hochstedter et al. 1990; van Staal et al. 1991;

Goodfellow et al. 1995; Shinjo et al. 1999; Leitch & Davies

2001). Their genesis has mostly been attributed to mantle

plumes, however, there are also occurrences where no plume

has been involved (e.g., Haase & Devey 1994).

The geochemical signatures of mafic dykes, accompanied

by the various mafic rock types in the Dorud-Azna region

indicate that magmas of at least two different compositions

originated from mantle sources in various time intervals.

The tectonic discrimination diagrams and Sr- and Nd isotopes

provide the broadest insight into the setting of this magmatism.

In the ternary Nb –Hf–Th diagram of Wood (1980), alkaline

mafic dykes plot in the field of within-plate alkaline basalts,

whereas the subalkaline dyke plots within the volcanic-arc

basalt field (Fig. 13a). The Dare-Hedavand metagabbro sam-

ples demonstrate the alkaline field, similar to alkaline mafic

dykes, along with enriched mid-ocean ridge basalt (E-MORB)

field. Because of the geochemical similarities and similar pro-

tolith ages, we assume a common origin of the alkaline mafic

dykes and the Dare-Hedavand metagabbro.

The cumulate Darijune gabbro plots in the field of volcanic-

arc basalt. Furthermore, the variation of Ti/V ratio is clearly

shown on the Ti/1000 vs. V diagram (after Shervais 1982) that

all the samples show transitional signatures, where the sub-

alkaline mafic dyke with a lower Ti/V ratio than the alkaline

dykes indicates MORB-affinity magmatism, the mafic dykes

as within-plate basalts (not shown), as well as mafic rocks

from the Amphibolite-Metagabbro unit plot in the MORB-

BAB (back-arc basin) fields overlapping with ocean island

alkaline basalts (Fig. 13b).

In the Nb/Yb versus Th/Yb tectonic discrimination diagram

of Pearce (2008), all the alkaline mafic dykes and various mafic

rocks of the Amphibolite-Metagabbro unit plot along the

enriched part of the MORB-OIB array (Fig. 13c). The subalka-

Fig. 12. Variation diagrams for incompatible trace elements and

element ratios for different magma compositions after Khudoley et al.

(2013). a — (Tb/Yb)

vs. (La/Sm)

diagram display two distinct

groups of mafic dykes; the discrimination line between magmas

sourced from melting of spinel peridotite mantle and garnet peridotite

mantle is based on Wang et al. (2002). b — (Nb/Yb)

vs. (Th/Yb)

diagram displaying all samples plot within the field of mantle-derived

melts marked with two different ranges; Upper Continental Crust

(UCC) and Lower Continental Crust (LCC) compositions are from

Taylor and McLennan (1985), and enriched mantle (EMI, EMII) and

HIMU (high µ where µ=

238

204

Pb) mantle values are from Condie

(2001). PM denotes normalization to Primitive Mantle values of

McDonough & Sun (1995). c — Nb/La vs. Nb/Th diagram showing

the trend of crustal contamination (after Zhang et al. 2014).

243

CARBONIFEROUS RIFTING IN THE CENTRAL SANANDAJ-SIRJAN ZONE (IRAN)

GEOLOGICA CARPATHICA

, 2017, 68, 3, 229 – 247

Fig. 13. Tectonic discrimination diagrams. a — V vs. Ti/1000 diagram after Shervais (1982); b — Hf–Nb–Th plot, with fields defined by Wood

(1980); c — Nb/Yb versus Th/Yb after Pearce (2008). The dashed field boundaries for TH: tholeiitic, CA: calc-alkaline, and Sh: shoshonitic

rocks are from convergent margins. The bold arrows in the bottom right are S: subduction component, C: crustal contaminant component,

W: within plate, and f: fractional crystallization vectors; d — Zr/Y–Nb/Y diagram (Fitton et al. 1997); DEP: deep depleted mantle; PM: primi-

tive mantle; DM: shallow depleted mantle; EN: enriched component; REC: recycled component; OPB: oceanic plateau basalt; OIB: ocean

island basalt; ARC: arc related basalt; NMORB: normal ocean ridge basalt (Symbols are as in Fig. 12).

line mafic dyke plots on a trend toward the area above the

mantle array in the arc-back-arc field. On the Zr/Y–Nb/Y

diagram (Fitton et al. 1997), the mafic dykes as well as all

other investigated mafic rocks groups plot above the ΔNb line

[ΔNb =1.74 + log (Nb / Y) −1.92 log (Zr/Y)], inside of the plume

field (Condie 2005) at relatively high Zr/Y values within the

OPB (oceanic plateau basalt) and OIB fields. However, the

subalkaline mafic dyke plots within depleted mantle with

lower Zr/Y values (Fig. 13d). Consequently, the subalkaline

mafic dyke is potentially related to the Darijune gabbro.

Concluding remarks

The REE patterns and some incompatible element ratios

reflect the geochemical diversity of the mantle sources prior to

the effects of subduction and display that the Dorud-Azna

mafic dykes, like metagabbros and Darijune gabbro contain

variable mixtures of depleted lithosphere mantle and enriched

(OIB-like) asthenospheric mantle sources. These geochemical

signatures for mafic dykes support the probably arc-related or

plume-related setting. In addition, the

Ar/

Ar amphibole

plateau age of ca. 321.32 ± 0.55 Ma of an alkaline mafic dyke

demonstrates the potentially similar origin as for the Carboni-

ferous Dare-Hedavand metagabbro and a close genetic rela-

tionship between these two types of rock types is, therefore,

assumed, probably reflecting the same process of lithospheric

or asthenospheric melts for their magma generation. A rela-

tionship of the alkaline mafic dykes to the Jurassic Darijune

gabbro can be excluded because of difference in age and geo-

chemical composition, although the geochemical signatures

(e.g., uprising from a depleted MORB-type asthenosphere) of

the subalkaline mafic dyke illustrate a possible relationship

with the cumulate Jurassic Darijune gabbro. In this case, the

alkaline dyke magma formed together with the Dare- Hedavand

metagabbro.

244 SHAKERARDAKANI, NEUBAUER, BERNROIDER, VON QUADT, PEYTCHEVA, LIU, GENSER, MONFAREDI and MASOUDI

GEOLOGICA CARPATHICA

, 2017, 68, 3, 229 – 247

Carboniferous back-arc rifting seems to be, therefore, the

most likely geodynamic setting for alkaline mafic dyke gene-

ration and emplacement. It has to be noted that more and more

evidence for Carboniferous and earliest Permian magma gene-

ration appear both in the Sanandaj-Sirjan Zone and Central

Iran, as well as the Alborz range and NE Iran (Fig. 1). In par-

ticular, Berberian & King (1981) suggested that Devonian-

Carboniferous basalts along the Sanandaj-Sirjan Zone are

clearly subjected to a rift setting during the Early Devonian

and Carboniferous extensional phase. Recently, Ayati (2015)

reported an alkaline to tholeiitic affinity of latest Devonian

basalts, for example, from the western Yazd block (Fig. 1),

which derived from a deep-seated source potentially indica-

ting initial stages of rifting.

However, in the last decade, more and more evidence has

appeared for the existence of Carboniferous to earliest Per-

mian orogenic events in Central Iran (Bagheri & Stampfli

2008; Zanchi et al. 2009a and b; Buchs et al. 2013; Karga-

ranbafghi et al. 2015), in the Sanandaj-Sirjan Zone (Advay &

Ghalamghash 2011; Bea et al. 2011; Moghadam et al. 2015)

but also in the Eastern Pontides in Turkey (Topuz et al. 2010;

Kaygusuz et al. 2012) and in the northern part of the Arabian

plate (Tavakoli-Shirazi et al. 2013; Frizon de Lamotte et al.

2013; Stern et al. 2014). Within the Palaeotethys, Bagheri &

Stampfli (2008) postulated “Variscan” terrane accretion in

Central Iran, also supported by abundant “Variscan” detritus

in Mesozoic sediments of Central Iran (Kargaranbafghi et al.

2015). In addition, evidence from northern (Talesh Moun-

tains) and northeastern Iran (Mashhad-Fariman area) indicate

the presence of a Late Palaeozoic, mainly Carboniferous

active margin of the Palaeotethys, which is the same as in

Central Iran (Nakhlak-Anarak units) developed during the

Eo-Cimmerian orogenic cycle (Ghazi et al. 2001; Zanchi et al.

2009a; Zanchetta et al. 2009, 2013). Within the SSZ, U–Pb

zircon ages of Gushchi A-granites and gabbronorites in NW

Iran indicate that the gabbronorites and granites were emplaced

synchronously at

320 Ma (Moghadam et al. 2015). Saccani

et al. (2013) report a U–Pb zircon age of

356.7 ± 3.4 Ma (Early Carboniferous) from

a leucogabbro dyke within the Misho Mafic

Complex (Fig. 1), which shows N- and P-MORB

affinities. Bea et al. (2011) found that the Khalifan

pluton of northwestern SSZ includes an A-type

peraluminous leuco granite with an intrusion age

of 315 ± 2 Ma. Honarmand et al. (2017) found

an Early Permian age (294.6 ± 2.7 Ma) for the

Hasan-Robat A-type granite. In the southeastern

part of the Sanandaj- Sirjan Zone, in the west of

Hajiabad, Ghasemi et al. (2002) show Late

Carbo niferous

Ar/

Ar mineral ages ran ging

from 330 to 300 Ma. Further, K/Ar ages of 310 ± 9

and 331 ± 5 Ma are reported by Sheikholeslami et

al. (2003) in the Neyriz area. All this evidence

argues that Carboniferous mafic magmatic com-

plexes are widespread in NW Iran (Fig. 1) and are

locally asso ciated with Carboniferous to earliest

Permian A-type anorogenic granites. We suggest, therefore,

that a major, potentially plume-related rift event is common in

NW Iran and it is potentially related to mafic flood basalt mag-

matism in the northernmost Arabian plate (Stern et al. 2014).

The alkaline mafic dykes and the Dare-Hedavand metagabbro

from the Dorud–Azna region in the central SSZ are the

southernmost occurrences of the mafic anorogenic magma-

tism within the SSZ and predate the suggested main stage

Permian rifting of the SSZ (Hassanzadeh & Wernicke 2016).

Our preferred interpretation is that the alkaline dykes of

the Dorud-Azna region formed during an initial stage of

potentially plume-related rif ting and are associated with the

intrusion of the Dare-Hedavand metagabbros (Fig. 14).

The plume might have induced heating and partial melting of

the lithospheric base. Heating also resulted in surface uplift.

Overheated mafic magma chambers at the base of the crust

might have caused melting of conti nental crust resulting in

A-type granites.

Furthermore, Kohn et al (1992) found a Late Devonian–

Early Carboniferous thermo-mechanical event that they related

to the “Variscan” orogeny, which is also known in the study

area (Shakerardakani et al., submitted). A so-called “Hercy-

nian unconformity” exists everywhere in the Arabian plate

(Frizon de Lamotte et al. 2013 and references therein). This

deformation and the associated “thermal event” are probably

independent from the “Variscan” orogeny in Europe and are

rather related to anorogenic magmatic events within the Arabian

plate and NW Iran (Frizon de Lamotte et al. 2013; Tavakoli-

Shirazi et al. 2013; Stern et al. 2014; Moghadam et al. 2015).

Consequently, we suggest that mafic magmatism postdates the

rising evidence for “Variscan” orogenic processes in the SSZ

and Arabian plate.

Acknowledgements: This paper is part of the PhD thesis of

Farzaneh Shakerardakani. FS acknowledges support through

a scholarship from the Afro-Asiatisches Institute Salzburg for

her PhD thesis at Salzburg University.

Fig. 14. Simplified tectonic model with the interpretation of formation of Carboni-

ferous alkaline mafic dykes and associated Dera- Hedavand metagabbro due plume

tectonics predating rifting.

245

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GEOLOGICA CARPATHICA

, 2017, 68, 3, 229 – 247

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