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International Geology Review
ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/tigr20
The Siah Cheshmeh-Khoy-Misho-Tabriz fault (NW
Iran) is a cryptic neotethys suture: evidence from
detrital zircon geochronology, Hf isotopes, and
provenance analysis
Ali Mohammadi , Jean-Pierre Burg & Marcel Guillong
To cite this article: Ali Mohammadi , Jean-Pierre Burg & Marcel Guillong (2020): The Siah
Cheshmeh-Khoy-Misho-Tabriz fault (NW Iran) is a cryptic neotethys suture: evidence from detrital
zircon geochronology, Hf isotopes, and provenance analysis, International Geology Review
To link to this article: https://doi.org/10.1080/00206814.2020.1845992
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ARTICLE
The Siah Cheshmeh-Khoy-Misho-Tabriz fault (NW Iran) is a cryptic neotethys
suture: evidence from detrital zircon geochronology, Hf isotopes, and
provenance analysis
Ali Mohammadi
a,b
, Jean-Pierre Burg
b
and Marcel Guillong
b
a
Eurasia Institute of Earth Sciences, Istanbul Technical University, Maslak, Turkey;
b
Department of Earth Sciences, ETH Zurich, Zurich,
Switzerland
ABSTRACT
Provenance analysis including sandstone modal compositions, heavy mineral assemblages, and
LA-ICP-MS U-Pb dating with in-situ Hf isotopic compositions of detrital zircons denes two types of
Late Cretaceous-Eocene turbiditic sequences separated by the Siah Cheshmeh-Khoy-Misho-Tabriz
Fault in the Azerbaijan Province, Northwest Iran: (1) Volcaniclastic turbidites to the southwest and
(2) terrigenous turbidites to the northeast. The rst appearance of Cr-spinel in Upper Cretaceous
sandstones of both sequences indicates the erosion of ophiolites at that time. The southwestern
volcaniclastic sandstones contain few Late Triassic and dominantly Late Cretaceous zircon grains
whose Hf isotopic compositions suggest a continental magmatic arc provenance. In the north-
eastern terrigenous sandstones, detrital zircon U-Pb ages cluster in ve main age groups from
Paleoproterozoic-Neoproterozoic with Hf isotopic compositions typical of continental crust to Late
Cretaceous-Palaeocene zircon grains with Hf isotopic compositions similar to those of the south-
west turbidites. The provenance characteristics dene, along with sedimentary structures, two
separate basins, a marginal basin to the northeast, and a deep oceanic basin to the southwest. We
conclude that the Siah Cheshmeh-Khoy-Misho-Tabriz Fault overprints a boundary between two
lithospheres separated by ophiolite outcrops; therefore, the Siah Cheshmeh-Khoy-Misho-Tabriz
Fault is a reworked suture. The Cretaceous arc, witnessed by clastic material, has been largely
eroded and/or subducted.
ARTICLE HISTORY
Received 7 July 2020
Accepted 24 October 2020
KEYWORDS
Tabriz fault; Khoy ophiolite
(NW Iran); turbidites; detrital
zircon; Hf isotopes;
neotethys suture
1. Introduction
The northeast-dipping subduction of the Neo-Tethys
Ocean beneath the Iranian Plate may have started as
early as the Triassic-Early Jurassic (e.g. Şengör and
Yilmaz 1981; Şengör et al. 2003; Alavi 2007; Zhang
et al. 2018). Subduction formed the Middle to Late
Jurassic magmatic arc in the Sanandaj-Sirjan Zone
(SSZ; Figure 1). Later on, in the Late Cretaceous, the
magmatic activity produced the so-called Urmia
Dokhtar Magmatic Arc (UDMA, Figure 1, as dened by
Alavi 2007), with island arc and continental arc seg-
ments, depending on the lithosphere on which the
UDMA was installed. Ophiolites were obducted onto
the Arabian passive margin in the Late Turonian and
collision between the Arabian and Iranian plates is
Chattian (e.g. Pirouz et al. 2017). Remnant parts of the
Neo-Tethys Ocean closed from Late Palaeocene to Early
Eocene times, occasionally involving subduction and
associated arc magmatism (Stöcklin 1974; Alavi 2007).
Such a complex closure of the Tethys Ocean nurtured
the idea that several oceanic basins may have left sev-
eral sutures, the Khoy Ophiolite Complex (KOC) marking
one of them (Figures 1, 2; e.g. Azizi and Jahangiri 2008;
Lechmann et al. 2018b). Still, the relationship of Tethys
oceanic remnants and related sutures in NW Iran (Siah
Cheshmeh-Khoy), Turkey (Izmir-Ankara-Erzincan), and
Armenia (Sevan-Akra) are almost unknown. The impli-
cation that there were dierent sedimentary basins on
both sides of such a suture had to be substantiated. For
this reason, we decided to date detrital zircon grains,
measure their Hf isotopic compositions, and perform
multi-proxy provenance studies of sandstones from
the widely distributed Late Cretaceous-Eocene turbidi-
tic successions in the Azerbaijan Province (NW Iran).
Provenance analysis including sandstone framework
and the heavy mineral study provides valuable informa-
tion on the composition of the source rocks to con-
strain the past tectonic setting of the basins (e.g.
Dickinson and Suczek 1979; Dickinson 1985; Zua
1985; Weltje and von Eynatten 2004). Detrital zircon
U-Pb ages and in-situ Hf isotopic compositions also
CONTACT Ali Mohammadi mohammadiali@itu.edu.tr Eurasia Institute of Earth Sciences, Istanbul Technical University, Maslak 34467, Turkey
Supplemental data for this article can be accessed here
INTERNATIONAL GEOLOGY REVIEW
https://doi.org/10.1080/00206814.2020.1845992
© 2020 Informa UK Limited, trading as Taylor & Francis Group
allow assessing crystallization ages and origin of mag-
mas of the source rocks (e.g., Košler et al. 2002; Sláma
et al. 2008). The map trace of the active Siah Cheshmeh-
Khoy-Misho-Tabriz Fault (SKMTF, Figures 1 and 2)
delineates the boundary between the two Cretaceous-
Eocene turbiditic domains. We, therefore, concur with
some previous authors (e.g. Alavi 2007; Jamali et al.
2012; Mesbahi et al. 2016; Lechmann et al. 2018b) that
the SKMTF overprints a suture zone, and add that the
Azerbaijan Magmatic Arc has been either deeply
eroded or subducted in the cryptic Siah Cheshmeh-
Khoy-Misho-Tabriz suture. We name the ‘Azerbaijan
Continental Magmatic Arc’ the missing Cretaceous-
Palaeocene magmatic arc to avoid confusion with the
essentially Eocene-Oligocene and still active Urmia
Dokhtar Magmatic Arc (e.g. Berberian et al. 1982; Alavi
1994; Regard et al. 2004; Omrani et al. 2008; Chiu et al.
2013; Zhang et al. 2018).
This work aimed to document in more detail the
Mesozoic history of the turbiditic sedimentary rocks
associated with the eastern KOC and those related to
the western KOC to clarify the general setting of these
two ‘ophiolitic complexes’ and turbiditic basins.
2. Geological framework of studied sandstones
Northwest Iran (Azerbaijan Province) is a part of the
Arabia-Eurasia collision zone (e.g. Vernant and Chery
2006; Allen et al. 2011). The Khoy, Maku, Serow, Tabriz-
Poldasht and Urumiyeh 1:250,000 geological maps (Alavi
and Bolourchi 1975; Aghanabati and Haghipour 1976;
Haghipour et al. 1978; Shahrabi et al. 1985; Eftekhar-
Nezhad et al. 1989) describe the geology of the eld
area (Figure 2), which has been subdivided into three
major zones: (1) the Central Iranian Block to the east, (2)
the Urmia Dokhtar Magmatic Arc to the south, and (3)
the Khoy Ophiolite Complex to the west (Figure 1;
Berberian and Berberian 1981; Khalatbari-Jafari et al.
2004).
The Central Iranian Block consists of a Precambrian
basement, Lower Palaeozoic (Cambrian-Ordovician)
sedimentary rocks, and Devonian-Carboniferous
Figure 1. Simplified tectonic map of Iran (modified after Stocklin 1968). Northwest frame: studied area.
2A. MOHAMMADI ET AL.
plutonic intrusions unconformably covered by Upper
Palaeozoic to Quaternary sedimentary successions (e.g.
Aghanabati and Haghipour 1976; Asadian and Eftekhar-
Nezhad 1993; Hassanzadeh et al. 2008; Azizi et al. 2011;
Bea et al. 2011; Saccani et al. 2013; Mohammadi et al.
2020 Figures 1, 2, 3). The Permo-Triassic platform lime-
stones and Jurassic siliciclastic sandstones are widely
exposed below Late Cretaceous-Eocene terrigenous
sandstones. The Late Cretaceous-Eocene turbidites are
successions of well-bedded, carbonate-rich terrigenous
Figure 2. (a) Simplified geological map of the Azerbaijan Basin. Stratigraphic ages according to the 1:250ʹ000 geological maps of Khoy,
Maku, Serow, Tabriz-Poldasht and Urumiyeh (.Alavi and Bolourchi 1975; Aghanabati and Haghipour 1976; Haghipour et al. 1978;
Shahrabi et al. 1985; Eftekhar-Nezhad et al. 1989) and locally new stratigraphic determinations. Abbreviations: EKOC: Eastern Khoy
Ophiolite Complex; WKOC: Western Khoy Ophiolite Complex. Green area: Northeastern basin, and Grey area: Southwestern basin, (b)
location of rift-related Jurassic granitoids and magmatic arc – related Cretaceous plutons (remnants of the Lost Azerbaijan Magmatic
Arc) adapted from Lechmann et al. (2018b).
INTERNATIONAL GEOLOGY REVIEW 3
sandstones, and shales (Figure 4(b,d)). The richness in
carbonate grains and hemipelagic sediments and abun-
dance of ichnofossils indicate sedimentation above the
carbonate compensation depth (CCD), most probably in
a marginal basin. Oligocene-Miocene sedimentary and
volcanic rocks unconformably overlie these older
sequences (Figure 3; Alavi and Bolourchi 1975;
Aghanabati and Haghipour 1976; Haghipour et al.
1978; Shahrabi et al. 1985; Eftekhar-Nezhad et al. 1989).
Calc-alkaline Eocene basaltic-andesitic lavas dominate
the still-active UDMA (Figures 1, 3; Chiu et al. 2013). The
Miocene to Quaternary magmatism has post-collisional
geochemical characteristics (e.g. Chiu et al. 2013;
Lechmann et al. 2018a). The Khoy Ophiolite Complex
(KOC, Figure 2) has been subdivided into two major
units: (1) The Eastern and (2) the Western Khoy
Ophiolite Complex (Hassanipak and Ghazi 2000;
Khalatbari-Jafari et al. 2003, 2004). The Eastern KOC
includes tectonic slices of partially metamorphosed lher-
zolites, harzburgites, and serpentinites.
40
K/
40
Ar ages of
amphiboles range from 189 to 102 Ma (Khalatbari-Jafari
et al. 2003). The western KOC is composed of non-
metamorphic peridotites, layered gabbros, diabase
dikes, extensive pillow lavas, and massive sheet ows.
Figure 3. Composite stratigraphic section of the Southwestern and Northeastern basins in NW Iran with local formations name and
comments on lithology. Black stars: sampled Cretaceous and Eocene turbiditic sequences.
4A. MOHAMMADI ET AL.
Lenses of Globotruncana-bearing hemipelagic limestone
interbedded with basaltic pillow lavas are Late Cretaceous
(Santonian-Campanian; Table S2).
40
K/
40
Ar ages of plagi-
oclases from layered gabbros range from 100 to 64.9 Ma
(Khalatbari-Jafari et al. 2004). The foraminiferal content of
hemipelagic layers and the stratigraphic context in the
eld area identied the olistostrome and turbidites cover-
ing the western KOC to be of Late Cretaceous-Eocene age
(Figure 3; Lechmann 2018). They include deep oceanic
volcaniclastic turbidites, volcanic breccias, and volcano-
sedimentary rocks (Figure 3). Turbidites are thin-layered,
well-bedded, ne to medium-grained volcaniclastic sand-
stones with shale intervals (Figure 4(a)). The scarcity of
hemipelagic sediments (E member of the Bouma cycle)
and lack of ichnofossils species sedimentation below the
CCD in the lower slope and abyssal parts of an oceanic
basin (Lechmann 2018).
2.1. The Siah Cheshmeh-Khoy-Misho-Tabriz fault
system
The NW-SE trending~ 350 km long and right-lateral
SKMTF strike-slip fault is described as a unied fault
system in northwest Iran (e.g. Berberian 1997;
Karakhanian et al. 2004; Djamour et al. 2011; Rizza et al.
2013; Su et al. 2017; Figure 2), continuing towards the
northwest as the right lateral Gailatu strike-slip fault in
Turkey (e.g. Karakhanian et al. 2004; Djamour et al. 2011).
This prominent fault system consists of several en
échelon segments with numerous splay faults having
normal components (Karakhanian et al. 2004) and
bounding three 10–12 km long and up to 4–5 km wide
pull-apart basin-type structures. The SKMTF fault system
separates the southwestern volcaniclastic basin from the
northeastern terrigenous basin (Figure 2). Near the city
of Khoy (Figure 2), the SKMTF forms a large depression
and joins the Misho-Tabriz Fault (Karakhanian et al.
2004).
The SKMTF experienced several strong historical and
recent earthquakes such as the M ∼ 7.7 Tabriz earth-
quake (1721) and the M ∼ 7.3 Siah Cheshmeh earth-
quake (1976; e.g. Ambraseys and Melville 2005; Siahkali;
Moradi et al. 2011; Solaymani; Azad et al. 2015). GPS data
suggest an average slip-rate of 7.3 ± 1.3 mm yr
−1
along
the Tabriz-Marand segment (e.g. Rizza et al. 2013;
Figures 1, 2) in agreement with the 7.3 mm yr
−1
average
slip-rate calculated from the oset, INSAR measurements
and dated morphotectonic features (Rizza et al. 2013;
Mesbahi et al. 2016; Haji-Aghajany et al. 2019). According
to Mesbahi et al. (2016), the Tabriz-Marand segment is
a reactivated suture. Lechmann et al. (2018b) based on
the location of the rifting-related Late Jurassic and
Figure 4. View of sampled outcrop with sampled place (a) southwestern volcanoclastic turbiditic sandstones (sample 8), (b)
northeastern terrigenous turbiditic sandstones (sample 12), and microphotographs (crossed polarized light) of representative samples:
(c) 8- volcanoclastic turbiditic sandstone, and (d) 12- terrigenous turbiditic sandstone. MQ: monocrystalline quartz, PQ: polycrystalline
quartz, Pl: plagioclase, VL: volcanic lithoclasts, SL: sedimentary lithoclasts, ML: metamorphic lithoclasts.
INTERNATIONAL GEOLOGY REVIEW 5
subduction-related Middle Cretaceous plutonic bodies
relative to the Siah Cheshmeh-Khoy segment of SKMTF
(Figure 2(b)) concurred with this interpretation and con-
cluded that the active Siah Cheshmeh–Khoy Fault reac-
tivates the former site of subduction.
3. Methods
Thirteen medium-grained turbiditic sandstones and
seventeen hemipelagic sediments (E-member of
Bouma cycle of turbiditic sandstones, samples with pre-
x 16Az in the ETH collection) have been studied (loca-
tion Figures 2, 3, 4; Tables S1, S2). Hemipelagic
sediments were carefully selected to avoid reworked
microfossil (foraminifera and radiolaria) in the strati-
graphic order of sandstone determinations (Table S2).
These microfossils mostly conrmed the published stra-
tigraphy, with sedimentation ages ranging from Late
Cretaceous to Eocene (Alavi and Bolourchi 1975;
Aghanabati and Haghipour 1976; Haghipour et al.
1978; Shahrabi et al. 1985; Eftekhar-Nezhad et al. 1989).
We used our stratigraphic determination where micro-
fossils diered from the published stratigraphy (Table
S2). Mineral separation, modal sandstone framework,
and heavy mineral identication techniques, measure-
ments of detrital zircon LA-ICP-MS U-Pb geochronology,
and ICP-MS (Nu Instrument Ltd.) in-situ Hf isotopic ratio
were carried out at ETH Zurich. Appendix 1 describes the
analytical methods.
4. Results
4.1. Modal sandstone composition
Modal compositions and sedimentary features charac-
terize two types of turbiditic sandstones that crop out
in geographically two separate regions: volcaniclastic
sandstones to the west-southwest and terrigenous
sandstones to the north-northeast of the study area
(Figures 2, 3 and 4)(a,b). Both sandstone types are
mainly litharenite, feldspathic litharenite, and lithic
arkose (Figure 5(a)). The terrigenous sandstones are
richer (>45%) in rounded quartz grains than volcani-
clastic sandstones (Figure 4(c,d); Table S3). In both
sandstone types, quartz grains are mostly mono-
crystalline (75%) and feldspars are dominantly plagio-
clase (>83%) with minor amounts of K-feldspar
(Figures 4(c,d), and 5(b)). Rock fragments are sedimen-
tary, volcanic, and low-grade to medium-grade phyl-
lites and schists in both terrigenous and volcaniclastic
sandstones (Figures 4(c,d), 5(c,d)). Volcanic rock frag-
ments of volcaniclastic sandstones are mainly basalt,
andesite, and volcanic glass.
In the terrigenous sandstones, framework composi-
tions contain more sedimentary lithic grains than volca-
nic lithic grains, except sample number 9 (Figure 5(c,d);
Table S3). In general, lithic grains in these sandstones
display high degrees of roundness (Figure 4(d)). This
feature suggests that the detritus sourced from the recy-
cling of older sediments or long-distance transport.
Sedimentary lithic grains generally consist of limestone,
dolomite, ne-grained sandstone, and siltstone. These
features also suggest that the terrigenous sandstones
are mainly recycled from older sedimentary rocks.
A recycled orogenic source with minor transitional-
dissected arc provenance is further inferred from stan-
dard provenance triangular diagrams (Dickinson 1985;
Figure (5e,f)).
Modal compositions of the volcaniclastic sandstones
show a dominance of angular volcanic lithic grains,
except sample number 10, which contains a higher
amount of sedimentary lithics. These features suggest
a transitional-dissected arc source (Figures 4(c), 5(e,f)).
Samples 14 and 15 are calcareous-turbidites dominantly
composed of carbonate lithics (~80% of total grain
count) with minor amount of quartz grains (< 10%) and
angular volcanic lithics (~10%). These calcareous-
turbiditic sandstones point to a local Late Cretaceous
carbonate platform inuenced by volcanic activity in
the northern margin of the southwestern volcaniclastic
basin.
4.2. Heavy minerals
The heavy mineral composition of the Azerbaijan turbi-
ditic sandstones (except sample 13, not considered here
because it contains too few heavy minerals) is character-
ized as follows: (1) Pyroxene, a frequent mineral in mac
to ultramac igneous rocks, is common in the volcani-
clastic sandstones (up to 93% of total heavy mineral
grains and only up to 4% of total heavy mineral grains
in terrigenous sandstones; Table S4 and Figure 6). (2)
Ultra-stable clastic minerals (zircon, tourmaline, rutile,
sphene, and monazite; ZTR = 6–57% of total grain
count) and apatite (up to 32%; Table S4 and Figure 6)
are typically derived from acidic to intermediate mag-
matic rocks and recycled siliciclastic sandstones. (3)
Metastable minerals (epidote group, garnet, staurolite,
andalusite, chloritoid, and serpentine; 2–48% of total
counted grains; Table S4 and Figure 6) occur mainly in
medium-high grade metamorphic rocks. (4) Cr-spinel (up
to 20% of total grain count; Table S4 and Figure 6)
indicates sourcing from oceanic ultramac rocks. Heavy
mineral compositions of the southwestern volcaniclastic
sandstones (high amounts of pyroxene and hornblende
groups) are typical of detritus mostly sourced from an
6A. MOHAMMADI ET AL.
Figure 5. Detrital composition and classification of Azerbaijan sandstones in provenance discrimination diagrams: (A) Sandstone
classification diagram (Folk 1980); (B) Qm-P-K diagram (Dickinson and Suczek 1979); (C) Lvh-Ls-Lm diagram (Dickinson 1985); (D) Qp-
Lvh-Lsm diagram (Dickinson and Suczek 1979); (E) QFL (Dickinson 1985) and (F) QmFLt (Dickinson 1985).
INTERNATIONAL GEOLOGY REVIEW 7
intermediate-basic magmatic arc (Figure 7; Table S4),
while the heavy mineral spectrum of the northeastern
terrigenous sandstones, dominated by ZTR minerals
(Figure 7; Table S4), indicates sediment supply from con-
tinental felsic rocks or recycled older sedimentary rocks.
4.3. Detrital zircon U-Pb age
Detrital zircon U-Pb dating by LA-ICP-MS was conducted
on eight samples (Figures 8, 9, 10, 11; Table S5, S6). Only
three samples out of four terrigenous sandstones and
ve samples out of nine volcaniclastic sandstones con-
tain enough zircon grains to be analysed. Generally,
sandstones from the terrigenous turbidites contain
high amounts of zircon grains. The results (Figures 9,
10) call attention to dierent zircon age distribution
patterns in northeastern (samples 1, 9, and 12) and
southwestern (samples 2, 3, 4, 6, and 7) sandstones.
Bimodal shapes of the detrital zircon allow categorizing
further the terrigenous sandstones with: (a) well to
Figure 6. Relative abundance of heavy mineral species. Assemblages arranged in stratigraphical order. Samples number with filled
black coloured stars indicates northeastern basin terrigenous sandstones.
Figure 7. Heavy mineral plot comparing ratios of metamorphic (MET; garnet, epidote group), volcanic (VOL; common hornblende,
pyroxene group), and stable minerals (ZTR; zircon, monazite, tourmaline, rutile, brookite, anatase, sphene). Apatite is not included.
Numbers indicate samples number.
8A. MOHAMMADI ET AL.
medium rounded, Proterozoic, Carboniferous, Triassic,
and Jurassic zircons and (b) euhedral Cretaceous and
Palaeocene detrital zircons.
Probability peaks of zircon age populations of terri-
genous sandstones are at 2503 Ma (Paleoproterozoic),
792 Ma and 565–621 Ma (Neoproterozoic), 344–347 Ma
(Early Carboniferous), 204–236 Ma (Late Triassic),
159–161.5 Ma (Middle-Late Jurassic), 93.4 Ma (Late
Cretaceous), and 59 Ma (Palaeocene; Figures 9, 10).
Proterozoic (540–2500 Ma) zircon grains make the lar-
gest population (≥ 66% of total zircon grains). Late
Ordovician and Palaeocene zircons occur only in sam-
ples 1 and 9, respectively (Table S5).
Detrital zircons with Triassic subhedral and
Cretaceous euhedral shape and magmatic zoning char-
acterize the southwestern volcaniclastic turbidites.
Zircons in the volcaniclastic sandstones are peaking
only at 229 Ma (a few Late Triassic), 113 Ma (Early Late
Cretaceous), and 92–95 Ma (Late Cretaceous) (Figures
10,11). There is no zircon older than Triassic and younger
than Late Cretaceous in these southwestern volcaniclas-
tic turbidites (Figure 10, Table S5).
4.4. Hf isotope ratio
Hafnium isotope ratio analysis was performed on 156
dated zircon grains from eight samples. In the
terrigenous sandstones, Paleoproterozoic zircon grains
have ɛ-Hf
(t)
from +2 to −11. Neoproterozoic zircons show
a larger variation in ɛ-Hf
(t)
from +11 to −19. Zircons with
Early Carboniferous ages show a small variation in ε-Hf
(t)
from +4 to
−4
. The only Late Triassic zircons show posi-
tive ε-Hf
(t)
values of +7.8. Zircons with Middle-Late
Jurassic age show a small variation of ε-Hf
(t)
values
from +5 to −5 and Late Cretaceous zircon grains show
the largest variation in ɛ-Hf
(t)
from +11 to −17 (Figure 12;
Table S7).
In the volcaniclastic sandstones, the Hf isotopic com-
position for Late Triassic zircons have only positive ε-Hf
(t)
values of +11 to +16 and Late Cretaceous zircon grains
show the largest variation in ɛ-Hf
(t)
from +13 to −22 and
even a grain with +19 (Figure 12; Table S7). Negative ɛ-
Hf
(t)
values suggest magmatic zircons with a continental
crust signature (e.g. Vervoort and Blichert-Toft 1999) and
positive ɛ-Hf
(t)
values suggest magmatic zircons with
depleted mantle signature (e.g. Patchett 1983).
5. Discussion
Sandstone modal framework, heavy mineral composi-
tions, and U-Pb geochronology with in-situ Hf isotope
data of detrital zircons enhance our knowledge of the
tectonic setting and sources of the Late Cretaceous-
Eocene sandstones in Azerbaijan Province (NW Iran).
Figure 8. Representative cathodoluminescence images of dated detrital zircons of the northeastern terrigenous (samples 01, 09, 12)
and southwestern volcaniclastic (samples 02, 03, 04, 06, 07) sandstones. C = coarse grain, M = medium grain, and F = fine-grain
zircons. White circles show the laser spots positions for U-Pb age analyses.
INTERNATIONAL GEOLOGY REVIEW 9
5.1. Sandstone framework analysis
The northeastern terrigenous sandstones are dominantly
composed of sedimentary lithic fragments with minor vol-
canic and metamorphic lithics (Figure 5(c,d)) which denote
mixed recycling and magmatic arc provenance. Rounded
lithic fragments (>80% of total grain count) and a high
amount of mono-crystalline quartz are consistent with
a recycled orogenic source (Figure 5(e,f)). The recycled
orogenic source was previously identied for Upper
Devonian sandstones in the northeastern terrigenous
basin (Najafzadeh et al. 2010). We reach the same
Figure 9. Detrital zircon
206
Pb/
238
U age population probability density diagrams with corresponding Concordia plots of concordant
detrital zircons of the samples 1, 9, and 12 from eastern turbiditic sandstones. Ages with discordance >5% are not considered.
10 A. MOHAMMADI ET AL.
conclusion for the Cretaceous-Eocene, terrigenous sand-
stones, which establishes the long recycling history of the
continental Central Iran Block, to the northeast of the Siah
Cheshmeh-Khoy-Misho-TabrizFault.
The southwestern volcaniclastic sandstones are dom-
inantly composed of angular volcanic lithic fragments
with subordinate sedimentary and metamorphic lithics
that specify a magmatic arc provenance (Figure 5(c,d)).
Figure 10. Detrital zircon
206
Pb/
238
U age population probability density diagrams with corresponding Concordia plots of concordant
detrital zircons of samples 2, 3, 4, 6, and 7 from western turbiditic sandstones. Ages with discordance >5% are not considered.
INTERNATIONAL GEOLOGY REVIEW 11
Figure 11. Zircon U-Pb age distribution pattern of Azerbaijan sandstone samples.
Figure 12. Time-corrected ɛ-Hf
(t)
values versus
206
U/
238
Pb ages (Ma) of Azerbaijan detrital zircons. Depleted Mantle evolution trend
(dashed line) from Griffin et al. (2000) and Chondritic Uniform Reservoir values (CHUR) from Blichert-Toft and Albarède (1997). The
evolved epsilon hafnium (ε-Hf
(t)
) data from zircons older than 2600 Ma are not shown.
12 A. MOHAMMADI ET AL.
Angular volcanic lithic fragments indicate a short trans-
port distance from the source to the sink.
5.2. Heavy mineral assemblage
The rst appearance of Cr-spinel in Turonian (Late
Cretaceous) sandstones (samples 3 and 12) indicates
that ultramac rocks, likely from the KOC (Figures 1, 2),
were being eroded at that time, with detritus reaching
both turbiditic basins.
Heavy mineral spectra of the northeastern terrigenous
sandstones are dominated by ZTR and apatite with minor
amounts of metamorphic minerals. Rounded shapes of
>90% zircon grains, tourmaline, and rutile indicate long
transport distances from the source to sink or recycling of
older sedimentary rocks. Metamorphic heavy minerals
most probably derive from the Proterozoic-Cretaceous
basement rocks of the source area in the Sanandaj-Sirjan
and Central Iran zones (Figure 2). The euhedral shape of
the Late Cretaceous zircon grains with magmatic zoning
typies a proximal synsedimentary magmatic rock source.
The heavy mineral assemblage of the volcaniclastic
turbidites is dominated by pyroxene and epidote groups,
minor zircon, and rare Cr-spinel (Figures 6, 7; Table 4).
Unlike in the northeastern turbidites, euhedral shapes of
pyroxene, epidote, and zircon grains disclose short trans-
port distances from the source to the sink. Pyroxene
indicates mac magmatic rocks as major provenance.
Epidote suggests medium-grade metamorphic rocks and
basic magmatic rocks as a secondary source.
5.3. Detrital zircon U-Pb ages and in-situ Hf
isotopes
5.3.1. Northeastern turbidites
Northeastern terrigenous sandstones are characterized
by a wide range of detrital zircon ages. The asynchro-
nous occurrence of zircons in the Late Cretaceous-
Eocene sandstones suggests repetitive recycling of
magmatic zircons (Figure 13). In contrast to the south-
western volcaniclastic turbidites, Cretaceous zircons
are minor components.
5.3.1.1. Proterozoic zircons.. Paleoproterozoic (-
1732–2570 Ma) and Neoproterozoic (541–1058 Ma) zir-
cons have ε-Hf
(t)
values (−19 to +11) typical of
continental crust (Figure 12; Table S5). Neoproterozoic-
Cambrian detrital zircons U-Pb ages have been
reported from Sanandaj-Sirjan and Central Iran zones
(Horton et al. 2008; Hassanzadeh et al. 2008), Makran
(Mohammadi et al. 2016a, 2017) and Sistan basins
(Mohammadi et al. 2016b). Neoproterozoic zircons of
granitic gneiss in the study area and granite and
orthogneiss of the Sanandaj-Sirjan basement, to the
south of the study area, yielded ε-Hf
(t)
from −7 to
+13 and −18.2 to +10.7 respectively (Nutman et al.
2013; Moghadam et al. 2015), similar to clastic zircons
of the Azerbaijan turbidites. The similarity in U-Pb
crystallization ages and Hf isotopic compositions indi-
cates that the basement of northeastern Azerbaijan
and the Sanandaj-Sirjan Zone are composed of con-
tinental fragments with Gondwana aliation, although
Figure 13. Azerbaijan sandstones stratigraphic age versus detrital zircon mean U-Pb age populations.
INTERNATIONAL GEOLOGY REVIEW 13
they are separated by the volcaniclastic turbidite basin
and the Khoy ophiolites.
5.3.1.2. Carboniferous zircons.. Carboniferous detrital
zircons (Figure 9; Table S5) with mixed positive and
negative ε-Hf
(t)
(−4 to +4.3; Figure 12; Table S6) are
typical for rift-related magmatic rock signatures (e.g.
Vervoort and Blichert-Toft 1999). Carboniferous conti-
nental break-up and rifting on the northern edge of
Gondwana is recorded by Sm-Nd isotopic compositions
and in-situ Hf isotopic compositions of Carboniferous
(zircon U-Pb) gabbroic and A-type granitic intrusions in
Azerbaijan (e.g. Bea et al. 2011; Saccani et al. 2013;
Moghadam et al. 2015; Mohammadi et al. 2020). Early
Carboniferous detrital zircons in the northeastern terri-
genous sandstones point to rifting-related magmatic
activity during the Carboniferous (e.g. Saccani et al.
2013; Moghadam et al. 2015; Mohammadi et al. 2020).
5.3.1.3. Triassic zircons.. The only in-situ ε-Hf
(t)
values
(+7.8; Figure 12; Table S6) obtained on Late Triassic
detrital zircons (Figure 9; Table S5) is an oceanic crust
signature. This scarce information suggests that the
Triassic detritus of terrigenous sandstones was supplied
from a local Triassic intermediate or mac protolith.
Supportively, Late Triassic diabase as a potential source
rock crop out in the north of the study area (northeast of
sample 16Az1, Figure 2; Eftekhar-Nezhad et al. 1989).
5.3.1.4. Jurassic zircons.. The ε-Hf
(t)
values (−5 to +4.7;
Figure 12; Table S6) of Jurassic detrital zircons (Figure 9;
Table S5) suggest rift-related magmatic rocks (e.g.
Vervoort and Blichert-Toft 1999) or recycled sediment
as the source. Lechmann et al. (2018b) reported conti-
nental rifting-related Middle Jurassic (154–159 Ma, zir-
con U-Pb age) granitic and granodioritic intrusions in the
NW Iran (Figure 2(b)). Most possibly these Jurassic intru-
sions supplied detrital zircons to the northeastern basin.
Furthermore, Azizi and Stern (2019), based on geochem-
istry of several Middle Jurassic magmatic intrusions in
the northern part of the Sanandaj-Sirjan Zone, con-
cluded that the Jurassic igneous activity occurred in
a continental rift setting. However, the other potential
source for the Jurassic zircons and detritus would be an
extensive Upper Triassic-Middle Jurassic siliciclastic suc-
cession, as it is known in the Azerbaijan region and
Alborz Mountain (Asadian and Eftekhar-Nezhad 1993).
According to Fürsich et al. (2009) the Middle Jurassic part
of the siliciclastic sequence indicates increasing subsi-
dence rates in a Neo-Tethys back-arc rift-basin, consis-
tent with extension recorded around the Central Iranian
Block (e.g. Şengör 1990; Hunziker et al. 2015).
5.3.1.5. Cretaceous zircons.. Unlike in the southwes-
tern turbidites, Cretaceous zircons are a minor popula-
tion in northeastern sandstones. Compared to the older
zircons, Cretaceous zircons are almost euhedral, indicat-
ing a short transport distance. Their magmatic zoning
and medium to high Th/U ratios (0.35–1.4) indicate an
intermediate to basic magmatic source (Teipel et al.
2004). Like for the southwestern turbidites, Cretaceous
zircons of the northeastern turbidites mostly have mixed
negative and positive ε-Hf
(t)
values (−8 to +11.5; Figure
12; Table S6) typical of a subduction-related continental
magmatic arc (e.g. Patchett 1983). Similar zircon ages
and Hf isotopic compositions of both western and east-
ern turbidites represent a common source, a continental
magmatic arc between two basins supplying Late
Cretaceous detritus in both basins. Lechmann et al.
(2018b) dated several plutonic bodies and associated
volcanic rocks with Middle Cretaceous (112–96 Ma, zir-
con U-Pb age) ages (Figure 2(b)). Based on geochemistry
and Sr-Rb and Nd-Sm isotopic data they concluded that
these rocks were produced in a continental arc setting
by subduction of the Khoy Ocean beneath the Central
Iranian continental block. Geographically, all Middle
Cretaceous plutonic rocks occur only to the east of the
Siah Cheshmeh-Khoy fault system. Furthermore, our
unpublished data shows Middle Cretaceous gabbroic
(97 Ma) and granitic (96 Ma) intrusions in the east of
the Misho-Tabriz fault system.
5.3.1.6. Palaeocene.. Palaeocene zircons as a major
population appear only in sample number 9 in the
northeastern sandstones (Figure 9). Their absence in
the other samples indicates that they derive from
a local source. Palaeocene volcanic rocks with andesitic
composition in the northeastern basin (Eftekhar-Nezhad
et al. 1989) is the possible source for these detrital
zircons.
5.3.2. Southwestern turbidites
The southwestern volcaniclastic turbidites are character-
ized by Triassic and Cretaceous detrital zircon ages. In
contrast to the northeastern terrigenous sandstones,
Cretaceous zircons are major components.
5.3.2.1. Triassic zircons.. Positive Hf isotopic composi-
tions (+11 to +16.5; Figure 12, Table S6) of Triassic (225–-
251 Ma; main peak at 229 Ma; Figure 10, Table S5)
detrital zircons indicate an oceanic crust signature, in
consistency with Th/U ratios (0.9–1.3; Table S5) of the
same detrital zircons (Teipel et al. 2004). The previously
mentioned Triassic diabases, to the north of the eld
area, are excluded as a possible source because they
are Late Triassic while the clastic zircons of the
14 A. MOHAMMADI ET AL.
southwestern turbidites are all Early Triassic. The geo-
graphically closest equivalent is the Early-Middle Triassic
magmatic-metamorphic belt in northern Turkey, attrib-
uted to the collision of a Triassic oceanic plateau, called
Nilüfer Unit, with the continental margin of Laurasia
(Okay 2000). We hypothesize that the analysed Triassic
zircons are sourced from this belt. However, the large
distance between the studied Azerbaijan basin and the
magmatic-metamorphic belt in Turkey leaves room for
other, yet unknown possible sources.
5.3.2.2. Cretaceous zircons.. Cretaceous zircons (-
123–81 Ma) display a major age peak at 92–95 Ma and
a subordinate peak at 113 Ma (Figure 10; Table S5).
Zircons with euhedral shapes, magmatic zoning, and
Th/U ratios between 0.32 and 1.8 (Table S5) experienced
a short transport distance and were supplied from inter-
mediate to mac magmatic rocks. They dominantly have
mixed negative and positive ε-Hf
(t)
values (−22 to +19.4;
Figure 12, Table S6) typical for subduction-related mag-
matism in continental arcs (e.g. Patchett 1983; Blichert-
Toft and Albarède 1997). The exposed Urmia Dokhtar
Magmatic Arc (present-day denition; e.g. Omrani et al.
2008) might represent a possible source. However, zir-
cons from this arc yield two main age intervals: Late
Cretaceous (81–72 Ma) and Middle Eocene-Late
Miocene (45–6 Ma; Chiu et al. 2013). The main Late
Early Cretaceous (113 Ma) and Late Cretaceous (92–-
95 Ma) age peaks of the detrital zircons we have mea-
sured in the southwestern volcaniclastic turbiditic
sandstones are older and not known in the UDMA
which, therefore, did not supply the southwestern
turbiditic detritus. Azizi and Jahangiri (2008) introduced
subduction-related Cretaceous calc-alkaline volcanic
rocks in the northern part of the Sanandaj-Sirjan Zone.
This might be another possible source for volcaniclastic
turbidites. However, the Neoproterozoic granite
(544 Ma) and granodiorite (551 Ma; Hassanzadeh et al.
2008) and the Late Carboniferous granites and gabbro-
norites (320 Ma; Shafaii Moghadam et al. 2015) and
granitoids (315 Ma; Bea et al. 2011) crop out in the
Northwestern Sanandaj-Sirjan zone. The lack of
Proterozoic and Palaeozoic zircon grains in volcaniclastic
turbidites exclude the northwestern Sanandaj-Sirjan
zone as a source of volcaniclastic detritus.
Instead, this detritus represents the deeply eroded
Cretaceous ‘Azerbaijan Continental Magmatic Arc’ pro-
duced by subduction beneath the Central Iranian Block
of a Neo-Tethys oceanic basin whose remnants could be
the western Khoy Ophiolites. Exposures of remnant arc
were identied by Lechmann et al. (2018b). Due to the
location of ‘Azerbaijan Magmatic Arc’ to the east of the
SKMTF system, the corresponding euhedral zircons and
detritus travelled a short distance to both northeastern
and southwestern basins.
6. Tectonic implications
Remnants of Late Cretaceous basins in northwestern Iran
are former branches of the Neo-Tethys Ocean (e.g.
Stöcklin 1974; Alavi 2007). Closure of remaining Neo-
Tethys branches occurred in Late Palaeocene-Early
Eocene, followed by the closure of the Neo-Tethys
along the Zagros Suture Zone in the Late Oligocene
Figure 14. Schematic model of Southwestern and Northeastern turbiditic basins and ‘Lost’ Azerbaijan Magmatic Arc; (a) in the Late
Cretaceous and (b) present time (A-A` profile of Fig. 1). Black arrows indicate sediment supply direction. SKMTF: Siah Cheshmeh-Khoy-
Misho-Tabriz Fault.
INTERNATIONAL GEOLOGY REVIEW 15
(e.g. Pirouz et al. 2017). Two separate Late Cretaceous-
Palaeocene subduction-related magmatic sequences
have been reported on both sides of the Tabriz Fault
(Jamali et al. 2012). Based on diverging petrogenesis and
reverse age zonation of these magmatic rocks (age
increasing on both sides with distance from the Tabriz
Fault), Jamali et al. (2012) suggested that the Tabriz fault
is the lithospheric boundary between these two mag-
matic sequences and related sedimentary basins (Figure
14(a)). Their interpretation is strengthened by several
Middle Cretaceous continental arc-related plutonic
bodies all along the eastern side of the SKMTF system
(Lechmann et al. 2018b; Figure 2(b)). The continuation of
these plutonic bodies towards the east consists of sev-
eral Upper Cretaceous, subvolcanic mac to intermedi-
ate magmatic rocks along the Tabriz Fault system
(Asadian and Eftekhar-Nezhad 1993; Asadian et al.
1995; Behrouzi et al. 1997; Mesbahi et al. 2017). In addi-
tion, the sequence of oceanic sediments in southwestern
volcaniclastic turbidites are associated with red-coloured
deep oceanic radiolarite (Khalatbari-Jafari et al. 2004).
These records are evidence for a deep oceanic basin
along SKMTF. This fault system could be a major, intra-
continental strike-slip contact like the large strike-slip
faults in Asia (e.g. Molnar and Tapponnier 1975).
However, the western Khoy Ophiolites suggest that
there was an oceanic basin; the whole-rock and mineral
geochemistry of EKOC indicate that the polymeta-
morphic EKOC is a part of the exhumed subcontinental
lithospheric mantle of the south passive margin of the
Central Iranian Block. In addition, the presence of ductile
and brittle normal faults along the boundary of the
lherzolitic upper mantle and the overlying crystalline
rocks in the EKOC are consistent with lithospheric thin-
ning and break-up of a continental margin (Lechmann
2018). Although, the whole-rock and mineral geochem-
istry, as well as an almost complete succession of harz-
burgites, isotropic and layered gabbros, pillow lavas with
interbedded pelagic limestone, suggests the formation
of WKOC in an oceanic lithosphere (Lechmann 2018;
Figure 14) where the volcaniclastic turbidites have
been accumulated. In that case, the SKMTF is
a reworked suture.
GPS interseismic velocities classied the Tabriz Fault
as an intra-mountain fault with a locking depth of
24 km (Vernant 2015). However, deep earthquakes
(>35 km) indicate that this fault is cutting the whole
crust. Our work shows that the SKMTF separates two
basins that evolved dierently on two separate tec-
tonic settings during Cretaceous-Eocene times: a deep
marine basin to the southwest, and an epicontinental
basin to the northwest. The scarcely preserved
Azerbaijan Magmatic Arc (Figure 14(a)) was placed
along the trace of the SKMTF, which makes it
a lithospheric boundary variously reactivated at dier-
ent times in the active Arabia-Eurasia collision zone.
Continued faulting into present days has created the
mapped tectonic contact between the continental
Miocene (to the northeast) and Quaternary (to the
southwest) sedimentary basins (Berberian and Arshadi
1976).
7. Conclusion
Sandstone frameworks show that the Siah
Cheshmeh-Khoy-Misho-Tabriz Fault juxtaposes two
Cretaceous-Eocene turbiditic basins formed in dier-
ent tectonic environments. To the northeast, terrige-
nous sandstones were sourced from metamorphic,
magmatic, and sedimentary rocks of a continental
basement as old as early Proterozoic and represent
a marginal basin. The provenance characteristics of
the southwestern and northeastern sedimentary
rocks are consistent with a deep oceanic basin (a
subordinate arm of Tethys Ocean) and Azerbaijan
marginal basin respectively. The southwestern turbi-
dites were mostly sourced from a ‘missing’
Azerbaijan Continental Magmatic and represent
a deep oceanic, forearc basin. Heavy mineral assem-
blages and Cr-spinel in both western and eastern
sandstones imply ophiolite as a subsidiary detrital
source since the Turonian. The last recorded activity
of the ‘lost’ Azerbaijan Continental Magmatic Arc
took place in the Middle Palaeocene and indicates
that the complete closure of the Khoy Ocean in the
study area occurred soon after. The Siah Cheshmeh-
Khoy-Misho-Tabriz Fault overprints a lithospheric
boundary along which the Cretaceous arc, mostly
witnessed by clastic material, has been removed by
erosion and/or subduction.
Acknowledgments
This study was funded by the Forschungsreserven/Burg J.-P
(grant no. 2-78159-99) and the Swiss National Fund, grants no.
2-77644-09 and 2-77979-14. We thank the Geological Survey of
Iran and Mohammad Faridi for logistic support during eld-
work. We sincerely thank Anna Lechmann for help in eld and
laboratory work. Thanks are due to Isabella Premoli-Silva for
fossil determination, Yannick Buret for help in Hf isotopes
processing, Karsten Kunze for his assistance on CL and BSC
imaging, and Remy Lüchinger for the thin sections.
Disclosure statement
No potential conict of interest was reported by the authors.
16 A. MOHAMMADI ET AL.
Funding
This work was supported by the Swiss National Fund [grants
no. 2-77644-09,grants no. 2-77979-14.]; Forschungsreserven/
Burg J.-P [grant no. 2-78159-99].
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Appendix 1
1. Sandstone thin section
Modal framework grain analysis of sandstones was per-
formed by applying the Gazzi-Dickinson method on thin sec-
tions stained for feldspars and carbonates (Dickson 1966;
Norman 1974). More than 300 detrital grains were counted in
each thin section for statistical reliability (Folk 1980). The Zua
method has been used to count lithic fragments (Zua 1985).
Minerals larger than 0.063 mm within rock fragments were
counted as monomineralic grains. Results were converted to
percentages for compositional comparison (table SI3; Weltje
and von Eynatten 2004). Data are displayed in ve standard
complementary triangular diagrams (QFL, QmFLt, LvhLsLm,
QmPK, and QpLvLsm; Folk 1980; Dickinson 1985).
2. Heavy mineral separation
Approximately 2–3 kg of fresh rock was collected from each
sample for heavy mineral analysis. To obtain transparent heavy
mineral fractions (density ≥ 2.9 g/cm3 and typically < 1% of the
bulk rock), sandstones were crushed with the SelFrag appara-
tus batch equipment (in ETH-Zurich) using high voltage (130–-
150 kV) pulse power technology, which liberates
morphologically intact minerals. From the < 2 mm sieved
fraction, carbonate was dissolved in warm (60–70°C) 10%
acetic acid. Heavy minerals were pulled out in separation
funnels (Mange and Maurer 1992) from 0.063 mm to 0.4 mm
sieve fractions using bromoform (density 2.88 g/cm
3
). The bulk
heavy mineral fractions were mounted in piperin (Martens
1932) between a glass slab and a cover. Identication and
quantication were carried out under a petrographic micro-
scope by applying the mid-point ribbon and eet counting
methods. At least 200 grains were counted per sample (Mange
and Maurer 1992; Table S4).
3. Detrital zircon separation and dating
Zircons were extracted from 2–3 kg fresh sandstones with
acoustic shockwaves produced in the SelFrag apparatus, ETH-
Zurich. Standard Holman-Wiley shaking tables and magnetic
separation methods (Frantz magnetic separator) delivered
mineral fractions enriched in zircons. Zircons of grain fraction
60–250 μm were handpicked under a binocular microscope
using the non-magnetic fraction. Selected grains were
mounted in U- and Pb-free resin, polished to expose the inter-
nal parts of the minerals, and coated with carbon. Suitable
zircon domains and inclusions identied with cathodolumines-
cence (CL) and back scattered-electron (BSE) images obtained
by split-screen on a CamScan CS44 Scandunk Viermalning
electron microscope (SEM; Tescan a.s., Brno) at ETH- Zurich,
then analysed with Laser Ablation Inductively Coupled Mass
Spectrometry (LA-ICP-MS) at ETH Zurich with the Excimer laser
(ArF 193 nm). Zircon’s characteristics were interpreted follow-
ing Corfu et al. (2003).
The Laser ablation ICP-MS analyses were performed at the
Institute of Geochemistry and Petrology, ETH Zurich using
a RESOlution (Australian Scientic Instruments/Applied
Spectra) laser ablation system equipped with a dual-volume
S-155 abFlation cell (Laurin Technic, Australia), coupled to an
Element XR (Thermo Scientic, Germany) sector-eld ICP-MS.
We used a laser repetition rate of 5 Hz, the energy density of ca.
2.5 J/cm2 and a spot diameter of 30 μm. The carrier gas
consisted of high-purity He (ca. 0.7 L/min) and make-up Ar
(ca. 1 L/min) from the ICP-MS. Data acquisition time per spot
was about 1 minute (30 s gas blank + 30 s ablation). The data
were processed with the Igor Pro Iolite v2.5 software
(Hellstrom et al. 2008), using the VizualAge data reduction
scheme (Petrus and Kamber 2012). Laser-induced element
fractionation (corrected after Paton et al. 2010), instrumental
mass discrimination and drift through the analytical session
were corrected by standard bracketing against zircon refer-
ence material GJ-1 (using isotope ratios recommended by
Horstwood et al. 2016). The quoted uncertainties for each
analysis correspond to the internal error and propagated
uncertainty based on the scattering of the primary reference
material (see Paton et al. 2010). The accuracy and reproduci-
bility within each run of analysis were monitored by periodic
measurements of zircon reference materials Plešovice (337 Ma;
Sláma et al. 2008) and 91,500 (1065 Ma; Wiedenbeck et al.
1995). More details about the analytical conditions, the data
from unknowns, and reference materials are provided in Table
S6. Concordia diagram was performed using ISOPLOT v.3.0
(Ludwig 2003). A concordant age is given by the overlapping
of the error ellipse with the Concordia age curve. In this study,
ages with discordance >5% are not considered for further
interpretation (Table S5). The frequency U-Pb age distribution
diagram or probability density plot described by Ludwig (2003)
includes a histogram representing the number of individual
zircon grains within a short age range and the probability
curve depicts the mean age peaks of the contained age popu-
lations in one sample.
4. In-situ Hafnium isotope ratios
In-situ Hf isotope analysis was performed on a Nu plasma
MC-ICP-MS (Nu instrument Ltd) attached to a 193 nm UV ArF
Excimer laser, at the ETH Zurich on the same zircon grains used
for U-Pb dating. Ablation was carried using He as a sweep gas
20 A. MOHAMMADI ET AL.
with a ow rate of 0.8–1.11 l/min and combined with Ar (~0.7 l/
min) using a 40 µm spot size and a 5 Hz laser pulse repetition
rate. The energy density used was 10–20 J cm
22
and each
ablation was preceded by a 40-second background measure-
ment, and ablated zircon was measured within 60 s. Lutetium
and Yb were analysed to correct for isobaric interferences on
176
Hf using
173
Yb/
176
Yb = 0.79618 and
175
Lu/
176
Lu = 0.026549
(Chu et al. 2002). The βHf and βYb mass bias coecients were
calculated using an exponential law from measured
179
Hf/
177
Hf
and
173
Yb/
171
Yb respectively and using natural abundance
reference values (
179
Hf/
177
Hf = 0.7325;
173
Yb/
171
Yb = 1.132685; (Chu et al. 2002). The Lu mass bias
fractionation was assumed to be the same as Yb. The accuracy
and precision of the data obtained was monitored through the
systematic measurements of the well-characterized Temora-2
(0.282686; Woodhead and Hergt 2005), Mud Tank (0.282507;
Woodhead and Hergt 2005), and Plešovice (0.282482; Sláma
et al. 2008) reference natural zircon samples with known Hf
isotopic compositions. The standard reference materials were
chosen to have a range in Yb/Hf ratios to test the accuracy of
the
176
Yb correction following the protocols of Fisher et al.
(2014). Repeated standard analysis yielded results for the ana-
lytical session: Temora-2 = 0.282683, n = 37;
Plešovice = 0.282474, n = 17; Mud Tank = 0.282494, n = 21,
which are good agreement with the published (table SI7;
Blichert-Toft and Albarède 1997).
INTERNATIONAL GEOLOGY REVIEW 21
