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Jurassic to Cenozoic Magmatic and Geodynamic
Evolution of the Eastern Pontides and Caucasus
Belts, and Their Relationship With the
Eastern Black Sea Basin Opening
Marc Hässig
1
, Robert Moritz
1
, Alexey Ulianov
2
, Nino Popkhadze
3
,
Ghazar Galoyan
4
, and Onise Enukidze
5
1
Department of Earth Sciences, University of Geneva, Geneva, Switzerland,
2
Institute of Earth Sciences, University of
Lausanne, Lausanne, Switzerland,
3
Al. Jalenidze Institute of Geology, Ivane Javakhishvili Tbilisi State University, Tbilisi,
Georgia,
4
Institute of Geological Sciences, National Academy of Sciences of Armenia, Yerevan, Armenia,
5
M. Nodia
Institute of Geophysics, I. Javakhishvili Tbilisi State University, Tbilisi, Georgia
Abstract The magmatic arcs of the Eastern Pontides and Lesser Caucasus lie in continuation from one
another. A comparison of the subduction‐related magmatic rocks outcropping throughout this segment
of the Northern Tethyan belt exhibits chronological disparities, questioning the common subduction history
of the Eastern Pontides and the Lesser Caucasus regions. New data and observations including
geochronological and geochemical data, relative to subduction‐to collision‐related magmatic rocks, argue a
novel paleogeographic reconstruction illustrating Mesozoic and Cenozoic evolution of this region.
Jurassic to Early Cretaceous arc magmatism runs mainly from the Sochi‐Ritsa/Bechasyn regions (Greater
Caucasus) toward the southeast to the Alaverdi region and further into the Lesser Caucasus. Late Cretaceous
and Cenozoic arc magmatism is evidenced throughout the Eastern Pontides extending through the
Bolnisi region to the Lesser Caucasus arc. East to west, Jurassic to Early Cretaceous and Late Cretaceous to
Cenozoic portions of arc split to the north and south of the Eastern Black Sea, respectively. Throughout
Cretaceous subduction, this segment of the magmatic arc of the Southern Eurasian margin was torn in two
due to the oblique opening of the Eastern Black Sea as a back‐arc to intra‐arc basin, from west to east.
This reconstitution implies that the Jurassic‐Early Cretaceous subduction‐related magmatic rocks of the
Greater Caucasus are remnant portions of the Eastern Pontides and Lesser Caucasus arcs. This infers the
emplacement of subduction‐to collision‐related magmatic rocks throughout the Mesozoic and Cenozoic
along the entire Southern Eurasian margin is solely due to a single long‐lasting north dipping subduction.
1. Introduction
The Eastern Pontides (EP) and the Lesser Caucasus (LC) regions are in continuation from one another, from
west to east, as illustrated by the similar geology of the EP and Somkheto‐Karabagh (SK) magmatic arcs, the
Izmir‐Ankara‐Erzincan suture (IAES) and Amasia‐Sevan‐Akera suture (ASAS) zones, and the correlated
ophiolitic nappe extending from northern Turkey to Armenia, Eastern Azerbaijan and northeastern Iran
(Figure 1). The geology of the EP and the LC is generally accepted as resulting from the successive subduction
of Paleotethyan and Neotethyan realms toward the north, below the Southern Eurasian margin along a com-
mon and continuous plate boundary (Adamia et al., 2011; Barrier et al., 2018; Dercourt et al., 1986; Khain,
1975; Saintot et al., 2006; Saintot & Angelier, 2002; Sosson et al., 2017; Stampfli et al., 2001). Yet
subduction‐related magmatic rocks cropping out throughout the EP and the LC are diachronic between
western and eastern sectors, respectively (Figure 2; ESD 1). The EP preserve mainly Late Cretaceous and
Cenozoic ages, with very few occurrences of Jurassic to Early Cretaceous magmatic rocks, whereas the LC
preserves Jurassic to Early Cretaceous, Late Cretaceous, and Cenozoic ages (Figure 2, and references
therein).
An alternative model considers the Black Sea as a relic of the Paleotethys realm, which mostly disappeared
due to a south dipping subduction just north of the EP and LC throughout the Mesozoic and the early
Cenozoic (Bektaş, 1986; Eyüboğlu, 2010, 2015; Eyüboğlu et al., 2013, 2017, 2018, 2019; Liu et al., 2018). In
this alternative setup, the Northern Neotethyan realm opened in a back‐arc configuration of this south
©2020. American Geophysical Union.
All Rights Reserved.
RESEARCH ARTICLE
10.1029/2020TC006336
Special Section:
Tethyan Dynamics: From
Rifting To Collision
Key Points:
•Early Jurassic‐early Cenozoic
subduction/collision‐related
magmatic arc emplacement is due to
a single north dipping subduction
•Early Cretaceous roll‐back and
Eastern Black Sea opening caused
gradual separation of a remnant arc
from the active supra‐subduction
zone
•Jurassic arc‐related magmatic of the
Greater Caucasus belong to the arc
of the north dipping subduction
below the Southern Eurasian
margin
Supporting Information:
•Supporting Information S1
•Figure S1
•Figure S2
•Figure S3
•Figure S4
•Table S1
•Table S2
•Table S3
Correspondence to:
M. Hässig,
marchassig@gmail.com
Citation:
Hässig, M., Moritz, R., Ulianov, A.,
Popkhadze, N., Galoyan, G., &
Enukidze, O. (2020). Jurassic to
Cenozoic magmatic and geodynamic
evolution of the Eastern Pontides and
Caucasus belts, and their relationship
with the Eastern Black Sea Basin
opening. Tectonics,39, e2020TC006336.
https://doi.org/10.1029/2020TC006336
Received 2 JUN 2020
Accepted 7 SEP 2020
Accepted article online 16 SEP 2020
HÄSSIG ET AL. 1of41
Figure 1. (a)Tectonic map of the Middle East‐Caucasus area, with main blocks and suture zones, after Hässig et al. (2019). Location of Figure 1b is indicated.
(b) Structural map of North‐Eastern Anatolia, the Lesser Caucasus, and Greater Caucasus, after Ali‐Zade (2005), Hässig, Rolland, Sosson, Galoyan, Müller, et al.
(2013) and Mauvilly et al. (2018), modified. The location of Figure 3 and the locations of additional published geochemical and chronological data to this study
are indicated by the red and white outlined numbers and letters: (1) this study, Figure 3; (2) Mederer et al. (2013); (3) Sadikhov and Shatova (2016, 2017);
(4) Calder et al. (2019); (5) Hess et al. (1995); (6) McCann et al. (2010); (7) Aydınçakır and Şen (2013); (8) Karsli et al. (2012); (9) Dokuz et al. (2010); (10) Moore
et al. (1980); (11) Eyüboğlu (2010); (12) Eyüboğlu et al. (2013) and Altherr et al. (2008); (13) Kaygusuz and Öztürk (2015); (14) Yilmaz‐Sahin (2005); (15)
Boztuğand Harlavan (2008); (16) Eyüboğlu, Santosh, and Chung (2011); (17) Kaygusuz and Aydinçakir (2011); (18) Kaygusuz et al. (2014); (19) Arslan and
Aslan (2006); (20) Kaygusuz et al. (2013); (21) Aydin (2014); (22) Kaygusuz et al. (2009, 2010) and Kaygusuz and Şen (2011); (23) Karsli et al. (2011); (24) Sipahi
et al. (2018); (25) Karsli et al. (2010); (26) Eyüboğlu et al. (2014); (27) Eyüboğlu, Santosh, Dudas, et al. (2011); (28) Topuz et al. (2011); (a: Bademli, b: Meydanlı,
c: Meşebaşı, d: Cakırbağ, e: Üzengili, f: Arslandede, g: Kozluk, h: Sorkunlu, i: Sarıçiçek, j: Kaletaş, k: Dölek, l: Çevrepınar, m: Tamdere, and n: Kösedağ)
Eyüboğlu et al. (2017); and (o: Ispur‐Ulutaş, p: Güzelyayla, q: Emeksen, and u: Elbeyli) Delibaşet al. (2016).
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Figure 2. Summary of geochronological data from this study (Alaverdi and Bolnisi districts) and published studies of magmatic rocks from the Eastern Pontides
the Lesser Caucasus and the Sochi‐Ritsa/Bechasyn region of the Greater Caucasus. Locations of the published studies indicated on Figure 1b.
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dipping Paleotethyan subduction just south of the EP and SK arcs (Eyüboğlu et al., 2019; Galoyan
et al., 2018). The relative absence of Jurassic and Early Cretaceous subduction‐related magmatism in the
EP is ascribed to back‐arc rifting in relation to slab roll‐back and break‐off during these times
(Bektaş, 1986; Eyüboğlu, 2010; Eyüboğlu et al., 2007). Afterwards, during the Late Cretaceous, north
dipping Northern Neotethyan subduction initiated south of the EP and SK arcs all the while south
dipping Paleotethyan subduction resumed north of the EP and SK arcs. In this model, Late Cretaceous to
Cenozoic magmatism throughout the EP and LC is due to both south and north dipping subductions, jointly.
This study provides new laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS) zircon
ages and geochemical characterisations of subduction‐related magmatic rocks of the Alaverdi district and of
the Bolnisi district, which is the northwestern extent of the SK arc of the LC and just northwest of the
Alaverdi district, respectively (Figures 1b and 3). Samples of the Alaverdi district have Jurassic ages and
those of the Bolnisi district have Late Cretaceous and Cenozoic ages. There are limited occurrences of
Figure 3. Geological map of the Bolnisi and Alaverdi districts, after Moritz et al. (2016) and Calder et al. (2019), respectively, modified. Locations of newly
analyzed samples are indicated.
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Jurassic and Early Cretaceous magmatism in the EP, yet ages argue a common subduction history of the EP
with the LC region from the Late Cretaceous on. Subduction‐related magmatic rocks north of the Eastern
Black Sea (EBS), within the Southern Slope (SS) of the Greater Caucasus (GC) and along the Main
Caucasus Thrust (MCT) in the Sochi‐Ritsa/Bechasyn regions provide evidence of Jurassic magmatism
(Hess et al., 1995; McCann et al., 2010). Furthermore, recent interpretations of seismic profiles of the EBS
(Nikishin et al., 2015a, 2015b) illustrate the presence of continental rift structures constrained to Early
Cretaceous times and successive sedimentary fill.
These new data, in conjunction with the magmatic record of the EP and SK, and the structure of the EBS
strongly support a model in which the emplacement of subduction‐related magmatic rocks along the entire
Southern Eurasian margin formed due to only one north dipping subduction. The relative scarcity of Jurassic
and Early Cretaceous magmatic rocks in the EP is attributed to the opening of the EBS basin. In this scenario
Paleotethyan and Neotethyan realms subduct toward the north below the Southern Eurasian margin along a
common and continuous plate boundary. Accompanying this north dipping Tethyan subduction south of
the EP and SK, opening of the EBS initiated during Cretaceous times in a back‐arc to intra‐arc configuration
north of the EP to the west toward the SK to east (McCann et al., 2010; Sosson et al., 2016, 2018;
Stephenson & Schellart, 2010, and references therein). In this scenario the Jurassic and Early Cretaceous
subduction‐related magmatic rocks originally emplaced as the EP were displaced farther north of the active
supra‐subduction zone (SSZ). Intra‐arc extension cut across and segmented the Northern Tethyan arc, con-
tinuing ultimately to the formation of the EBS. Ongoing subduction is responsible for the development of an
active arc separated from a remnant one by an oblique marginal basin. Similar models have already been
proposed in the Mediterranean and northeast of New Zealand. There, respectively, Corsica and Sardinia
represent remnant relics of the current Apennine arc separated by the Tyrrhenian basin (Karig, 1972;
Sartori, 2003) and the remnant Colville and active Kermadec arcs are separated by the Havre Trough
(Wright, 1997; Wright et al., 1996).
2. Geological Framework
2.1. Tauride‐Anatolide Platform and South Armenian Block
The Tauride‐Anatolide Platform (TAP) and South Armenian Block (SAB) originated as slivers of
peri‐Gondwanan continental lithosphere, which rifted northward away from the northern margin of
Gondwana since Triassic times (e.g., Barrier & Vrielynck, 2008; Dercourt & Vrielynck, 1993; Speranza
et al., 2012). Distinct Neotethyan suture zones defined by the ophiolitic belts outline successive accretion
and collision events involving the closure of Tethyan oceanic basins (Figure 1a). The precise timing of con-
tinental collision events associated with the final closure of both Northern and Southern Neotethyan Oceans
still remains controversial. The closure of the Northern Neotethys and collision of the SAB and TAP with
Eurasia was not synchronous along its southern margin (Sosson et al., 2016). In the LC, collision is inter-
preted as being Late Cretaceous (~73–71 Ma) (Meijers et al., 2015; Rolland, Billo, et al., 2009; Rolland,
Galoyan, et al., 2009) and ranging between Paleocene and early Eocene farther west in the EP (Hippolyte
et al., 2017; Okay & Nikishin, 2015; Robertson, Parlak, & Ustaömer, 2013; Robertson, Parlak, Ustaömer,
Taslı, et al., 2013; Şengör & Yilmaz, 1981).
2.2. IAES and ASAS Zones
Suture zones are defined in our study region by ophiolite belts and ophiolitic mélanges. The ophiolitic rocks
of the TAP‐SAB define three main belts (Figure 1a); from north to south (1) the northern ophiolitic belt
marking its limit with the Southern Eurasian margin characterized by the EP and SK arcs, (2) the
Median or Tauride ophiolite belt limiting the Tauride and Anatolide Terranes, and (3) the Southern or
Peri‐Arabic ophiolitic belt separating the SAB from the Arabian Block. The northern ophiolite belt defines
the IAES‐ASAS. The IAES‐ASAS represents remnants of the Northern Neotethys Ocean, constituting a con-
tinuous limit between the EP and SK arcs with the TAP‐SAB (Çelik et al., 2011; Hässig, Rolland, Sosson,
Galoyan, Sahakyan, et al., 2013; Hässig et al., 2017; Okay & Tüysüz, 1999; Rolland et al., 2016;
Sarıfakıoğlu et al., 2017; Sosson et al., 2010, 2016). An Eocene succession of terrigenous clastic and silicic
volcanic rocks typically unconformably covers the TAP‐SAB, the IAES‐ASAS suture zone, and the EP and
LC (SK) magmatic arcs, marking complete suturing (Lordkipanidze et al., 1988; Sosson et al., 2010).
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2.3. EP and LC Arcs
The general consensus is that the EP and LC arcs are subduction‐related magmatic arcs formed during the
Mesozoic and Cenozoic. Two main models for their formation currently prevail: (1) one Tethyan north dip-
ping subduction zone, with Paleotethyan followed by Neotethyan oceanic lithospheres subducting below the
EP and SK arcs along the Southern Eurasian margin throughout Mesozoic and Early Cenozoic times (Rice
et al., 2006, 2009; Robertson & Dixon, 1984; Şengör & Yilmaz, 1981; Ustaömer & Robertson, 2010; Yılmaz &
Boztuğ, 1996), and (2) two Tethyan subductions, one south dipping featuring Paleotethyan oceanic litho-
sphere subducting below the EP and SK arcs throughout the Mesozoic‐Early Cenozoic and one north
dipping featuring Neotethyan oceanic lithosphere subducting below the same EP and SK arcs during the
Late Cretaceous‐Early Cenozoic (Bektaşet al., 1999; Eyüboğlu, 2015; Eyüboğlu et al., 2019).
2.3.1. EP
The Pontides are an east‐west striking mountain belt in northern Turkey, located between the Black Sea and
the IAES, to the north and south, respectively (Figure 1a). The EP arc is distinguished as the eastern section
of the mountain belt. This portion of the Pontides is mainly composed of Late Paleozoic and Mesozoic accre-
tionary complexes as well as Mesozoic and Cenozoic magmatic rocks (Figure 1b) (Okay, 1997; Sayıt&
Göncüoglu, 2013).
High‐temperature/low‐pressure gneiss and schist with Early Carboniferous metamorphic ages are the oldest
rocks of the pre‐Jurassic basement (Dokuz, 2011; Dokuz et al., 2015; Nzegge et al., 2006; Okay et al., 2006;
Topuz et al., 2004, 2007). These basement rocks are overlain by Early to Middle Jurassic sedimentary rocks
interbedded with rare basalt and pyroclastic rocks (Dokuz et al., 2006; Kandemir, 2004; Kandemir &
Yılmaz, 2009; Şen, 2007).
Based on our compilation, the majority of the geochronologic data (Figure 2; ESD 1) show that the EP
recorded two major magmatic pulses during the Late Cretaceous (~90–75 Ma) and the early Eocene
(~55–40 Ma). In addition, a small set of data indicate magmatism during the Early Cretaceous
(~140–130 Ma) and even as old as the Jurassic (Figure 2; ESD 1). The Jurassic and Early Cretaceous intru-
sions have a calc‐alkaline composition and are attributed to a subduction environment (Boztuğ&
Harlavan, 2008; Delibaşet al., 2016). The Late Cretaceous magmatic rocks are calc‐alkaline, high‐K
calc‐alkaline, and shoshonitic, including localized adakitic compositions attributed to subduction as well,
generally attributed to north dipping Paleotethyan and Northern Neotethyan domains (Akin, 1979;
Altherr et al., 2008; Çinku et al., 2010; Delibaşet al., 2016; Karsli et al., 2010, 2011; Okay &
Tüysüz, 1999; Şengör et al., 2003; Topuz et al., 2007; Ustaömer & Robertson, 2010; Yilmaz et al., 1997).
Alternatively, Eyüboğlu (2010), Eyüboğlu, Santosh, and Chung (2011), Eyüboğlu, Santosh, Dudas,
et al. (2011), and Eyüboğlu et al. (2013, 2014) favor a south dipping subduction of the Paleotethyan domain
during Jurassic and Early Cretaceous times.
The early Eocene magmatism has high‐K calc‐alkaline and shoshonitic compositions, with arc and adakitic
signatures, variably attributed to a subduction setting (Aydınçakır&Şen, 2013; Karsli et al., 2011; Kaygusuz
& Öztürk, 2015; Topuz et al., 2011; Yilmaz‐Sahin, 2005) or to a continent‐continent collision environment
between the TAP and the EP (Aliyazicioğlu, 1999; Boztuğet al., 2004; Çoban, 1997; Dokuz et al., 2015;
Dokuz et al., 2019; Eyüboğlu, Santosh, Chung, 2011; Karsli et al., 2007, 2010, 2011; Okay &
Şahintürk, 1997; Okay et al., 1997; Şen et al., 1999; Şengör & Yilmaz, 1981; Tokel, 1977; Topuz et al., 2005,
2011; Yilmaz & Boztuğ, 1996). The EP arc was further uplifted above sea level during the Oligocene, as evi-
denced by the absence of marine sedimentary rocks and solely the presence of detrital material dated to that
time (Topuz et al., 2011). Miocene and younger magmatism is preserved as partly eroded subaerial volca-
noes, ascribed to postcollisional magmatism (Robinson, Banks, et al., 1995).
2.3.2. LC
The LC is a region spanning from southern Georgia to northwestern Iran, across Armenia and Azerbaijan. It
is composed of the LC (SK) arc, the ASAS, and the SAB, respectively from north to south (Figure 1a). The SK
arc has a northwest‐southeast strike. It is limited to the northeast by the Transcaucasus basin. The SK arc is
defined by Mesozoic‐early Cenozoic magmatic series (Figures 1b and 2) (Adamia et al., 1977, 1981, 1987;
Knipper, 1975; Maghakyan et al., 1985; Ricou et al., 1986).
The basement rocks of the LC, outcropping as the Akhum and Asrikchai massifs, are interpreted as Late
Paleozoic (Hess et al., 1995; Shengelia et al., 2006). Proterozoic and Paleozoic ages have also been
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described in the Georgian Loki and Khrami massifs at the northwestern most extent of the SK arc, argued to
represent portions of the same basement (Gamkrelidze & Shengelia, 2007; Zakariadze et al., 2007). The
assemblages composing this arc unconformably overlay these basement rocks (Sosson et al., 2010).
Our compilation of geochronologic data (Figure 2; ESD 1), featuring new data from the Bolnisi district in
southern Georgia and the Alaverdi district in northeastern Armenia (Figures 1b and 3), show that the LC
recorded three major magmatic pulses: (1) Jurassic to Early Cretaceous (~190–130 Ma), (2) Late
Cretaceous (~90–80 Ma), and (3) early Eocene (~55–40 Ma). The Jurassic and Early Cretaceous series
features intrusions, which have tholeiitic to calc‐alkaline compositions, as well as volcanoclastic and sedi-
mentary rocks are attributed to a subduction environment (Calder et al., 2019; Mederer et al., 2013;
Sadikhov & Shatova, 2016, 2017). The Late Cretaceous magmatic rocks are calc‐alkaline, high‐K calc‐alka-
line and shoshonitic in composition. Their formation is attributed to northward subduction of the
Northern Neotethys.
2.3.2.1. Alaverdi District, NE Armenia
The Alaverdi district (Figure 3) represents the northeastern portion of the Jurassic SK arc in Armenia. It is
mainly characterized by Middle Jurassic magmatic rocks interlayered with sedimentary rocks, followed by
Late Jurassic and Cretaceous volcanic and sedimentary rocks, themselves discordantly covered by
Eocene and Quaternary rock units (Calder et al., 2019; Ghazaryan, 1971; Lebedev & Malkhasyan, 1965;
Mederer et al., 2014; Melkonyan, 1976; Sopko, 1961). The lowermost portion of the Middle Jurassic
unit is composed of lava with basaltic, basaltic‐andesitic, and andesitic compositions (Bagdasaryan &
Melkonyan, 1968). These are conformably covered by tuff and lava breccia of basaltic andesitic, andesitic,
and dacitic composition, in turn covered by pyroclastic and lava flows of dacitic and rhyolitic composition
(Sopko, 1961). Basaltic‐andesitic and andesitic tuff breccia interlayered with sandstone and limestone con-
stitute the Late Jurassic to Early Cretaceous rock sequence (Calder et al., 2019; Ghazaryan, 1971;
Sopko, 1961). Unconformably covering this succession are Eocene sedimentary rocks, lava flows and basal-
tic, andesitic, and rhyolitic tuff (Karapetyan et al., 1982).
Several intrusions crosscut this succession of magmatic and sedimentary rocks (ESD 1), most importantly
are those of Haghpat, circa 161 Ma (Melkonyan et al., 2014) and the composite Shnogh‐Koghb, dated at circa
164 Ma, 156 Ma (Melkonyan & Ghoukassyan, 2004), and 152.9 Ma (Calder et al., 2019). Much less important
in size yet more numerous, multiple basaltic, andesitic, dacitic, and rhyolitic dikes and sills (ca. 155 Ma;
Calder et al., 2019) as well as subvolcanic bodies crosscut the Middle Jurassic units.
2.3.2.2. Bolnisi District, South Georgia
The Bolnisi district (Figure 3) is located at the northeastern most portion of the SK in southern Georgia,
mainly characterized by Late Cretaceous volcanic and sedimentary sequences emplaced between the
Khrami and Loki massifs (Adamia et al., 2011; Zakariadze et al., 2007). These sequences are divided into
six suites interpreted to be between Cenomanian and Maastrichtian in age, unconformably overlain by
Maastrichtian limestone and marl (Apkhazava, 1988; Gambashidze, 1974; Gugushvili et al., 2014;
Popkhadze et al., 2014). An early Eocene formation consisting of terrigenous sedimentary rocks and middle
Eocene volcanic rocks unconformably covers the older rocks. In turn, these formations are conformably
overlain by late Eocene shallow‐marine clastic rocks.
2.4. GC and EBS Basins
2.4.1. GC
The GC belt extends from the northern margin of the EBS basin to the western margin of the South Caspian
basin (Figure 1). Its formation resulted from multiple deformation phases during the Phanerozoic,
particularly throughout Mesozoic and Cenozoic times (Adamia, 1980; Adamia et al., 1977, 2010;
Dotduyev, 1989; Gamkrelidze, 1986; Khain, 1984; McCann et al., 2010; Milanovsky, 1991; Milanovsky
et al., 1984; Milanovsky & Khain, 1963; Muratov et al., 1984; Nikishin, Cloetingh, Bolotov, et al., 1998;
Nikishin, Cloetingh, Brunet, et al., 1998; Nikishin et al., 2001; Philip et al., 1989; Razvetaev, 1977, 1989).
Throughout the Mesozoic and early Cenozoic, several basins developed within the Eurasia Plate, principally
the GC basin which initiated its opening in the Early‐Middle Jurassic (Adamia et al., 1981, 2011; Barrier &
Vrielynck, 2008; Dercourt et al., 1986; Khain, 1975). Its inversion and subsequent formation of the GC
mountain belt is related to accretion of Gondwanan and Tethyan terranes along the southernmost edge of
the East European Platform (Adamia et al., 2011; Dercourt & Vrielynck, 1993; Dercourt et al., 2000;
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Sengör, 1989; Zonenshain et al., 1990). The GC has a doubly verging structure with two fold‐and‐thrust com-
plexes on either side of a nascent axial zone (Khain, 1997; Mosar et al., 2010; Sholpo, 1993; Sosson
et al., 2016).
The southern edge of the axial zone is marked by the north dipping MCT (Figure 1b) (Somin, 2011). The
MCT thrusts Variscan basement composed of Late Paleozoic arc‐related granitic plutons, migmatite, and
both orthogneiss and paragneiss (Nalivkin, 1973) atop the SS. The Variscan basement is covered by
Cambrian and Devonian rocks and is in turn unconformably overlain by Late Jurassic and Cretaceous shelf
carbonate (Nalivkin, 1976). The upper portion north of the MCT contains a complex system of top‐to‐north
thrust sheets of Jurassic to middle Miocene sedimentary and magmatic rocks (Cowgill et al., 2016;
Gudjabidze & Gamkrelidze, 2003; Nalivkin, 1976). South of the MCT, the SS is a of top‐to‐south thrust
system dominated by thrust sheets of Early Jurassic to Pleistocene magmatic and sedimentary rock, featur-
ing Paleogene olistostromes with Cretaceous olistoliths (Adamia et al., 2011; Banks et al., 1998; Cowgill et
al., 2016; Dotduyev, 1987; Forte et al., 2010, 2013, 2014; Kandelaki & Kakhazdze, 1957; Philip et al., 1989;
Vincent et al., 2007). The SS is limited to the south by late Miocene to Plio‐Pleistocene sedimentary rocks
of the Transcaucasus basin (Figure 1b) (Adamia et al., 2010; Forte et al., 2010, 2013).
2.4.2. EBS
The Black Sea consists of a roughly east‐west orientated depression, north of the Pontides, south of the
Crimean Mountains, and southwest of the GC belt (Figure 1) (Dercourt & Vrielynck, 1993; Dercourt
et al., 2000; Nikishin et al., 2003; Okay et al., 1994; Robinson & Kerusov, 1997; Stephenson &
Schellart, 2010). Currently, the Black Sea features a ~2 km deep abyssal plain separated into two large
depressions, the Western Black Sea and EBS basins (Robinson et al., 1996; Sydorenko et al., 2017;
Zonenshain & Pichon, 1986). They are filled with a thickness of up to 14 km of Mesozoic and Cenozoic
detrital and sedimentary rocks (Graham et al., 2013; Nikishin et al., 2015a, 2015b; Sydorenko et al., 2017;
Yegorova & Gobarenko, 2010). These depressions affect a basement of unconstrained composition and
tectonic age. The basins formed during the Cretaceous and the early Cenozoic (Cloetingh et al., 2003;
Finetti, 1988; Khriachtchevskaia et al., 2010; Letouzey et al., 1977; Okay et al., 2013; Spadini et al., 1996;
Stephenson & Schellart, 2010; Vincent et al., 2005). Various scenarios have been proposed to explain their
origin (Bektaş, 1986; Eyüboğlu et al., 2014; Okay et al., 1994, 2013; Stephenson & Schellart, 2010), but it
remains a subject of debate due to poor understanding of the nature of their basement.
Interpretations of a recent seismic survey (Nikishin et al., 2015a, 2015b) infer that the present EBS basin is
underlined by a basement featuring WNW‐ESE striking normal faults dipping toward its center, parallel
to its northern and southern margins. This argues for the presence of tilted blocks analog to a rifted
continental crust bordering an oceanic crust (Bosworth, 1985; Cloetingh et al., 2003; Spadini et al., 1996).
Late Barremian to Albian sedimentary sequences are deformed by this faulting. In the northern portion of
the EBS basin small Albian‐Cenomanian volcanoes seal these normal faults (Nikishin et al., 2015a).
Santonian‐Maastrichtian sedimentary rocks cover the center of the depression, as well as onlap over loca-
lized synnormal faulting Cenomanian‐Santonian sedimentary fill (Afanasenkov et al., 2008; Nikishin
et al., 2008; Robinson, Spadini, et al., 1995; Shillington et al., 2008). Santonian‐Campanian submarine volca-
noes 100 km north of the present‐day Turkish coast are imaged and are argued to be related to a major E‐W
trending normal fault system (Nikishin et al., 2003). Paleocene, Eocene, and Oligocene turbiditic sequences
constitute the upper part of the sedimentary fill (Letouzey et al., 1977; Shillington et al., 2008). Numerous
compressional and transpressional structures appear along the periphery of the EBS basin in the later
Cenozoic sedimentary record (Khriachtchevskaia et al., 2010).
3. Sampling
Throughout field studies, a total of 37 representative samples of magmatic rocks was collected, 18 from the
Alaverdi district and 19 from the Bolnisi district (Figure 3). From the Alaverdi district, 13 of the samples are
intrusive magmatic rocks ranging from pinkish granites with coarse‐grained phaneritic textures (marked
by potassium feldspar, plagioclase, quartz, and biotite), light gray colored quartz‐monzonite with
coarse‐grained phaneritic to porphyritic textures (marked by less quartz than granites, large fledspar, plagi-
oclase, and biotite), granodiorite with fine‐grained phaneritic textures (marked by high plagioclase content),
and fine‐grained phaneritic diorite (marked by less quartz content). Each sample represents a specific
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occurrence of repetitive subparallel metric wide/thick dikes or sills. The five remaining samples are
extrusive magmatic rocks, which outcrop as lava flows of basalt, andesite, or rhyolite.
From the Bolnisi district, five samples are intrusive magmatic rocks including two grayish granites (one with
a coarse‐grained and one with a fine‐grained phaneritic textures marked by large feldspar, plagioclase,
quartz, and biotite), two phaneritic granodiorites and one porphyritic quartz‐monzonite, which outcrop as
metric to decametric wide dikes along regional faults or along the contacts between trachytic‐rhyolitic
bodies and Cretaceous volcano‐sedimentary sequences. The remaining 14 samples are extrusive magmatic
rocks with aphanitic to porphyritic textures and compositions ranging from dacitic, to trachydacitic, and
to rhyolitic. These outcrop as isolated hills and mounds devoid of greenery among well vegetated
Cretaceous volcano‐sedimentary deposits.
All 37 samples were analyzed for whole‐rock geochemical composition. Twelve samples from the Alaverdi
district (all intrusive with massive textures) and 13 from the Bolnisi district (4 intrusive and 9 extrusive,
all with massive textures) underwent LA‐ICP‐MS zircon U‐Pb dating.
4. U‐Pb Zircon Geochronology
Twenty‐five samples, 12 from the Alaverdi district and 13 from the Bolinsi district, were chosen for U‐Pb
dating by LA‐ICP‐MS. The analytical techniques and methods used to obtain these new data are presented
in ESD 2. All of the spot analyses were carried out on zircons exhibiting clear magmatic zonations as seen on
SEM‐CL zircon images presented in ESD 3, as well as locations of spot analyses. The detailed results are
listed in ESD 4a and 4b and summarized in Table 1 and ESD 1. The results can be divided into three distinct
age groups; (1) Late Jurassic, (2) Late Cretaceous, and (3) Eocene (Figure 2).
4.1. Late Jurassic Magmatism
Of the 12 dated samples of the Alaverdi district all yield Late Jurassic ages (Oxfordian‐Tithonian, ca.
161–145 Ma). The ages range from 158.1 ± 3.1 Ma for AR‐17‐32 to 146.6 ± 1.6 Ma for AR‐17‐33 (Table 1).
Table 1
Summary of New U‐Pb Zircon LA‐ICP‐MS Datings With (WGS84) GPS Locations
District Sample
Longitude
(°E)
Latitude
(°N) Rock type
Weighted mean age (Ma)
206
Pb/
238
U(1σ) MSDW
Alaverdi AR‐17‐26 44.71123 41.16792 granite 151.20 ± 1.20 1.40
AR‐17‐29 44.81810 41.17585 qtz‐monzonite 150.70 ± 1.50 1.18
AR‐17‐30 44.81810 41.17585 granite 151.40 ± 1.40 1.20
AR‐17‐32 44.83866 41.13926 granite 158.10 ± 3.10 0.32
AR‐17‐33 44.83839 41.14796 diorite 146.60 ± 1.60 1.02
AR‐17‐37 44.71552 41.06629 granite 154.20 ± 1.40 0.54
AR‐17‐38 44.64733 41.15480 granite 152.39 ± 0.85 1.30
AR‐17‐39 44.62370 41.21660 rhyolite 151.73 ± 0.89 1.04
AR‐17‐40 44.65309 41.15228 rhyolite 151.22 ± 0.79 0.66
AR‐17‐41 44.61661 41.09775 qtz‐monzonite 153.70 ± 1.30 0.12
AR‐17‐42 44.63350 41.09814 rhyolite 156.50 ± 1.30 1.20
AR‐17‐43C 44.73980 41.07568 granodiorite 154.00 ± 1.70 0.59
Bolnisi G‐17‐02 44.39766 41.30809 granodiorite 159.00 ± 1.40 0.66
G‐17‐04 44.60182 41.41674 dacite 83.90 ± 2.40 1.40
G‐17‐05 44.62590 41.42062 rhyolite 84.36 ± 0.80 1.40
G‐17‐06 44.62908 41.42801 rhyolite 83.31 ± 0.90 1.20
G‐17‐09 44.56591 41.45947 rhyolite 83.99 ± 0.84 1.60
G‐17‐12 44.75991 41.36993 rhyolite 81.64 ± 0.94 0.63
G‐17‐16 44.53088 41.34752 granite 52.04 ± 0.70 1.11
G‐17‐17 44.53122 41.34672 granodiorite 52.29 ± 0.56 0.96
G‐17‐19 44.52847 41.35770 granite 52.85 ± 0.89 0.26
G‐17‐20 44.71418 41.37173 rhyolite 83.70 ± 1.30 0.40
G‐17‐21 44.69194 41.35037 trachydacite 85.30 ± 1.10 1.30
G‐17‐22 44.42693 41.40975 rhyolite 83.70 ± 1.10 1.50
G‐17‐24 44.37200 41.46363 rhyolite 83.60 ± 1.30 0.43
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When considering their uncertainties, the U‐Pb ages overlap with each other between 161.2 and 149.2 Ma,
except for Sample AR‐17‐33 (Figure 2). This is further emphasized by the 159 ± 1.4 Ma age of Sample
G‐17‐02, which crops out along a major fault marking the northern limit of the Loki massif in the southern
portion of the Bolnisi district (Figure 3). In addition, similar ages of magmatic rocks are reported by Calder
et al. (2019) for the Alaverdi district (Figure 2).
4.2. Late Cretaceous Magmatism
Nine of the 13 samples of the Bolnisi district are dated Late Cretaceous. U‐Pb ages range between
85.3 ± 1.1 Ma for G‐17‐21 and 81.64 ± 0.94 Ma for G‐17‐12 (Table 2). Emplacement of a major magmatic
complex ascribed to part of the LC is bracketed between 86.4 and 80.7 Ma when considering the overlapping
uncertainties of the individual dates (Figure 2). The Late Cretaceous magmatic event has not been identified
so far in the Alaverdi district (Figure 3).
4.3. Eocene Magmatism
Eocene ages have only been determined for magmatic rocks outcropping in the central portion of the Bolnisi
district (Table 1 and Figure 3). Three samples yield ages between 52.85 ± 0.89 Ma for G‐17‐19 and
52.29 ± 0.56 Ma for G‐17‐17. Taking into account the uncertainties of these ages, we can constrain the empla-
cement of a magmatic complex over a short period of time between 53.74 and 51.34 Ma (Figure 2).
5. Whole‐Rock Geochemistry by XRF and LA‐ICP‐MS
Thirty‐seven samples, 18 from the Alaverdi district and 19 from the Bolinsi district, were chosen for
geochemical analyses. All analyzed samples present minimal alteration, and care was taken to remove
weathered surfaces prior to sample preparation. The detailed results are listed in Table 2, and sample
locations are plotted in Figure 2.
The sampled magmatic rocks from the Alaverdi district can be divided into two groups on the basis of SiO
2
contents, varying between basic (46 to 54 wt.%) and acid (64 to 77 wt. %) compositions (Table 2 and Figure 4).
The basic rocks exhibits generally higher Al
2
O
3
(14–20 wt.%), TiO
2
(0.44–1.28 wt.%), CaO (8–11 wt.%), FeOt
(8.4–12 wt.%), and MgO (4.4–8.2 wt.%) compared to the felsic rocks (Al
2
O
3
:12–16 wt. %, TiO
2
:
0.06–0.72 wt.%, CaO: 0.20–4.4 wt.%, FeOt: 1.33–5.0 wt.%, and MgO: 0.04–2.5 wt.%). Binary plots for most
of the major elements show a good anticorrelation of Al
2
O
3
, TiO
2
, CaO, FeO
t
,P
2
O
3
, and MgO contents
decreasing with increasing SiO
2
(ESD 5). Contrary to those from the Alaverdi district, the samples from
the Bolnisi district exhibit a near linear spread in SiO
2
, with rather acid values (62–79 wt.%; Table 2 and
Figures 4 and 5). As those of the Alaverdi district, most of the major elements show a good anticorrelation
with SiO
2
, especially in Al
2
O
3
(18 to 11 wt.%), TiO
2
(0.73 to 0.14 wt.%), CaO (4.1 to 0.15 wt.%), Fe
2
O
3
(56.0 to 0.97 wt.%), and MgO (2.5 to 0.01 wt.%) (ESD 5).
On the AFM diagram (Figure 5), most of the samples from both Alaverdi and Bolnisi districts plot in the
calc‐alkaline field, even though a small group of samples from the Alaverdi district plots along the limit with
the tholeiitic domain and one Late Cretaceous sample from the Bolnisi district plots well within it. The
sample group along the limit between the calc‐alkaline and tholeiitic domains corresponds to those with
lower SiO
2
contents. As in the binary diagrams of SiO
2
against major element oxides, the plots of all of
the samples of the Alaverdi district define a clear trend with decreasing FeO
tot
/MgO and increasing
Na
2
O+K
2
O within the calc‐alkaline field (Figure 5). Again, similarly to the binary diagrams, the plots of
the samples from the Bolnisi district overlap and continue this trend.
The classification diagram based on the immobile elements Th and Co (Figure 6), which are supposed to
avoid the effects of metasomatism and metamorphism (Hastie et al., 2007), shows comparable results. All
the samples from both districts plot within the calc‐alkaline or high‐K and shoshonitic domains except for
the basalt sampled in the Alaverdi district (Figures 6 and 7). This sample, which plots in the tholeiitic field
of the AFM diagram, is identified as an island arc tholeiite. As in the previous diagrams, the five basic
samples of the Alaverdi district plot in a separate cluster compared to the acid ones. Once more, as with
the binary diagrams and the AFM diagram, the plots of the samples from the Bolnisi district overlap with
the acid samples of the Alaverdi district.
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Table 2
Geochemical Whole Rock Data for Magmatic Rocks From the Alaverdi and Bolnisi Districts, Including Isotopic Data (Sr and Nd)
District Alaverdi Alaverdi Alaverdi Alaverdi Alaverdi Alaverdi Alaverdi Alaverdi Alaverdi
Sample AR‐17‐26 AR‐17‐28 AR‐17‐29 AR‐17‐30 AR‐17‐31 AR‐17‐32 AR‐17‐33 AR‐17‐34 AR‐17‐35
Longitude (°E) 44.71123 44.78142 44.81810 44.81810 44.83866 44.83866 44.83839 44.63589 44.65680
Latitude (°N) 41.16792 41.16649 41.17585 41.17585 41.13926 41.13926 41.14796 41.12922 41.13445
Age Late Jurassic Late Jurassic
Late
Jurassic Late Jurassic Late Jurassic Late Jurassic Late Jurassic Late Jurassic Late Jurassic
rock type granite basalt qtz‐monzonite granite diorite granite diorite gabbro granodiorite
SiO
2
% 73.81 50.56 63.79 76.17 53.06 74.36 51.26 46.30 66.30
TiO
2
% 0.13 1.28 0.63 0.19 0.44 0.06 0.89 0.86 0.72
Al
2
O
3
% 13.59 13.90 15.73 12.31 15.60 13.78 20.20 16.10 15.52
FeOt % 2.53 12.08 4.47 1.33 11.45 1.44 8.40 11.01 4.99
MnO % 0.03 0.12 0.07 0.02 0.15 0.01 0.15 0.17 0.07
MgO % 0.16 8.16 2.06 0.17 6.45 0.04 4.37 7.96 0.84
CaO % 1.41 7.95 3.65 0.42 10.84 0.75 9.21 9.88 1.48
Na
2
O % 3.25 2.11 5.10 3.79 1.26 3.73 4.23 1.69 6.80
K
2
O % 2.09 0.82 2.12 4.16 0.18 5.37 0.29 0.08 0.19
P
2
O
5
% 0.02 0.14 0.16 0.03 0.06 0.01 0.22 0.13 0.22
LOI % 2.88 2.80 1.35 0.55 0.49 0.24 0.94 5.74 2.69
Total % 99.89 99.99 99.13 99.13 100.09 99.78 100.17 99.96 99.83
Ba ppm 274 306 339 588 42.1 1332 112 26 65
Rb ppm 46 12 50 88 2.3 53 3.7 1.1 5.2
Sr ppm 70 250 285 57 252 99 508 188 78
Ta ppm 2.4 0.6 1.0 0.8 0.1 5.7 0.17 0.34 0.63
Th ppm 10 1.7 5.7 7.2 0.9 9.6 0.5 1.7 2.8
Zr ppm 291 85 178 123 31 176 38 70 161
Nb ppm 35 8.9 12.8 13 1.0 63 3.2 5.4 10.0
Y ppm 53 19 24 9.4 13 43 16 19 27
Hf ppm 8.5 2.3 4.8 3.8 1.0 6.8 1.0 1.8 3.9
V ppm
Cr ppm 18 323 42 20 594 26 54 252 17.6
Ni ppm
Co ppm 1.5 53 14 3.9 31 5.9 22 45 11
U ppm 2.7 0.3 1.0 1.5 0.2 2.1 0.16 0.38 0.82
Sc ppm 9.0 28 14 6.6 46 4.7 25 32 14
Cu ppm 15 82 48 18 23 26 31 80 24
Zn ppm 170 115 66 43 76 29 75 97 93
Pb ppm 12 2.5 6.2 9.8 1.4 6.2 2.0 1.7 2.8
Cs ppm 0.4 0.33 0.6 0.7 0.1 0.3 0.3 0.5 0.4
La ppm 47 11 23 19 4.4 17 8.5 10 16
Ce ppm 89 21 44 40 8.6 35 18 20 33
Pr ppm 9.3 2.5 4.8 3.3 1.0 4.1 2.3 2.4 4.1
Nd ppm 37 12 19 11 4.9 18 11 10 18
Sm ppm 7.9 3.2 3.9 1.8 1.3 4.9 2.8 2.4 4.2
Eu ppm 1.1 1.1 0.98 0.42 0.42 0.85 1.2 0.86 1.3
Gd ppm 7.9 3.6 4.1 1.6 1.7 5.6 3.2 2.9 4.7
Tb ppm 1.4 0.56 0.63 0.24 0.27 1.1 0.47 0.50 0.70
Dy ppm 9.3 3.8 4.3 1.6 2.1 7.3 3.2 3.5 4.9
Ho ppm 2.0 0.72 0.84 0.32 0.46 1.4 0.63 0.72 1.00
Er ppm 5.9 2.0 2.5 1.0 1.4 4.7 1.8 2.2 2.9
Tm ppm 0.90 0.27 0.38 0.15 0.21 0.75 0.23 0.30 0.42
Yb ppm 6.3 1.7 2.8 1.3 1.6 5.5 1.6 2.1 2.8
Lu ppm 0.97 0.25 0.39 0.22 0.24 0.81 0.24 0.31 0.44
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Table 2
Continued
District Alaverdi Alaverdi Alaverdi Alaverdi Alaverdi Alaverdi Alaverdi Alaverdi Alaverdi Bolnisi
Sample AR‐17‐36 AR‐17‐37 AR‐17‐38 AR‐17‐39 AR‐17‐40 AR‐17‐41 AR‐17‐42 AR‐17‐43A AR‐17‐43C G‐17‐01
Longitude
(°E) 44.71255 44.71552 44.64733 44.62370 44.65309 44.61661 44.63350 44.73980 44.73980 44.39766
Latitude (°N) 41.11475 41.06629 41.15480 41.21660 41.15228 41.09775 41.09814 41.07568 41.07568 41.30809
Age
Late
Jurassic
Late
Jurassic
Late
Jurassic
Late
Jurassic
Late
Jurassic Late Jurassic
Late
Jurassic
Late
Jurassic
Late
Jurassic Late Jurassic
rock type granite granite granite rhyolite rhyolite
qtz‐
monzonite rhyolite andesite granodiorite
qtz‐
monzonite
SiO
2
74.92 66.71 77.01 74.23 71.37 64.18 68.93 51.60 65.78 63.96
TiO
2
0.39 0.44 0.14 0.26 0.32 0.51 0.42 0.82 0.51 0.72
Al
2
O
3
12.15 14.44 12.93 14.56 14.69 15.88 14.97 17.31 15.37 14.61
FeOt 3.98 2.37 1.62 1.63 3.04 4.48 3.37 10.82 4.30 5.64
MnO 0.04 0.05 0.02 0.01 0.01 0.09 0.08 0.23 0.06 0.13
MgO 0.77 0.29 0.26 0.11 0.43 2.52 0.94 4.35 1.99 2.50
CaO 0.29 4.39 0.37 0.20 0.24 2.02 2.58 10.24 4.17 1.76
Na
2
O 5.63 6.14 3.28 5.78 5.44 5.69 3.80 3.18 4.06 5.88
K
2
O 0.28 0.59 2.06 1.36 2.26 1.83 3.68 0.36 1.83 1.85
P
2
O
5
0.08 0.14 0.02 0.07 0.10 0.17 0.12 0.12 0.14 0.16
LOI 1.66 4.43 1.67 1.14 1.36 2.79 0.96 0.38 1.18 1.99
Total 100.17 99.98 99.39 99.35 99.25 100.15 99.86 99.41 99.38 99.20
Ba 45 62 99 101 245 412 554 57 294 335
Rb 4.5 14 60 21 38 25 92 3.1 38 39
Sr 64 62 36 32 148 197 335 179 240 142
Ta 0.35 0.52 1.6 0.93 1.2 0.46 1.3 0.12 0.63 0.62
Th 2.2 4.0 6.9 4.4 4.8 4.8 11 1.1 4.6 4.9
Zr 118 146 217 142 195 126 200 58 153 199
Nb 5.6 6.3 20 12 17 6.2 17 2.0 7.4 10
Y 3317301930 14 2121 18 31
Hf 3.7 3.8 6.2 4.0 5.3 3.3 4.9 1.7 4.1 5.4
V 4.0 12 19 81 38 278 73
Cr 23 31 12 14 14 32 19 31 25 34
Ni 4.2 7.0 6.1 17 6.3 13 14
Co 5.1 6.8 0.49 0.87 2.9 11 5.1 28 9.0 13
U 0.67 0.60 2.0 0.85 1.3 1.0 2.4 0.51 0.74 1.2
Sc 19 12 7.2 5.9 6.4 11 9.5 34 12 20
Cu 53 27 17 6.4 4.8 8.0 8.2 54 86 32
Zn 52 53 30 20 49 57 52 111 33 77
Pb 2.9 2.2 2.1 2.5 1.7 4.3 12 2.4 3.0 6.2
Cs 0.1 0.4 0.8 0.8 0.8 0.6 0.4 0.2 0.4 0.3
La 5.8 14 30 19 25 16 31 6.5 18 18
Ce 15 26 61 36 45 31 56 14 34 38
Pr 2.1 2.8 6.5 4.0 5.5 3.3 5.7 1.9 3.7 4.6
Nd 11 12 24 15 22 13 20 9.0 14 19
Sm 3.3 2.4 5.1 3.0 4.9 2.6 3.7 2.7 2.9 4.6
Eu 0.82 0.90 0.55 0.56 0.89 0.82 0.94 0.96 0.83 1.1
Gd 4.5 2.8 5.0 2.9 5.0 2.5 3.5 3.3 3.0 5.0
Tb 0.78 0.43 0.77 0.48 0.80 0.39 0.54 0.55 0.48 0.80
Dy 5.9 2.8 5.5 3.5 5.4 2.5 3.6 3.8 3.1 5.3
Ho 1.3 0.56 1.2 0.73 1.1 0.50 0.76 0.80 0.65 1.1
Er 4.1 1.8 3.5 2.3 3.4 1.4 2.3 2.4 1.9 3.3
Tm 0.65 0.25 0.56 0.36 0.50 0.20 0.37 0.35 0.31 0.51
Yb 4.6 1.9 4.1 2.6 3.5 1.4 2.6 2.4 2.1 3.5
Lu 0.72 0.28 0.59 0.40 0.53 0.22 0.39 0.37 0.34 0.52
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Table 2
Continued
District Bolnisi Bolnisi Bolnisi Bolnisi Bolnisi Bolnisi Bolnisi Bolnisi Bolnisi Bolnisi
Sample G‐17‐02 G‐17‐04 G‐17‐05 G‐17‐06 G‐17‐07 G‐17‐09 G‐17‐12 G‐17‐20 G‐17‐21 G‐17‐22
Longitude
(°E) 44.39766 44.60182 44.62590 44.62908 44.65448 44.56591 44.75991 44.71418 44.69194 44.42693
Latitude (°N) 41.30809 41.41674 41.42062 41.42801 41.41711 41.45947 41.36993 41.37173 41.35037 41.40975
Age
Late
Jurassic
Late
Cretaceous
Late
Cretaceous
Late
Cretaceous
Late
Cretaceous
Late
Cretaceous
Late
Cretaceous
Late
Cretaceous
Late
Cretaceous
Late
Cretaceous
rock type granodiorite dacite rhyolite rhyolite rhyolite rhyolite rhyolite rhyolite trachydacite rhyolite
SiO
2
66.33 70.76 78.94 78.21 73.04 76.63 70.38 73.60 66.60 73.62
TiO
2
0.39 0.63 0.15 0.14 0.23 0.15 0.29 0.32 0.39 0.17
Al
2
O
3
14.97 15.64 11.01 11.02 14.05 12.14 14.88 13.36 18.13 13.17
FeOt 5.96 4.89 0.97 1.21 2.06 1.31 2.38 1.89 1.93 1.55
MnO 0.08 0.18 0.01 0.01 0.03 0.01 0.03 0.01 0.00 0.03
MgO 2.37 0.08 0.01 0.09 0.15 0.13 0.45 0.50 0.20 0.57
CaO 0.59 0.30 0.97 0.93 1.91 0.15 0.55 0.37 0.36 0.29
Na
2
O 2.83 0.02 2.71 2.54 4.76 0.64 2.75 5.39 6.63 3.48
K
2
O 1.49 0.15 3.92 4.63 2.99 6.53 7.12 2.64 3.94 5.18
P
2
O
5
0.17 0.13 0.02 0.03 0.07 0.03 0.09 0.08 0.09 0.05
LOI 4.16 7.06 0.73 1.33 0.80 2.22 1.11 0.93 1.12 1.02
Total 99.33 99.83 99.44 100.13 100.09 99.93 100.04 99.09 99.38 99.14
Ba 61 182 578 535 464 475 1965 439 539 723
Rb 40 3.0 71 79 60 124 120 42 65 103
Sr 72 88 83 79 150 35 46 76 66 47
Ta 0 0.70 0.53 0.55 0.59 0.71 0.74 0.53 0.56 0.67
Th 3.5 4.9 7.3 8.0 4.8 8.3 4.5 4.3 5.6 6.7
Zr 90 205 100 101 129 82 339 160 203 90
Nb 5.4 12 5.0 5.8 8.6 6.2 11 8.0 6.7 9.7
Y 6.5 56 9.5 9.5 16 6.1 20 22 32 10
Hf 2.4 5.7 2.7 2.8 3.7 2.3 6.9 4.6 5.1 2.3
V 18 19 13
Cr 21 17 32 31 27 31 27 14 14 20
Ni 6.4 4.3 7.9
Co 5.8 4.0 1.7 1.6 4.2 2.7 2.7 1.7 0.6 1.2
U 0.69 1.9 1.4 1.3 1.4 1.2 1.7 1.0 1.8 1.7
Sc 12 19 7.2 7.2 11 6.5 8.3 11 12 5.8
Cu 1743 4.8 4.3 25 7.6 4.2 3.7 6.5 7.0 8.1
Zn 160 116 53 45 66 37 101 35 21 20
Pb 3.5 39 7.9 7.6 9.7 7.1 3.7 5.5 6.2 8.8
Cs 3.5 0.6 0.63 0.85 0.57 3.6 0.69 0.31 0.23 0.80
La 8.5 18 16 18 15 19 25 14 21 22
Ce 20 39 26 29 31 29 48 29 45 43
Pr 2.4 4.7 2.6 2.8 3.0 2.5 5.1 3.5 4.6 3.8
Nd 10 20 9.5 9.0 12 7.7 19 15 18 13
Sm 2.0 5.3 1.8 1.7 2.2 1.4 3.4 3.6 3.8 1.9
Eu 0.60 1.6 0.41 0.34 0.62 0.30 0.91 0.86 0.98 0.48
Gd 1.8 7.2 1.7 1.5 2.3 1.1 3.2 3.7 4.2 1.7
Tb 0.24 1.4 0.26 0.24 0.40 0.18 0.48 0.60 0.71 0.26
Dy 1.4 9.8 1.6 1.6 2.7 1.1 3.3 4.0 5.3 1.7
Ho 0.27 2.0 0.31 0.31 0.63 0.22 0.67 0.86 1.1 0.34
Er 0.8 5.3 1.1 1.0 1.9 0.7 2.3 2.6 3.4 1.1
Tm 0.12 0.74 0.19 0.16 0.34 0.12 0.37 0.41 0.53 0.17
Yb 0.86 4.9 1.3 1.3 2.4 0.8 2.9 3.0 3.8 1.3
Lu 0.14 0.73 0.21 0.19 0.36 0.14 0.52 0.46 0.57 0.21
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Table 2
Continued
District Bolnisi Bolnisi Bolnisi Bolnisi Bolnisi Bolnisi Bolnisi Bolnisi
Sample G‐17‐23 G‐17‐24 G‐17‐11 G‐17‐16 G‐17‐17 G‐17‐18 G‐17‐19 G‐17‐25
Longitude (°E) 44.38380 44.37200 44.69035 44.53088 44.53122 44.53225 44.52847 44.36521
Latitude (°N) 41.47557 41.46363 41.25616 41.34752 41.34672 41.34659 41.35770 41.45368
Age Late Cretaceous Late Cretaceous Eocene Eocene Eocene Eocene Eocene Eocene
rock type rhyolite rhyolite trachydacite granite granodiorite dacite granite trachydacite
SiO
2
71.84 78.44 62.09 68.45 67.67 68.29 69.00 63.56
TiO
2
0.37 0.24 0.73 0.22 0.21 0.23 0.21 0.57
Al
2
O
3
12.46 11.66 17.28 16.13 15.51 16.10 15.94 16.41
FeOt 3.55 2.14 3.80 2.09 1.85 2.11 2.02 5.30
MnO 0.19 0.03 0.15 0.05 0.05 0.05 0.05 0.08
MgO 1.10 0.11 0.86 0.92 0.50 0.86 1.00 1.29
CaO 0.64 0.33 1.53 3.17 4.09 3.68 1.03 2.88
Na
2
O 5.72 5.66 6.35 5.56 4.49 5.03 5.55 5.21
K
2
O 1.06 1.09 5.45 1.34 0.97 1.24 2.12 3.01
P
2
O
5
0.22 0.06 0.15 0.08 0.08 0.08 0.07 0.14
LOI 1.91 0.53 1.08 1.14 3.76 1.55 2.06 1.05
Total 99.04 100.29 99.47 99.15 99.18 99.21 99.05 99.50
Ba 272 412 2161 447 247 390 437 651
Rb 13 12 93 33 19 26 56 61
Sr 70 129 253 680 422 640 342 262
Ta 0.50 0.60 2.7 0.22 0.21 0.22 0.23 0.45
Th 4.0 5.8 24 1.5 1.5 1.4 1.5 4.6
Zr 182 166 428 60 60 59 61 76
Nb 7.7 9.5 51 3.5 3.3 3.3 3.6 5.6
Y 25 21 32 4.1 3.8 4.1 3.5 14
Hf 5.3 4.5 9.6 1.6 1.7 1.7 1.7 2.0
V 2.5 10 36 33 37 35 164
Cr 13 20 45 28 21 24 27 22
Ni 3.7 11 9.1 7.2 6.6 8.7 42
Co 1.1 1.3 3.0 4.5 3.5 4.2 4.1 11
U 1.0 1.2 5.2 0.80 0.77 0.71 0.59 1.0
Sc 15 9.6 8.9 8.3 7.6 7.9 7.5 17.9
Cu 5.2 8.2 5.9 14 5.1 12 17 11
Zn 81 28 102 35 31 37 33 46
Pb 4.1 5.9 20 9.6 7.5 8.6 8.5 4.5
Cs 0.08 0.17 0.31 1.3 1.5 0.51 4.0 0.83
La 17 20 144 8.2 7.9 7.9 4.8 18
Ce 37 41 229 14 14 14 14 27
Pr 4.7 4.5 24 1.4 1.4 1.4 1.1 3.2
Nd 21 18 80 5.2 5.1 5.0 4.1 13
Sm 5.1 3.5 12 0.94 0.91 0.95 0.79 2.5
Eu 1.1 0.68 3.0 0.36 0.34 0.34 0.25 0.85
Gd 4.8 3.4 8.6 0.88 0.88 0.81 0.72 2.6
Tb 0.72 0.56 1.1 0.12 0.12 0.11 0.11 0.37
Dy 4.7 3.9 6.4 0.7 0.7 0.8 0.7 2.4
Ho 1.0 0.82 1.2 0.15 0.14 0.15 0.12 0.50
Er 3.1 2.5 3.3 0.4 0.4 0.4 0.4 1.4
Tm 0.48 0.38 0.51 0.06 0.05 0.07 0.06 0.21
Yb 3.4 2.8 3.6 0.5 0.4 0.4 0.4 1.5
Lu 0.53 0.44 0.51 0.07 0.07 0.07 0.07 0.23
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The Th/Yb versus Ta/Yb diagram (Pearce, 1982, 1983) for volcanic rocks shows a range of compositions from
those which have higher Th/Yb values than depleted mantle to those with higher Th/Yb values than
enriched mantle (Figure 8), often attributed to addition of nonconservative elements such as Th during
subduction (Pearce et al., 1984).
The samples from both the Alaverdi and Bolnisi districts have similar extended trace element spider diagrams
displaying a subduction‐related signature (Figure 9). These include general relatively high concentrations of
large ion lithophile elements (LILE), lower contents of high field strength elements (HFSE), and negative Nb
and Ta, and positive U, K, and Pb anomalies. In addition, various degrees of negative Eu anomalies, variable
light rare earth element (LREE) enrichments, and more pronounced depleted medium and heavy rare earth
element (MREE and HREE) patterns in chondrite‐normalized diagrams (Figure 10) are present for all the
samples of both the Alaverdi and Bolnisi districts. In the La/Yb versus Th/Yb diagram (Figure 11) the
Alaverdi and Bolnisi district samples plot in continuation from one another from the primitive island arc,
to the island arc, to the continental arc fields. In the Sr/Y versus Y diagram (ESD 6), the samples from both
the Alaverdi and Bolnisi districts tend to overlap, even though it could be argued that they also plot in
continuation from one another from the normal volcanic‐arc field to the beginning of the adakite field. Let
it be pointed out that even if samples of both districts have comparable chemical compositions, four samples
from the Bolnisi district of Eocene age plot well in the adakite field, whereas none of the samples from the
Alaverdi district do.
6. Discussion and Geodynamic Implications
6.1. Jurassic to Early Cretaceous Subduction‐Related Magmatism: EP‐GC‐LC
When juxtaposing the ages of the various subduction‐related magmatic rocks throughout the study region, it
becomes apparent that a significant portion of the rocks corresponding to the Jurassic history of the EP is
missing or underrepresented (Figure 2), as the EP form a continuous synchronous arc with the SK arc
(Figure 1a). Along the northern margin of the EP, subduction‐related tuff and basaltic rocks ascribed
to Early and Middle Jurassic times by stratigraphic positioning have been described (Şen, 2007).
Outcropping in the Sochi‐Ritsa/Bechasyn region of the SS, between the MCT and the northern margin of
the EBS, subduction‐related magmatic rocks ascribed to Early and Middle Jurassic times also crop out
(McCann et al., 2010).
In comparison with the SK, samples of the EP, Bolnisi, and the Sochi‐Ritsa/Bechasyn regions plot along
similar tends in geochemical diagrams (Table 3). This implies that magmatic rocks of each of these regions
are the products of comparable processes and can be attributed to a similar arc setting. The extended trace
and rare earth element plots show generally similar patterns. Enrichment in LILE and negative anomalies
in Ta‐Nb are common in all samples from all three sectors (EP, GC, and LC), diagnostic of
subduction‐related magmatism (Figure 9). Samples from the GC and SK sectors feature well marked positive
Hf anomalies, whereas the samples of the EP tend to be devoid of positive anomalies, even one sample
Figure 4. Geochemical classification and discrimination diagrams. (a) Total alkali versus silica (TAS) diagram.
(b) Simplified QAPF diagram (Streckeisen, 1976) showing the normative mineralogical composition of intrusive rocks.
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Figure 5. AFM plot (Irvine & Baragar, 1971).
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Figure 6. Th‐Co discrimination diagram for island arc rocks (Hastie et al., 2007).
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Figure 7. Zr versus Y diagram (Barrett & MacLean, 1994, 1999).
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Figure 8. Ta/Yb versus Th/Yb tectonic emplacement diagram (Pearce, 1982). Mafic rocks are represented with red contours.
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showing a negative Hf anomaly. Rare earth element patterns (Figure 10) of Early‐Middle Jurassic rocks of
the LC and some rocks of Late Jurassic‐Early Cretaceous age of the SK show no, to limited, LREE
enrichment. Jurassic‐Early Cretaceous rocks of the GC and EP sectors and Bolnisi region exhibit
well‐marked LREE enrichment. These differences may be due to localized involvement of residual, minor
phases in the downgoing slab, fluid and sediment addition, and partial melting in the mantle wedge
(Dokuz et al., 2010). Th/La versus (Ce/Ce*)
Nd
diagrams (ESD 7) show that the subducted sediment
component varies between continental and volcanic detritus, except for those of Bolnisi composed solely
of continental detritus, with no correlation of Hf anomalies or LREE enrichment. In the La/Yb versus Th/
Yb diagram (Figure 11) Jurassic‐Early Cretaceous samples from the GC (Sochi‐Ritsa/Bechasyn), LC (SK,
Kapan, and Bolnisi) and EP plot within the primitive island arc field. Likewise, these samples plot in a
common field in the Sr/Y versus Y diagram, as normal arc (ESD 6), further supporting a common
geodynamic evolution among the considered regions.
In light of all the geochemical data (Table 3), it appears that the Jurassic‐Early Cretaceous subduction‐related
magmatic rocks of the GC (Sochi‐Ritsa/Bechasyn), LC (SK, Kapan, and Bolnisi), and EP share similar
geochemical characteristics from site to site and age to age, implying similarity of magmatic processes may
be involved, if not a continuous arc setting. The geographic positioning of the EP and SK arcs, and the
Bolnisi and Kapan regions further supports a common geodynamic process responsible for their formation
(Figure 1a). Yet the Sochi‐Ritsa/Bechasyn region is located over 100 km farther north with respect to the
Figure 9. Primitive mantle‐normalized extended trace element spider diagrams, normalization with respect to Sun and McDonough (1989). References featuring
adakite‐like rocks, according to ESD 6, are underlined. Adakite‐like rocks from the Bolnisi district are represented with red contours.
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EP and the SK. It is separated from the EP by the EBS and from the SK by the Transcaucasus. The Jurassic
subduction‐related magmatic rocks of the GC present a general NW‐SE distribution, in continuity with
those of the SK (Figure 3). The position of these subduction‐related magmatic rocks found along the SS of
the GC can in no way be linked to the formation of the GC belt itself. The GC was formed by top‐to‐south
lithospheric thrusting (Cowgill et al., 2016; Mauvilly et al., 2018; McCann et al., 2010) during the Cenozoic
(Mosar et al., 2010; Nikishin et al., 2010). The structure of the GC implies that any related magmatism
would be farther north of the MCT, not to its south. Furthermore, the ages of these rocks (Early and
Middle Jurassic; Hess et al., 1995; McCann et al., 2010) do not coincide with the timing of the compression
within the GC belt (Mauvilly et al., 2018; Mosar et al., 2010).
6.2. Late Cretaceous Subduction‐Related Magmatism: EP‐LC
An important contribution of this study is the confirmation of Late Cretaceous subduction‐related volcanic
rocks associated to the LC in its northernmost part, in the Bolnisi district (Figure 3). Previously, there were
only little chronometric data available documenting Late Cretaceous subduction‐related magmatic rocks in
the Bolnisi district. In comparison, the Late Cretaceous magmatic evolution is well documented for the EP
arc (Figure 2 and Table 3). By contrast, no Cretaceous or younger subduction‐related magmatic rocks have
been documented for the SS of the GC.
Figure 10. Rare earth element chondrite‐normalized diagrams. Normalization with respect to Sun and McDonough (1989). References featuring adakite‐like
rocks, according to ESD 6, are underlined. Adakite‐like rocks from the Bolnisi district are represented with red contours.
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Figure 11. La/Yb versus Th/Yb volcanic rock discrimination diagram (Condie, 1989).
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Table 3
Summary of Observations and Tendencies Described in Figures 5–12
Region diagrams Figure Jurassic‐Early Cretaceous Late Cretaceous Paleogene
Greater Caucasus
Sochi‐Ritsa/
Bechasyn
major
elements
versus
SiO
2
ESD 5 CaO, Fe
2
O
3
, and MgO
negative trends with
increasing SiO
2
Al
2
O
3
positive trend with
increasing SiO
2
for SiO
2
< 55 wt%
Al
2
O
3
negative trend with
increasing SiO
2
for SiO
2
> 55 wt%
two TiO
2
and P
2
O
5
negative
trends with
increasing SiO
2
high angle for SiO
2
< 55 wt%
low angle for SiO
2
> 55 wt%
AFM 5 subduction‐related
calc‐alkaline trend
Th versus
Co
6 island arc tholeiite,
calc‐alkaline, high‐K,
and shoshonitic
Zr versus
Y
7 tholeiitic, transtional,
calc‐alkaline,
and shoshonitic
Th/Yb
versus
Ta/Yb
(Pearce
diagram)
8 depleted to enriched mantle
source affected by
subduction component
plotting as volcanic arc
extended
trace
elements
9 enrichment in LILE
negative anomalies
in Ta‐Nb
rare earth
elements
10 LREE enrichment
negative Eu anomalies
La/Yb
versus
Th/Yb
11 primative island arc
to island arc
Sr/Y
versus
Y
ESD 6 normal arc
Th/La
versus
(Ce/Ce*)
Nd
ESD 7 continental and
volcanic detritus
Lesser Caucasus
Somkheto‐
Karabagh
major
elements
versus
SiO
2
ESD 5 CaO, Fe
2
O
3
, and MgO
negative trends with
increasing SiO
2
Al
2
O
3
positive trend
with increasing SiO
2
for SiO
2
< 55 wt%
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Table 3 Continued
Region diagrams Figure Jurassic‐Early Cretaceous Late Cretaceous Paleogene
Al
2
O
3
negative trend
with increasing
SiO
2
for SiO
2
> 55 wt%
two TiO
2
and P
2
O
5
negative trends with
increasing SiO
2
high angle for
SiO
2
< 55 wt%
low angle for
SiO
2
> 55 wt%
AFM 5 subduction‐related
calc‐alkaline
trend
Th versus Co 6 island arc tholeiite
to calc‐alkaline
to high‐K and
shoshonitic
Zr versus Y 7 tholeiitic, transtional,
calc‐alkaline and
shoshonitic
Th/Yb
versus
Ta/Yb
(Pearce
diagram)
8 depleted to enriched
mantle source affected
by subduction
component plotting
as volcanic arc
extended
trace
elements
9 enrichment in LILE
negative anomalies
in Ta‐Nb
rare earth
elements
10 LREE enrichment as
well as no LREE
enrichment
negative Eu
anomalies
La/Yb
versus
Th/Yb
11 primative island
arc to island arc
Sr/Y
versus Y
ESD 6 normal arc
Th/La
versus
(Ce/Ce*)
Nd
ESD 7 continental and
volcanic detritus
Kapan major
elements
versus
SiO
2
ESD 5 Al
2
O
3
, TiO
2
, CaO,
Fe
2
O
3
,P
2
O
5
, and
MgO negative
linear trend with
increasing SiO
2
two TiO
2
negative
trends with
increasing SiO
2
high angle for
SiO
2
< 55 wt%
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Table 3 Continued
Region diagrams Figure Jurassic‐Early Cretaceous Late Cretaceous Paleogene
low angle for
SiO
2
> 55 wt%
AFM 5 subduction‐related
calc‐alkaline trend
Th versus
Co
6 island arc tholeiite
to calc‐alkaline
Zr versus
Y
7 tholeiitic, transitional
and calc‐alkaline
Th/Yb
versus
Ta/Yb
(Pearce
diagram)
8 depleted to enriched
mantle source affected
by subduction
component plotting
as volcanic arc
extended
trace
elements
9 enrichment in LILE
negative anomalies
in Ta‐Nb
rare earth
elements
10 no LREE enrichment
negative Eu anomalies
La/Yb
versus
Th/Yb
11 primative island arc
to island arc
Sr/Y
versus Y
ESD 6 normal arc
Th/La
versus
(Ce/Ce*)
Nd
ESD 7 continental and
volcanic detritus
Bolnisi major
elements
versus
SiO
2
ESD 5 Al
2
O
3
, TiO
2
, CaO,
Fe
2
O
3
,P
2
O
5
, and
MgO negative linear
trend with
increasing SiO
2
Al
2
O
3
, TiO
2
, CaO,
Fe
2
O
3
,P
2
O
5
, and
MgO negative
linear trend with
increasing SiO
2
Al
2
O
3
, TiO
2
, CaO, Fe
2
O
3
,
P
2
O
5
, and MgO negative
linear trend with
increasing SiO
2
AFM 5 subduction‐related
calc‐alkaline trend
, subduction‐related
calc‐alkaline trend
Th versus
Co
6 calc‐alkaline calc‐alkaline calc‐alkaline and high‐K
and shoshonitic
Zr versus
Y
7 calc‐alkaline and
shoshonitic
calc‐alkaline and
shoshonitic
shoshonitic
Th/Yb
versus
Ta/Yb
(Pearce
diagram)
8 MORB to enriched
mantle source affected
by subduction component
plotting as volcanic arc
MORB to enriched
mantle source
affected by
subduction
component plotting
as volcanic arc
enriched mantle source
affected by subduction
component plotting as
volcanic arc
extended
trace
elements
9 enrichment in LILE enrichment in LILE enrichment in LILE
negative anomalies
in Ta‐Nb
negative anomalies
in Ta‐Nb
negative anomalies in
Ta‐Nb
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Table 3 Continued
Region diagrams Figure Jurassic‐Early Cretaceous Late Cretaceous Paleogene
rare earth
elements
10 LREE enrichment LREE enrichment LREE enrichment
negative Eu
anomalies
negative Eu anomalies
La/Yb versus
Th/Yb
11 island arc island arc and
continental
margin arc
continental margin arc
Sr/Y versus Y ESD 6 normal arc normal arc normal arc and adakite‐like
Th/La versus
(Ce/Ce*)Nd
ESD 7 continental detritus continental
detritus
volcanic detritus
Eastern Pontides
east major
elements
versus
SiO
2
ESD 5 Al
2
O
3
, TiO
2
, CaO,
Fe
2
O
3
,P
2
O
5
, and
MgO negative
linear trend with
increasing SiO
2
Al
2
O
3
, TiO
2
, CaO,
Fe
2
O
3
,P
2
O
5
, and
MgO negative
linear trend with
increasing SiO
2
Al
2
O
3
, TiO
2
, CaO, Fe
2
O
3
,
P
2
O
5
, and MgO negative
linear trend with
increasing SiO
2
AFM 5 subduction‐related
calc‐alkaline trend
subduction‐related
calc‐alkaline trend
subduction‐related
calc‐alkaline trend
Th versus
Co
6 calc‐alkaline to
high‐K and
shoshonitic
high‐K and
shoshonitic
tholeiitic and calc‐alkaline
Zr versus
Y
7 transitional,
calc‐akaline, and
shohonitic
calc‐alkaline and
shoshonitic
tholeiitic, transtional,
calc‐alkaline and
shoshonitic
Th/Yb
versus
Ta/Yb
(Pearce
diagram)
8 depleted mantle to
MORB source
affected by
subduction component
plotting as volcanic arc
enriched mantle
source affected
by subduction
component
plotting as
volcanic arc
depleted to enriched mantle
source affected by
subduction component
plotting as volcanic arc
extended
trace
elements
9 enrichment in LILE enrichment in LILE enrichment in LILE
negative anomalies
in Ta‐Nb
negative anomalies
in Ta‐Nb
negative anomalies in
Ta‐Nb
rare earth
elements
10 LREE enrichment LREE enrichment flat to LREE enrichment
primative island arc and
island arc
negative Eu anomalies negative Eu
anomalies
La/Yb
versus
Th/Yb
11 island arc continental margin
arc
normal arc
Sr/Y
versus Y
ESD 6 normal arc
continental detritus
normal arc
Th/La
versus
(Ce/Ce*)
Nd
ESD 7 continental detritus continental and volcanic
detritus
mid major
elements
versus
SiO
2
ESD 5 Al
2
O
3
, TiO
2
, CaO,
Fe
2
O
3
,P
2
O
5
, and
MgO negative
linear trend with
increasing SiO
2
Al
2
O
3
, TiO
2
, CaO, Fe
2
O
3
,
P
2
O
5
, and MgO negative
linear trend with
increasing SiO
2
AFM 5
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Table 3 Continued
Region diagrams Figure Jurassic‐Early Cretaceous Late Cretaceous Paleogene
subduction‐related
calc‐alkaline trend
subduction‐related
calc‐alkaline trend
Th versus
Co
6 high‐K and
shoshonitic
calc‐alkaline and high‐K
and shoshonitic
Zr versus
Y
7 calc‐alkaline and
shoshonitic
transtional, calc‐alkaline
and shoshonitic
Th/Yb
versus
Ta/Yb
(Pearce
diagram)
8 enriched mantle
source affected by
subduction
component
plotting as
volcanic arc
MORB to enriched mantle
source affected by
subduction
component plotting as
volcanic arc
extended
trace
elements
9 enrichment in LILE enrichment in LILE
negative anomalies
in Ta‐Nb
negative anomalies in
Ta‐Nb
rare earth
elements
10 LREE enrichment LREE enrichment
negative Eu anomalies
La/Yb
versus
Th/Yb
11 continental margin arc island arc and continental
margin arc
Sr/Y
versus Y
ESD 6 normal arc normal arc and adakite‐like
Th/La
versus
(Ce/Ce*)
Nd
ESD 7 continental detritus continental and volcanic
detritus
west major
elements
versus
SiO
2
ESD 5 Al
2
O
3
, TiO
2
, CaO,
Fe
2
O
3
,P
2
O
5
, and
MgO negative linear
trend with increasing
SiO
2
Al
2
O
3
, TiO
2
, CaO, Fe
2
O
3
,
P
2
O
5
, and MgO
negative linear trend
with increasing SiO
2
AFM 5 subduction‐related
calc‐alkaline trend
subduction‐related
calc‐alkaline trend
Th versus
Co
6 calc‐alkaline and
high‐K and
shoshonitic
calc‐alkaline and high‐K
and shoshonitic
Zr versus
Y
7 tholeiitic, calc‐alkaline,
and shoshonitic
transtional, calc‐alkaline
and shoshonitic
Th/Yb
versus
Ta/Yb
(Pearce
diagram)
8 depleted to enriched
mantle source
affected by
subduction
component plotting
as volcanic arc
enriched mantle source
affected by
subduction
component plotting
as volcanic arc
extended trace
elements
9 enrichment in LILE enrichment in LILE
negative anomalies in
Ta‐Nb
negative anomalies in
Ta‐Nb
10 LREE enrichment LREE enrichment
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The Late Cretaceous samples of the northernmost LC at Bolnisi (Figure 3) and EP arc fall along similar
trends in geochemical diagrams (Table 3), akin to those of the Jurassic‐Early Cretaceous samples. This indi-
cates that the magmatic rocks of each of these two regions formed due to a comparable process not only dur-
ing Late Cretaceous times but since Jurassic times. We ascribe the emplacement of these magmatic rocks to a
similar arc setting from the Jurassic to at least the Late Cretaceous. Yet the Th‐Co discrimination diagram
(Figure 6) shows a subduction‐related calc‐alkaline trend for the samples of Bolnisi, while the samples from
the EP exhibit high‐K and shoshonitic compositions. Only the samples from the western EP partially overlap
with those of Bolnisi in the calc‐alkaline field. Zr versus Y diagrams (Figure 7) contradict this by showing
Bolnisi, eastern EP, and mid‐EP samples plot both as calc‐alkaline and shoshonitic.
In light of all the geochemical data (summarized in Table 3), the Late Cretaceous subduction‐related mag-
matic rocks of the LC and the EP share similar characteristics. The geographic positioning of the EP and
LC arcs in lateral continuation from one another along the northern Tethyan suture further supports a com-
mon geodynamic process responsible for their formation (Figure 1a). Yet important variations in subduction
dynamics and margin evolution are apparent along the strike of the intercontinental boundary, suggestive of
particular subduction dynamics along mid and east EP compared to west EP and Bolnisi regions (i.e., slab
roll‐back).
6.3. Paleocene‐Eocene Subduction‐and Collision/Post Collision‐Related Magmatism: EP‐LC
Early Paleogene ages are well represented in the LC (Bolnisi and Kapan) and EP (Figure 2). In the LC, ages
are constrained to Eocene times, whereas in the EP magmatism ranges from the beginning of the Paleocene
and throughout the Eocene. Magmatism in the Kapan region coincides with the early magmatic evolution of
Meghri‐Ordubad pluton which is linked to the subduction of the southern branch of the Neotethys (Mederer
et al., 2013; Moritz et al., 2016; Rezeau et al., 2016, 2017), which is not within the scope of this study. There is
no real consensus for the signification of the Paleocene‐Eocene magmatism of the entire EP. It has been
ascribed not only to subduction‐related settings (Eyüboğlu et al., 2013, 2017; Eyüboğlu, Santosh, &
Chung, 2011; Eyüboğlu, Santosh, Dudas, et al., 2011) but also to collision (Karsli et al., 2011; Yilmaz‐
Sahin, 2005) and post‐collision‐related settings (Aydınçakır&Şen, 2013; Kaygusuz & Öztürk, 2015; Topuz
et al., 2011).
Paleogene samples from the LC and EP show similar trends as those of the Jurassic‐Early Cretaceous and
Late Cretaceous magmatic rock samples (summarized in Table 3). This suggests that the magmatic rocks
of each of these two regions formed due to similar continuous process since the Jurassic times. The emplace-
ment of these magmatic rocks can be attributed to a common setting evolving from Jurassic to at least
Eocene times. Samples from Bolnisi show the most evolved compositions while those from the EP show a
distinct E‐W partition, from the least evolved compositions for the Eocene east EP, to the most evolved com-
positions for the Paleocene‐early Eocene (Ypresian) in the west EP region. In the La/Yb versus Th/Yb dia-
gram (Figure 11), the Paleocene‐early Eocene (Ypresian) samples of Bolnisi are characterized as
continental margin arc, as are those of the mid‐EP and west EP. Mid‐Eocene (Lutetian) samples from the
east EP and the mid‐EP have island arc compositions, while samples from the east EP extend to primitive
island arc compositions. In the Sr/Y versus Y diagram (ESD 6), the samples of the LC and EP sectors plot
Table 3 Continued
Region diagrams Figure Jurassic‐Early Cretaceous Late Cretaceous Paleogene
rare earth
elements
negative Eu anomalies negative Eu anomalies
La/Yb versus
Th/Yb
11 primitive island arc,
island arc, and
continental
margin arc
continental margin arc
Sr/Y versus Y ESD 6 normal arc normal arc and
adakite‐like
Th/La versus
(Ce/Ce*)Nd
ESD 7 continental and
volcanic detritus
continental and
volcanic detritus
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as normal arc and adakite‐like rocks. Samples emplaced during the Paleocene‐early Eocene (Ypresian)
corresponding to mid‐EP have normal magmatic arc tendencies, while the Bolnisi and west EP regions plot
as adakite‐like rocks. Mid‐Eocene (Lutetian) samples of the mid‐EP show normal arc tendencies, while those
of east EP plot within the overlap of the normal arc and adakite‐like fields.
In addition to their geographic positioning, the Paleogene magmatic rocks of the LC and the EP share similar
characteristics. Yet important variations geochemical tendencies (Table 3) indicative of variations in subduc-
tion dynamics and margin evolution are apparent along the strike of the intercontinental boundary, the
IAES and ASAS. The Eocene magmatic rocks of the Bolnisi district are coeval with the early Eocene
adakite‐like magmatism of the EP (Figure 2). Thus, both the EP and the Bolnisi district share a common
early Eocene collision and postcollisional magmatic evolution, suggestive of asthenospheric upwelling in
relation to slab dynamics (i.e., slab break‐off, roll‐back, or tearing). The drawing of asthenospheric flow
and the accompanying renewed metasomatism of rejuvenated mantle is in accordance with previous studies
concerning the Eocene magmatism of the EP (Dokuz et al., 2019; Eyüboğlu, Santosh, Dudas, et al., 2011;
Karsli et al., 2010, 2011; Topuz et al., 2005, 2011). However, in addition to mantle‐related processes and slab
melting, we cannot discount magmatic differentiation in a thickened lower crust, that could also have
generated the adakite‐like high Sr/Y ratios (Chapman et al., 2015; Chiaradia, 2015; Mamani et al., 2010;
Richards & Kerrich, 2007).
6.4. Subduction Polarity
Two models subsist concerning the subduction, or subductions, responsible for the magmatism of the EP
and LC. The first describes that the magmatic arcs result from the subduction of Tethyan realms
(Paleotethys and Northern Neotethys) toward the north below the South Eurasian margin, from Triassic
times until collision during the Paleogene (Adamia et al., 2011; Barrier et al., 2018; Dercourt et al., 1986;
Saintot et al., 2006; Saintot & Angelier, 2002; Sosson et al., 2017; Stampfli et al., 2001). In this model, the
BS opened in an intra‐arc to back‐arc setting. The second features a south dipping subduction of
Paleotethys throughout the Mesozoic to early Cenozoic times below the northern margin of the Pontides
(Bektaş, 1986; Eyüboğlu, 2010, 2015; Eyüboğlu et al., 2007, 2013, 2017, 2018; Liu et al., 2018). Recent mod-
ifications of this model also feature two subduction zones, a south and north dipping subductions along the
northern and southern margins of the Pontides, respectively, during latest Cretaceous to early Cenozoic
times (Eyüboğlu et al., 2019). This second model implies that the BS represents a relic of the Paleotethys
ocean, which mostly disappeared through south dipping subduction. In the latter model, the northern
Neotethyan realm opened in a back‐arc setting related to the south dipping Paleotethyan subduction.
Typical subduction polarity evidence such as obducted ophiolites, subduction/obduction‐related meta-
morphic rocks as well as radiolarite originating from abyssal parts of disappeared oceanic domains are only
present along the southern margin of the EP and LC's SK arcs, not along their northern margin (Asatryan
et al., 2010, 2011, 2012; Celik, 2007; Çelik et al., 2006, 2011; Danelian et al., 2007, 2008, 2010, 2012, 2014;
Galoyan, 2008; Galoyan et al., 2009; Hässig, Rolland, Sosson, Galoyan, Müller, et al., 2013; Hässig,
Rolland, Sosson, Galoyan, Sahakyan, et al., 2013; Hässig et al., 2017, 2019; Okay & Tüysüz, 1999; Okay
et al., 2001, 2006; Parlak & Delaloye, 1999; Parlak et al., 2012; Robertson, 2002; Robertson, Parlak,
Ustaömer, Taslı, et al., 2013; Rolland, Billo, et al., 2009; Rolland et al., 2010; Sosson et al., 2010; Topuz
et al., 2013). Furthermore, the seismic profiles of the BS (Nikishin et al., 2015a., 2015b) do not show any
structures suggestive of south dipping subduction or the existence of any basins older than Cretaceous.
On the contrary, this imagery is interpreted as portraying a rifted continental crust with a central oceanic
crust stratigraphically covered by a sedimentary fill indicating basin opening and deepening throughout
the Cretaceous (Afanasenkov et al., 2008; Görür, 1988; Nikishin et al., 2008; Robinson, Spadini, et al., 1995;
Shillington et al., 2008), followed by subsidence throughout the Paleogene (Letouzey et al., 1977; Shillington
et al., 2008).
The geochronological and geochemical data concerning the Sochi‐Ritsa/Bechasyn region of the GC's SS
(summarized in Table 3; McCann et al., 2010) strongly support a common history with the EP in spite of
being over 100 km farther north along the northern margin of the BS. In the second model (south dipping
subduction beneath the EP), a common origin for subduction‐related magmatic rocks of the GC with those
of the LC and EP is incompatible. In this model the GC and LC‐EP ensemble are on alternate sides of the
subduction: the GC north of the sinking slab, while the LC and EP on the overriding margin. The
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HÄSSIG ET AL. 29 of 41
significance of the geodynamic evolution of the Jurassic subduction‐related magmatic rocks of the GC, and
their similarities with those of the LC and EP arcs, is not considered nor discussed. The model proposing
solely a north dipping subduction and an opening of the BS as an intra‐arc to back‐arc basin implies that
the Jurassic subduction‐related magmatic rocks of the GC were originally part of the same continuous
belt from the LC's SK to the EP. Here, the Jurassic subduction‐related magmatic rocks of the GC
represent part of the missing Jurassic subduction‐related magmatic rocks of the EP.
6.5. Geodynamic Reconstruction: Cutting an Arc in Two
Major and trace element geochemistry (summarized in Table 3) generally supports a model derived from
regional geology and geochronology where the subduction‐related magmatic rocks of the EP and LC arcs
as well as those of the Sochi‐Ritsa/Bechasyn regions of the GC are all relics of a common arc. This underlines
ongoing evolution of a common interplate boundary from Jurassic to Eocene times with evolution and
maturing of a common arc throughout, including the opening of a related intra‐arc to back‐arc basin
(Figure 12). In light of the new and available geochemical and geochronologic data, it appears that the stu-
died subduction‐related magmatic rocks exhibit coherent, continuous, and similar tendencies throughout
the EP, LC, and GC regions during the Jurassic‐Early Cretaceous, throughout the EP and LC during the
Late Cretaceous. Considering EP and LC Paleogene evolution, analyses of the data manifest differences in
subduction, collision, and postcollision dynamics.
The Jurassic‐Early Cretaceous subduction‐related magmatic rocks of the GC (Sochi‐Ritsa/Bechasyn), LC
(SK, Kapan, and Bolnisi), and EP to represent relics of a common and continuous arc along the
Southern Eurasian Margin (Figure 12a). The arc was deformed and divided throughout its formation
due to the opening of marginal basins (Figures 12b–12d) and even further afterward due to collisional
processes (Stephenson & Schellart, 2010). Division of an active arc into two segments (one active and
another inactive “remnant”) separated by an oblique marginal basin has been proposed in the
Mediterranean where Corsica and Sardinia represent remnant relics of the current Apennine arc
Figure 12. Middle Toarcian (ca. 180 Ma) to present‐day palaeotectonic evolution of the NE Anatolian‐Lesser Caucasus region. Maps modified from DARIUS
Programme paleotectonic maps of the Central Tethyan Realm (Barrier et al., 2018) featuring new interpretations.
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separated by the Tyrrhenian basin (Karig, 1972; Sartori, 2003). Also, northeast of New Zealand, studies
have shown that the Colville arc represents a remnant relic of the active Kermadec arc, displaced from
above the active SSZ by the opening of the Havre Trough (Wright, 1997; Wright et al., 1996). The
Sochi‐Ritsa/Bechasyn region of the GC and the EP are separated by the EBS (Figures 12d and 12e).
The Sochi‐Ritsa/Bechasyn region and the LC are separated by the Transcaucasus. Only the EP and the
LC arcs form a continuous structure along the Southern Eurasian Margin from Late Cretaceous to
Eocene times (Figure 12f), considered today to be the Northern Tethyan arc.
There are no reported Cretaceous subduction‐related magmatic rocks in the GC (Hess et al., 1995; McCann
et al., 2010). This strongly argues migration of the Sochi‐Ritsa/Bechasyn region away from the active SSZ
becoming remnant portions of the Tethyan arc during, or shortly preceeding, these times (McCann
et al., 2010; Figures 12d and 12e). Incidentally, the complete rifting and supposed formation of oceanic crust
of the EBS basin is constrained to the beginning of the Late Cretaceous (Nikishin et al., 2015a, 2015b;
Sheremet et al., 2017). Generally, Late Cretaceous magmatic rocks from the LC and the EP share similar
characteristics (Table 3), comparable arc formation settings (Figure 12f). Yet Late Cretaceous magmatic
rocks of the EP, particularly for the east EP and mid‐EP, have high‐K and shoshonitic tendencies
(Figures 6 and 7 and Table 3), a more enriched mantle source (Figure 8), and a clearly marked normal con-
tinental arc affinity (Figure 11, ESD 6). We argue that these characteristics indicate roll‐back of the north
dipping subducting Tethyan slab below EP, especially the east EP and mid‐EP. This roll‐back and the open-
ing of the EBS were coeval. This roll‐back induced the upwelling of asthenospheric mantle in the back‐arc
domain and caused magma underplating in the lower crust significantly weakening it (Göğüş, 2015; Kim
et al., 2015; Menant et al., 2016). Additionally, due to this roll‐back, dehydration of the subducting slab con-
tributed to greater mantle source metasomatism. As a consequence, many Late Cretaceous plutons across
the EP formed along the active continental margin in an intra‐arc extensional environment (Boztuğ
et al., 2004; Boztuğ& Harlavan, 2008; Karsli et al., 2010, 2012; Kaygusuz et al., 2009).
Characteristics of Paleogene magmatic rocks in the EP and LC exemplify disparities in collision and postcol-
lision dynamics as well as their timing (Figures 12g–12i). The Paleogene magmatism of the LC (Bolnisi) is
dated to the early Eocene (Figure 2). In the LC the end of collision and subsequent suturing of the interplate
domain is constrained to the beginning of the Eocene (Lordkipanidze et al., 1988; Sosson et al., 2010;
Figure 12h). The magmatic rocks there have high‐K and shoshonitic to calc‐alkaline tendencies (Figures 6
and 7 and Table 3), originating from an enriched mantle source (Figure 8), with normal arc and
adakite‐like characteristics (ESD 6) emplaced in a mature continental margin (Figure 11). Following entry
of the SAB in the subduction zone, we propose a rapid evolution from roll‐back of the subducting slab just
prior or during suturing to break‐off just after the said suturing in the LC. This model accounts for the coex-
istence of early Eocene high‐K and shoshonitic magmatism with adakite‐like tendencies. After the end of the
roll‐back, slab‐pull provoked slab break‐off (Sperner et al., 2001) of the north dipping Northern Neotethyan
oceanic lithosphere from the SAB (Sokol et al., 2018).
For the EP, the emplacement of the magmatic rocks is divided into two phases: (1) Paleocene‐early Eocene
(Ypresian) and (2) mid‐Eocene (Lutetian). Paleocene‐early Eocene (Ypresian) rocks of the mid‐EP and west
EP rocks exhibit calc‐alkaline and shoshonitic tendencies (Figures 6 and 7) originating from an enriched
mantle source (Figure 8) with adakite‐like characteristics (ESD 6) emplaced in a continental margin setting
(Figure 11). The mid‐Eocene times (Lutetian) rocks of the mid‐EP samples present calc‐alkaline and shosho-
nitic tendencies, but also transitional and some even tholeiitic (Figures 5 and 6) originating from a mantle
ranging from mid‐ocean ridge basalt (MORB) to more enriched mantle source (Figure 8) with normal arc
characteristics in a mature arc setting (Figure 11). East EP samples for this second Paleogene phase are quite
different. They exhibit calc‐alkaline and tholeiitic tendencies (Figures 5 and 6) from a mantle ranging from
depleted MORB to a more enriched mantle source (Figure 8) with normal arc characteristics in a primitive
arc setting (Figure 11).
It has been proposed that adakite‐like tendencies and the restoration of a primitive arc setting with
less evolved magmatic rocks are due to either slab break‐off (Aydınçakır&Şen, 2013; Boztuğ&
Harlavan, 2008; Dokuz et al., 2010; Karsli et al., 2011) or subduction of a mid‐oceanic ridge (Eyüboğlu
et al., 2013, 2017; Eyüboğlu, Santosh, & Chung, 2011), creating an asthenospheric window. Alternative mod-
els include the initiation of a second subduction creating a convergent double‐sided subduction system along
both southern and northern margins of the EP (Eyüboğlu et al., 2019) or orogenic root collapse of the EP
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(Kaygusuz & Öztürk, 2015). In light of the evolution derived from the LC, we favor a model featuring the slab
break‐off or westward tear propagation.
In the favored model, roll‐back first occurred in the EP during Cretaceous times while in the LC it
occurred during the early Eocene. Paleogeographic reconstructions (Barrier et al., 2018; Okay et al., 2001;
Sosson et al., 2010) argue that interplate Northern Neotethyan suturing occurred in the LC before sutur-
ing in the EP (Figures 12g–12i). This, in turn, argues that break‐off occurred first in the LC and propa-
gated westward to the EP. The resulting westward propagating window would be responsible for the
adakite‐like magmatism. Following this tearing‐off of the single north dipping oceanic lithospheric slab,
below the LC and EP, and complete suturing in the LC, a short‐lived magmatic pulse occurred
(Figure 12i). Resulting underplating (roll‐up) and dehydration of the remaining oceanic slab attached
to the underthrusted TAP and subsequent mantle wedge metasomatism occurred in the east EP and
mid‐EP during mid‐Eocene times.
7. Conclusions
By incorporating new data and observations with geochronological and geochemical data, including paleo-
geographic reconstructions, a Mesozoic and Cenozoic model is proposed for the evolution of the Caucasus,
EP, and Black Sea regions. The Jurassic‐Early Cretaceous segments of the arc run from the Sochi‐Ritsa/
Bechasyn region southeast to the Alaverdi district, continuing to the rest of the SK arc of the LC
(Figure 1b). The Late Cretaceous to Cenozoic portion of the arc is evidenced throughout the EP continuing
to the Bolnisi district. From east to west, Jurassic and Late Cretaceous to Cenozoic portions of the arc split to
the north and south side of the EBS, respectively. The scarcity of Jurassic and Early Cretaceous arc‐related
magmatic rocks in the EP and Early Cretaceous to Cenozoic arc‐related magmatic rocks in the GC
(Figure 2) argues for basin opening and important arc migration (formation of a remnant arc) during these
times.
The current position of the magmatic rocks and structure of the GC, the LC, the EP, and the EBS best support
a model in which they result from a single and continuous north dipping subduction of the northern branch
of the Neotethys with opening of the EBS basin in a back‐arc to intra‐arc setting throughout the Mesozoic
and Early Cenozoic. This model implies that there is no south dipping Paleotethyan subduction north of
the EP and LC.
Based on all the available geological data, we propose the following model for the evolution of the southern
Eurasian margin, along which a magmatic arc was formed during north dipping subduction of the
Paleotethyan and Northern Neotethyan domains, affected by collisional to postcollisional tectonics and
magmatism after accretion with the SAB and TAP (Figure 12):
1. Jurassic‐Paleogene magmatic rocks were emplaced due to a single north dipping interplate boundary
(subduction‐collision).
2. Emplacement of Jurassic‐Early Cretaceous subduction‐related magmatic arc.
3. Cretaceous roll‐back and onset of the EBS basin opening. The previously emplaced Jurassic magmatic
rocks in the EP region are transposed farther north of the SSZ due to the EBS basin opening.
4. Break‐off and suturing in the LC during the early Eocene.
5. Propagation of break‐off tearing westward to the EP.
Data Availability Statement
Data availability complies with FAIR Data guidelines. New data for this research are included in this paper
(Supporting information files are available at https://doi.org/10.5281/zenodo.4028469). Preexisting data sets
for this research are available in the references compiled in the caption of Figure 1(Altherr et al., 2008; Arslan
& Aslan, 2006; Aydin, 2014; Aydınçakır&Şen, 2013; Boztuğ& Harlavan, 2008; Calder et al., 2019; Delibaş
et al., 2016; Dokuz et al., 2010; Eyüboğlu, 2010; Eyüboğlu, Santosh, Dudas, et al., 2011; Eyüboğlu, Santosh,
& Chung, 2011; Eyüboğlu et al., 2013; Eyüboğlu et al., 2014; Eyüboğlu et al., 2017; Hess et al., 1995; Karsli
et al., 2010, 2011, 2012; Kaygusuz et al., 2009, 2010; Kaygusuz & Şen, 2011; Kaygusuz & Aydinçakir, 2011;
Kaygusuz et al., 2013, 2014; Kaygusuz & Öztürk, 2015; McCann et al., 2010; Mederer et al., 2013; Moore et
al., 1980; Sadikhov & Shatova, 2016, 2017; Sipahi et al., 2018; Topuz et al., 2011; Yilmaz‐Sahin, 2005).
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Acknowledgments
This work was carried out within the
scope of the Swiss National Science
Foundation projects 200020_168996
and 200021_188714. Funding for
fieldwork was provided by the
Fondation Ernst et Lucie Schmidheiny
and the Fonds Général de l'Université
de Genève. We thank Jean‐Marie
Boccard and Fabio Capponi for their
involvement in sample preparation and
data acquisition, as well as Agathe
Martignier for her involvement in
SEM‐CL image acquisition invaluable
for the U‐Pb dating.
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