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At the crossroads of the Lesser Caucasus and the Eastern Pontides: Late Cretaceous to early Eocene magmatic and geodynamic evolution of the Bolnisi district, Georgia

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The Bolnisi district is a distinct tectonic zone of the Lesser Caucasus, which is considered to represent the eastern extremity of the Turkish Eastern Pontides. Late Cretaceous, low-K, calc-alkaline to high-K rhyolite of the Mashavera and Gasandami Suites is the predominant rock type of the district, and is accompanied by subsidiary dacite, and rare high-alumina basalt and trachyandesite of the Tandzia Suite. The Mashavera and Gasandami rhyolite and dacite have yielded U-Pb LA-ICP-MS and TIMS zircon ages between 87.14 ± 0.16 and 81.64 ± 0.94 Ma, which are in line with the Coniacan-Santonian ages of radiolarian fauna of the Mashavera Suite. The felsic rocks of the Mashavera and Gasandami Suites were deposited during a ~6.6 m.y.-long silicic magmatic flare-up event, which together with the Tandzia Suite mafic rocks, documents Late Cretaceous bimodal magmatism in an extensional tectonic setting. Trace element data indicate that high Y-Zr, low- to high-silica rhyolite and dacite, and low Y-Zr high-silica rhyolite have been erupted, respectively, from coeval deep and shallow crustal reservoirs. The rocks of the bimodal magmatic event are overlain by high-K volcanic rocks of the Campanian Shorsholeti Suite, which have been erupted during slab roll-back and steepening, from magmas produced by deep melting of a metasomatised mantle. Eocene postcollisional felsic intrusions crosscut the Late Cretaceous rock. The Coniacian to early Campanian bimodal magmatism, and the subsequent high-K magmatism of the Bolnisi district are contemporaneous and share geochemical characteristics with the Late Cretaceous magmatism of the Eastern Pontides. It documents the existence of a Late Cretaceous regional silicic magmatic province, and subsequent high-K magmatism during slab steepening. This regional magmatic evolution coincided with the opening of the Black Sea and the Adjara-Trialeti basins. This evolution was coeval with the wanning stages of northern Neotethyan subduction, after a ~40 m.y.-long magmatic lull along the southern Eurasian convergent margin. Early Eocene adakite-like magmatism affected both the Bolnisi district and the Eastern Pontides, demonstrating a common postcollisional magmatic evolution.
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Research Article
At the crossroads of the Lesser Caucasus and the Eastern Pontides: Late
Cretaceous to early Eocene magmatic and geodynamic evolution
of the Bolnisi district, Georgia
Robert Moritz
a,
,Nino Popkhadze
b
, Marc Hässig
a
, Titouan Golay
a
, Jonathan Lavoie
a
,Vladimer Gugushvili
b
,
Alexey Ulianov
c
, Maria Ovtcharova
a
, Marion Grosjean
a
, Massimo Chiaradia
a
, Paulian Dumitrica
c
a
Department of Earth Sciences, University of Geneva, Rue des Maraîchers 13, 1205 Geneva, Switzerland
b
Al. Janelidze Institut of Geology, I. Javakhishvili Tbilisi State University, 0186 Tbilisi, Georgia
c
Institut of Earth Sciences, University of Lausanne, Géopolis, 1015 Lausanne, Switzerland
abstractarticle info
Article history:
Received 19 August 2020
Received in revised form 26 October 2020
Accepted 9 November 2020
Available online xxxx
Keywords:
Northern Neotethys
Lesser Caucasus
Silicic magmatic are-up
Bimodal and high-K magmatism
Eastern Pontides
The Bolnisi district is a distinct tectonic zone of the Lesser Caucasus, which is considered to represent the eastern
extremity of the Turkish Eastern Pontides. Late Cretaceous, low-K, calc-alkaline to high-K rhyolite of the
Mashavera and Gasandami Suites is the predominant rock type of the district, and is accompanied by subsidiary
dacite, andrare high-aluminabasalt and trachyandesite of the Tandzia Suite.The Mashavera and Gasandami rhy-
olite and dacite have yielded U-Pb LA-ICP-MS and TIMS zircon ages between 87.14 ± 0.16 and 81.64 ± 0.94 Ma,
which are in line with the Coniacan-Santonianages of radiolarianfauna of the Mashavera Suite. The felsic rocksof
the Mashavera and Gasandami Suites were deposited during a ~6.6 m.y.-long silicic magmatic are-up event,
which together with the TandziaSuite mac rocks, documents Late Cretaceous bimodal magmatism in an exten-
sional tectonic setting. Trace element data indicate that highY-Zr, low- to high-silica rhyolite and dacite,and low
Y-Zr high-silica rhyolite have been erupted, respectively, from coeval deep and shallow crustal reservoirs. The
rocks of the bimodal magmatic event are overlain by high-K volcanic rocks of the Campanian Shorsholeti Suite,
which have been erupted during slab roll-back and steepening, from magmas produced by deep melting of a
metasomatised mantle.Eocene postcollisional felsic intrusions crosscut the Late Cretaceous rock.
The Coniacian to early Campanian bimodal magmatism, and the subsequent high-K magmatism of the Bolnisi
district are contemporaneous and share geochemical characteristics with the Late Cretaceous magmatism of
the Eastern Pontides.It documents the existence of a LateCretaceous regional silicic magmatic province, and sub-
sequent high-K magmatism during slab steepening. This regional magmatic evolution coincided with the open-
ing of the Black Sea and the Adjara-Trialeti basins. This evolution was coeval with the wanning stages ofnorthern
Neotethyan subduction, after a ~40 m.y.-long magmatic lull along the southern Eurasian convergent margin.
Early Eocene adakite-like magmatism affected both the Bolnisi district and the Eastern Pontides, demonstrating
a common postcollisional magmatic evolution.
© 2020 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction
The Bolnisi district in southern Georgia is a distinct tectonic zone
along the Tethyan orogenic belt, which is located at the crossroads of
the Lesser Caucasus and the Eastern Pontides (Fig. 1). Both belts belong
to the southern Eurasian margin, which evolved from a Mesozoic sub-
duction environment to a collision and post-collision setting during
the latest Cretaceous and Cenozoic (Okay and Şahintürk, 1997;Sosson
et al., 2010;Adamia et al., 2011;Rolland et al., 2011;Okay et al., 2013).
The Bolnisi district is part of the Lesser Caucasus, where it sits at
the northern tip of the Jurassic-Cretaceous sedimentary-volcanic
Somkheto-Karabagh belt (Fig. 1). Because of its particular geological
setup consisting of voluminous Late Cretaceous silicic and subsidi-
ary mac magmatic rocks in a horst and graben setting (Adamia
et al., 2011;Apkhazava, 1988;Gugushvili, 2015;Gugushvili, 2018;
Gugushvili and Omiadze, 1988;Popkhadze et al., 2014), the Bolnisi dis-
trict is singled out as a separate tectonic zone within the Lesser Caucasus.
Furthermore, the Late Cretaceous and the post-collisional Eocene
magmatism of the Bolnisi district (Fig. 2) have been correlated with
Lithos xxx (2020) xxx
Corresponding author.
E-mail address: robert.moritz@unige.ch (R. Moritz).
LITHOS-105872; No of Pages 23
https://doi.org/10.1016/j.lithos.2020.105872
0024-4937/© 2020 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Contents lists available at ScienceDirect
Lithos
journal homepage: www.elsevier.com/locate/lithos
Please cite this article as: R. Moritz, N. Popkhadze, M. Hässig, et al., At the crossroads of the Lesser Caucasus and the Eastern Pontides: Late
Cretaceous to early Eocene m..., Lithos, https://doi.org/10.1016/j.lithos.2020.105872
the magmatic environment of the Eastern Pontides, along a trend ex-
tending towards Artvin in northeastern Turkey, but which is concealed
by Oligocene to Quaternary rocks (Fig. 1;Yilmaz et al., 2000, 2014;
Adamia et al., 2010, 2011;Hässig et al., 2020). This underlines the special
tectonic signicance of the Bolnisi district, as a link between the Lesser
Caucasus and the Eastern Pontides.
Voluminous silicic magmatism occurs in various tectonic settings re-
lated to subduction, continental break-up or continental hot spots
(e.g., Barker et al., 2020;Deering et al., 2008;Smith et al., 2005). Such
voluminous silicic magmatism, also referred to as ignimbrite are-ups,
is an outstanding geological event duringan orogenic evolution, mainly
developed in extensional or rift settings and characterized by bimodal
magmatism, where hot upwelling mantle triggers crustal melting
(Gravley et al., 2016). Therefore, we consider that the voluminous and
silicic nature of the magmatism of the Bolnisi district records a distinct
geological event, which must have profoundly affected the northern
part of the Lesser Caucasus during Late Cretaceous subduction of the
northern Neotethys, and which can be correlated with contemporane-
ous events in the Eastern Pontides, and subsequent Cenozoic collisional
and post-collisional evolution of the southern Eurasian margin.
While numerous studies have been carried out on the Late Creta-
ceous and Cenozoic magmatism evolution of the Eastern Pontides, and
its relationship with respect to the Black Sea (e.g., Eyüboğlu et al.,
2011,Eyüboğlu et al., 2014;Özdamar, 2016;Dokuz et al., 2019;
Kandemir et al., 2019;Aydin et al., 2020), there is a paucity of knowl-
edge about the Georgian Bolnisi district, which hinders any regional in-
terpretation and understanding with respect to the Eastern Pontides. In
this contribution, following an overview of its geological setting, we
present new lithogeochemicaland radiogenic isotope data of Late Creta-
ceous and Eocene magmatic rocks of the Bolnisi district, complemented
by U-Pb zircon age data of the magmatism, and fauna ages of sedimen-
tary and volcano-sedimentary host rocks. Together with published data,
the new geochemical and radiometric age data allow us to reconstruct
the Late Cretaceous to Eocene magmatic and geodynamic evolution
of the Bolnisi district, and discuss its link with the Turkish Eastern
Pontides.
2. Regional geological setting
The Lesser Caucasus consists of three main tectonic zones (Fig. 1;
Sosson et al., 2010;Adamia et al., 2011): (1) the Somkheto-Karabakh
belt and its southern extension, the Kapan block, belong to a
northwest-oriented Jurassic-Cretaceous magmatic arc, related to the
subduction of the northern Neotethys along the Eurasian margin
(Fig. 1;Kazmin et al., 1986;Lordkipanidze et al., 1989;Rolland et al.,
2011;Mederer et al., 2013); the Bolnisi district under study is located
at the northernmost extremity of the Somkheto-Karabakh belt
(Fig. 1); (2) in the southwest, the Gondwana-derived South Armenian
block consists of Proterozoic metamorphic basement rocks and Devo-
nian to Paleocene sedimentary and volcanic cover rocks (SAB in
Fig. 1), which is interpreted as the northeastern extension of the
Tauride-Anatolide platform (TAP in Fig. 1;Barrier and Vrielynck, 2008;
Sosson et al., 2010;Robertson et al., 2013;Meijers et al., 2015), and
which has been affected by extensive Cenozoic magmatism (Kazmin
et al., 1986;Moritz et al., 2016b;Rezeau et al., 2016, 2017); and
(3) the Jurassic-Cretaceous Amasia-Sevan-Akera ophiolite belt outlines
Fig. 1. Geological map of the Lesser Caucasus and Eastern Pontides, and adjoining areas (after Adamia and Gujabidze, 2004;ssig et al., 2013;Mederer et al., 2014;Delibaşet al., 2016;
Kandemiret al., 2019). Late Cretaceous areas of the Eastern Pontidesreferred to in this study:1 - Özdamar (2016),2-Eyüboğlu et al. (2014),3-Eyüboğlu (2010),4Aydin et al. (2020).
R. Moritz, N. Popkhadze, M. Hässig et al. Lithos xxx (2020) xxx
2
Fig. 2. Geologyof the Bolnisi district(after Vashakidze,2001;Vashakidze andGugushvili, 2006),and location of samples datedby U-Pb geochronology(this study and Hässig et al., 2020).
R. Moritz, N. Popkhadze, M. Hässig et al. Lithos xxx (2020) xxx
3
the suture zone between the Somkheto-Karabakh belt and the South
Armenian block (ASASZ in Fig. 1;Rolland et al., 2009a, 2009b), and is
correlated with the Izmir-Ankara-Erzincan suture zone (IAESZ in
Fig. 1), located between the Eastern Pontides and the Tauride-
Anatolide platform (Hässig et al., 2013).
Closure of the northern Neotethys and collision of the Gondwana-
derived South Armenian block and Tauride-Anatolide platform with
the Eurasian margin was diachronous. It is interpreted as late Campa-
nian to early Maastrichtian (~7371 Ma) in the Lesser Caucasus
(Meijers et al., 2015;Rolland et al., 2009a, 2009b), whereas the
Tauride-Eastern Pontides collision is generally interpreted as Paleocene
to early Eocene (Hippolyte et al., 2017;Kandemir et al., 2019;Karsli
et al., 2011;Okay and Şahintürk, 1997;Robertson et al., 2013;Şengör
and Yilmaz, 1981;Topuz et al., 2005, 2011), although Rice et al.
(2006) suggest collision as early as the Campanian-Maastrichtian, and
Dokuz et al. (2019) during the early Maastrichtian. This evolution was
coeval with progressive opening of the Black Sea, beginning during
the Late Cretaceous in the west and propagating eastward with time
(Hippolyte et al., 2017;Nikishin et al., 2011;Sosson et al., 2016).
These interpretations are consistent with northward subduction of the
northern Neotethys (Adamia et al., 2011;Aydin et al., 2020;Hässig
et al., 2013, 2020;Karsli et al., 2010;Özdamar, 2016;Rolland et al.,
2011;Sosson et al., 2010;Yilmaz et al., 2000), although other authors
advocate a south-verging Mesozoic subduction (Eyüboğlu, 2010;
Eyüboğlu et al., 2011, 2014).
3. Geological setting of the Bolnisi district
Late Cretaceous rocks predominate in the Bolnisi district. They crop
out between horsts of the Transcaucasian crystalline basement, named
Khrami and Loki massifs (Fig. 2;Zakariadze et al., 2007), which belong
to the Variscan belt of the Black Sea region (Okay and Topuz, 2017).
Both massifs consist of Late Proterozoic and Paleozoic metamorphic
and sedimentary rocks, crosscut by Neoproterozoic to Late Carbonifer-
ous, Jurassic and Cretaceous magmatic intrusions. Early Jurassic
volcaniclastic rocks, and Late Jurassic to Early Cretaceous limestone
and volcanoclastic rocks overlay unconformably the basement rocks
(Adamia et al., 2011;Zakariadze et al., 2007).
The Late Cretaceous rock sequence is known as the Bolnisi Group, and
unconformably overlies the Jurassic and older crystalline basement rock
units (Adamia et al., 2011).TheLateCretaceousrocksaredominatedby
calc-alkaline rhyolitic to rhyodacitic volcanic and sub-volcanic rocks, and
are accompanied by subsidiary dacitic, andesitic and basaltic rocks, and
sedimentary rocks (Adamia et al., 2011;Gugushvili, 2015;Gugushvili,
2018;Kazmin et al., 1986;Lordkipanidze et al., 1989). At Madneuli
(Fig. 2), a porphyritic granodiorite-quartz diorite crosscut by drilling
yielded a whole-rock K-Ar age of 8889 Ma (Gugushvili and Omiadze,
1988;Rubinstein et al., 1983). Rhyolitic domes at Madneuli have
whole-rock K-Ar ages of 8485 Ma, and 7271 Ma at Sakdrisi and
Beqtakari. Pyroclastic rocks from Sakdrisi yielded K-Ar ages of 77.6 Ma
(Fig. 2;Gugushvili, 2004).
The Bolnisi Group has been subdivided into ve to seven rock suites
with debated stratigraphic age interpretations(Fig. 3;Gambashidze and
Nadareishvili, 1980, 1987;Gambashidze, 1984;Apkhazava, 1988;
Vashakidze, 2001;Gugushvili, 2015,Gugushvili, 2018). The lower
lithostratigraphic sequences are predominantly exposed next to the
Khrami and Loki massifs (Fig. 2), and consist of sedimentary rocks and
rhyolitic tuff of the Cenomanian Ophreti and Tserakvi Suites, and an-
desitic to rhyolitic tuff and andesite and basalt lava ows interlayered
with limestone and marl of the early Turonian Digverdi Suite. The over-
lying Mashavera Suite is interpreted as late Turonian-Coniacian
(Vashakidze, 2001) or late Turonian-Santonian (Fig. 3;Gambashidze,
1984;Gambashidze and Nadareishvili, 1987;Apkhazava, 1988;
Gugushvili, 2015,Gugushvili, 2018). Migineishvili and Gavtadze
(2010) reinterpreted the Mashavera Suite as Campanian based on
nannofossil determinations (Fig. 2). The main lithologies of the
Mashavera Suite consist of subvolcanic magmatic bodies, domes/extru-
sions (Fig. 4a), lava ows with local columnar jointing textures, abun-
dant pyroclastic rocks, including surge and fall deposits, tuff (Fig. 4b),
and ignimbrite (Fig. 4cd). Hyaloclastite with pillow-like shapes are ev-
idence for a subaqueous environment (Fig. 4e), which is also supported
by interlayered siltstone, limestone, radiolaria-bearing beds (Fig. 4b)
and turbidite sequences. Locally, phreatomagmatic breccia crosscut
the volcanic and sedimentary rocks (Popkhadze et al., 2014, 2017,
2019).
The upper part of the Bolnisi Group consists of the Santonian
to Campanian Tandzia, Gasandami and Shorsholeti Suites (Fig. 3;
Gambashidze and Nadareishvili, 1980, 1987;Gambashidze, 1984;
Apkhazava, 1988;Gugushvili, 2015,Gugushvili, 2018). The Tandzia
Suite is absent in some stratigraphic reconstructions (Fig. 3,see
Vashakidze, 2001), and it is not reported as a separate unit in Fig. 2.
The Tandzia Suite includes aphyric to porphyritic basalt, andesitic ba-
salt and trachybasalt lava ows (Fig. 4f), tuff, and dikes (Fig. 4gh),
with sedimentary rock interlayers. It crops out in the northwestern
part of the study area, southwest of Beqtakari (Fig. 2). The Gasandami
Suite is made up of rhyolitic and rhyodacitic lava, subvolcanic intru-
sions, dikes, pyroclastic rocks (Fig. 4h), and limestone and volcano-
sedimentary rocks in its upper part. The uppermost Shorsholeti Suite
consists of porphyritic trachyandesite and basaltic trachyandesite lava
(Fig. 4i), interlayered with pyroclastic rocks and limestone. It crops
out in the western part of the study area, south of the Khrami massif
(Fig. 2).
The rhyolite and dacite of the Mashavera and Gasandami Suites pre-
dominate in the Bolnisi district (Fig. 2). The original mineral assemblage
of the Late Cretaceous magmatic rocks has been replaced by regional
prehnite-pumpellyite to low grade greenschist facies metamorphic
and propyllitic alteration minerals. Felsic rocks generally contain a
ne-grained quartz and feldspar matrix, and feldspar phenocrysts
with remnants of polysynthetic or Carlsbad twinning. Feldspar is re-
placed by muscovite-sericite, carbonate, epidote, zoïsite and prehnite-
pumpellyite. In intermediate to mac rocks, ferromagnesian minerals
are replaced by chlorite, epidote and carbonate. In pyroclastic rocks,
pumice and ame (Fig. 4d) are replaced by chlorite, iron oxydes, quartz,
carbonate, and prehnite-pumpellyite.The Bolnisi district is a majormin-
ing district (Fig. 2;Migineishvili, 2005;Gugushvili, 2004;Moritz et al.,
2016a). Therefore, several locations have also been affected by intense
hydrothermal alteration, including silicication, and variable potassic
(muscovite or K-feldspar), carbonate, argillic and epidote-zoïsite
alteration.
Shallow-marine to hemipelagic limestone, marl and conglomerate
of the Campanian to MaastrachtianTetritskaro Suite (Fig. 3), and Paleo-
cene marl, sandstone, and turbiditic terrigenous clastic rocks cover the
Bolnisi Group in the northern part of the study area (Fig. 2). They are de-
void of volcanic rocks, therefore it is concluded that magmatic arc activ-
ity had ceased by the Maastrichtian (Adamia et al., 2011;Yilmaz et al.,
2000). To the west and north, the Late Cretaceous rocks of the Bolnisi
Group are covered by Eocene volcanic rocks (Fig. 2), and in the northern
part by late Eocene shallow marine, turbiditic sedimentary rocks of the
Adjara-Trialeti belt related to rifting of the Black Sea (Adamia et al.,
2010;Lordkipanidze et al., 1979). Eocene rhyolitic subvolcanic intru-
sions, granodiorite and plagiogranite crosscut the Late Cretaceous
rocks in the central part of the study area (Figs. 2 and 4a). The youngest
units consist of Oligo-Miocene euxinic sedimentary rocks and Miocene
to Quaternary molasse and basaltic to rhyolitic volcanic rocks (Adamia
et al., 2011).
4. Results
4.1. Radiolaria determinations and age constraints of the Mashavera Suite
For stratigraphic age determination of the Mashavera Suite, radiolar-
ian fauna was extracted from beds interlayered with volcaniclastic
R. Moritz, N. Popkhadze, M. Hässig et al. Lithos xxx (2020) xxx
4
mudstone, ne-grained and pumice- to crystal rich tuff (Fig. 4b; see Ap-
pendix A), cropping out in the eastern part of the Madneuli mine
(Fig. 2). The stratigraphic age of the radiolarian fauna was determined
based on zonations for the Cenomanian-Maastrichtian (Pessagno Jr.,
1976), the late Barremian-early Turonian (ODogherty, 1994), and the
Cenomanian-Campanian (Bragina, 2004). Our determinations were
compared to Coniacian fauna from Deva Beds, Romania, and
Coniacian-Santonian fauna from Masirah Island, Oman (P. Dumitrica,
unpublished data). The most compelling age constraint is based on the
genus Alievium, which is characterized by an evolutionary trend of the
three main spines of the shell from the Turonian to Campanian
(Pessagno Jr., 1976). The spines changed from a completely three-
bladed morphology for Alievium superbum (Squinabol) in the
Turonian-lowermost Coniacian, to three-bladed proximally and conical
distally for Alievium praegallowayi Pessagno in the Coniacian-early
Santonian, and to completely conical for Alievium gallowayi Pessagno
in the Santonian-late Campanian.
The radiolarian fauna collected at Madneuli (Figs. 2 and 4b) contains
only Alievium praegallowayi (Fig. 5a). Neither Alievium superbum nor
Alievium gallowayi are present. Therefore, this fauna places the sample
from the Mashavera Suite within the Coniacian-early Santonian
Alievium praegallowayi radiolarian zone, and excludes a Campanian
age. The Coniacian age is also supported by the presence of Dictyomitra
formosa Squinabol sensu Pessagno 1976 (Fig. 5b), Archaeodictyomitra
Fig. 3. Comparison of U-Pb zirconages of magmatic rocks of the Bolnisi district with U-Pb and
40
Ar/
39
Ar ages of magmaticrocks from the Eastern Pontides, and stratigraphic age interpre-
tations of the Bolnisi Group. Numerical ages (Ma) of the stratigraphic stages arefrom the InternationalChronostratigraphic Chart (2016). Late Cretaceous volcanic rocks from the Eastern
Pontides: a = Kandemir et al.(2019), 92.1 Ma and 88.8 Ma (
40
Ar/
39
Ar), location 4 in Fig. 1;b=Eyüboğlu et al. (2014), 91.1 and83.1 Ma (SHRIMP U-Pb),location 2 in Fig.1;c=Revan et al.
(2017), 88.1 Ma in Tunca (LA-ICP-MS U-Pb); d = Aydin et al. (2020), 86.5 and 83.0 Ma (SHRIMPU-Pb), location 4 in Fig. 1;e=Özdamar (2016), 81.3 Ma (LA-ICP-MS U-Pb),and 86.0 and
75.3 Ma (
40
Ar/
39
Ar), location 1 in Fig. 1;f=Eyüboğlu (2010),80.9Ma(
40
Ar/
39
Ar), location 3 in Fig. 1. Early Eoceneadakite-like rocks from the Eastern Pontides: g = Dokuz et al.(2013),
54.4 Ma (
40
Ar/
39
Ar); h = Karsli et al. (2011), 53.6 and 51.3Ma (
40
Ar/
39
Ar); i = Eyüboğlu et al. (2011), 53.2Ma (LA-ICP-MS U-Pb); j = Topuz et al. (2005), 52.1 and 51.8 Ma (
40
Ar/
39
Ar);
k=Topuz et al. (2011), 51.5 and 51.3 Ma (
40
Ar/
39
Ar), and 51.1 Ma (LA-ICP-MS U-Pb); l = Karsli et al. (2010), 50.3 and 47.4 Ma (
40
Ar/
39
Ar). Abbreviations: Fm = formation, volc =
volcanic rocks.
R. Moritz, N. Popkhadze, M. Hässig et al. Lithos xxx (2020) xxx
5
R. Moritz, N. Popkhadze, M. Hässig et al. Lithos xxx (2020) xxx
6
squinaboli Pessagno (Fig. 5c), Dictyomitra napaensis Pessagno (Fig. 5d),
Pseudoaulophacus praeoresensis Pessagno (Fig. 5e), and Pseudoau-
lophacus circularis Bragina (Fig. 5f). These species also occur in the
Coniacian of the Deva Beds, Romania (P. Dumitrica, unpublished data).
Identication of Crucella irwini Pessagno at Madneuli (Fig. 5g) sup-
ports a middle-late Turonian to Coniacian age (Pessagno Jr., 1976).
The presence of Pseudodictyomitra nakasekoi Taketani (Fig. 5h) and
Pseudodictyomitra sp. A (Fig. 5i) at Madneuli is consistent with a
Turonian-Coniacian age, as illustrated by the Parapedhi Formation of
Cyprus (Bragina and Bragin, 2006).
4.2. Whole-rock major and trace element geochemistry
The analytical procedures are described in the electronic Appendix
A. The sample locations are shown in Fig. 2, and their major and trace el-
ement compositions are presented in the electronic Appendix B. Rock
samples for petrogenetic interpretations were collected outside of min-
eralized areas. Therefore, data trends in diagrams have petrogenetic sig-
nicance, and are devoid of hydrothermal alteration effects. Major oxide
data were normalized to a 100% volatile-free basis.
Samples from the Gasandami Suite and most of those of the
Mashavera Suite samples plot in the rhyolite eld in the total alkali-
silica diagram, and a few Mashavera Suite samples plot in the dacite
and trachydacite elds (Fig. 6a). The Tandzia Suite samples are basaltic,
trachybasaltic and basaltic trachyandesitic in composition, and the
Shorsholeti Suite samples plot across the trachyandesitic and basaltic
trachyandesitic elds (Fig. 6a). The Eocene intrusive rocks are grano-
dioritic to granitic in composition (Fig. 6a). Based on the immobile-
element classication diagram, Gasandami and Mashavera Suite
samples are mostly rhyodacitic/dacitic, except a few andesite and rhyo-
lite samples, and a larger group plotting in the trachyandesitic eld
(Fig. 6b). The Shorsholeti Suite and Eocene intrusive samples are
Fig. 4. a: Late Cretaceous rhyolitic subvolcanic intrusions of theMashavera Suite and early Eocene intrusion withan adakitic-like composition (sample BO-07-08), north ofQvemo Bolnisi
village (Fig. 2), inset: texture of the early Eocene intrusion with feldspar and ferromagnesian phenocrysts in a ne-grained matrix; b: ne-grained and pumice- to crystal-rich tuff of the
MashaveraSuite interbedded with volcaniclastic mudstone and sandstone, and radiolaria-bearing beds, southeastern Madneuli open pit (Fig. 2); c: Mashavera Suite outcrop with juxta-
posed highY-Zr rhyolite (dark-coloured, sample SA-18-06) and lowY-Zr rhyolite (light-coloured) dated at 83.3 Ma (sampleSA-18-05, location1 in Fig. 2, southwest of Sakdrisi), thecon-
tact between both rhyolite types is wavy and is evidence that they were still in a plastic and hot state when they were deposited next to each other; d: welded ignimbrite with ame
texture (sample BO-07-33A, location 15 in Fig. 2 at Fakhalo); e: hyaloclastite with pillow-like shapes in the southeastern part of the Madneuli open pit (Fig. 2); f: basalt breccia owbe-
longingto the Tandzia Suite (sampleBO-07-26, northof Tandzia village);g: mac dike of the Tandzia Suite (sample BO-09-15A) crosscutting volcano-sedimentary rocksof the Mashavera
Formation (southwest of location3 in Fig. 2); h: mac dikeof the Tandzia Suite(sample SA-18-02A) crosscutting pumicetuff of the Gasandami Suite (Sakdrisi openpit IV, Fig. 2); i: basaltic
trachyandesite of the Shorsholeti Suite (sample BO-07-39, NNW of Dmanisi village, Fig. 2), inset: feldspar phenocrysts in a ne-grained matrix of the basaltic trachyandesite.
Fig. 5. Radiolaria fauna sampled in the Madneuli mine (location in Fig. 2), see radiolarian-bearing beds hosted by tuff (Fig. 4b). The white horizontal scale bar is 50 μmlong.
R. Moritz, N. Popkhadze, M. Hässig et al. Lithos xxx (2020) xxx
7
trachyandesite, and the Tandzia Suite samples plot in the andesitic/ba-
saltic and sub-alkaline basaltic elds (Fig. 6b).
The Marshavera Suite, Tandzia Suite and Eocene intrusions belong
mostly to the calc-alkaline series and partly to the low-K/tholeiite series,
and a few Marshavera dacite samples are high-K calc-alkaline (Fig. 6c).
The Gasandami Suite has a dominantly low-K/tholeiitic composition
(Fig. 6c). The trachyandesitic Marshavera Suite and the Shorsholeti
Suite samples fall in the high-K calc-alkaline to shoshonite elds
(Fig. 6c). A dominantly calc-alkaline and felsic to intermediate composi-
tion of the samples of this study is supported by the Th vs. Co diagram
(Fig. 6d), except for the Tandzia samples, which plot in the macrock
eld, and the Shorsholeti samples, which are high-K calc-alkaline and
shoshonitic. Tandzia and Shorholeti Suite samples have the highest
TiO
2
,Al
2
O
3
,Fe
2
O
3
, MgO and CaO concentrations, which is consistent
with their mac to intermediate composition (Fig. 7ae). The
Gasandami Suite and Eocene intrusions have the highest Na
2
Oconcen-
trations (Fig. 7f), which t with their predominant low-K composition
(Fig. 6c).
Two distinct groups of Mashavera Suite rhyolite are identied
based on different trace element compositions. One group has
distinctly higher Rb and lower Y and Zr concentrations (Fig. 7gi),
and partly lower Nb, and higher Ba and Th concentrations than the
other one (Fig. 7jl). Therefore, in the remaining part of this contri-
bution, we will distinguish high Y-Zr and low Y-Zr rhyolite types
for the Mashavera Suite (Figs.2,69), which are also clearly visible
in outcrops (Fig. 4c). In the immobile-element classication dia-
gram (Fig. 6b), the Mashavera low Y-Zr rhyolite samples plot mostly
as trachyandesite, and the high Y-Zr rhyolite samples plot as
rhyodacite/dacite. The high Rb concentration of the Mashavera low
Y-Zr rhyolite (Fig. 7g) is consistent with their high-K calc-alkaline
and shoshonitic composition (Fig. 6c). The Mashavera low Y-Zr rhyo-
lite has SiO
2
concentrations above 75 wt% (Fig. 6a, c), therefore it
qualies as high-silica rhyolite (Gualda and Ghiorso, 2013). By con-
trast, the Mashavera high Y-Zr rhyolite falls in a broader range of
SiO
2
concentrations from 71 to 78 wt%, and consists of both low-
and high-silica rhyolite (Fig. 6a, c). The Mashavera dacite and the
Gasandami rhyolit e samples have trace element concen trations over-
lapping with those of the Mashavera Suite high Y-Zr rhyolite (Fig. 7g
l), and therefore are also labeled as Mashavera Suite high Y-Zr dacite
and Gasandami Suite high Y-Zr rhyolite (Figs. 2, 69).
Fig. 6. Geochemica l classication diagrams for magmatic rocks from theBolnisi district; a: TAS classication (Le Maître,2002), with equivalentnames of coarse-grainedintrusive rocks in
brackets (Middlemost, 1994); b: classication based on immobile elements (Winchester and Floyd, 1977); c: K
2
O (wt%) vs. SiO
2
(wt%) (Pecerillo and Taylor, 1976); d: Th (ppm) vs. Co
(ppm) (Hastie et al., 2007). High- and low-silica rhyolite boundary in 6a and 6c from Gualda and Ghiorso (2013).
R. Moritz, N. Popkhadze, M. Hässig et al. Lithos xxx (2020) xxx
8
Mashavera Suite high Y-Zr rhyolite and dacite and the Gasandami
Suite have similar primitive, mantle-normalized trace element spider
and chondrite-normalized rare earth element (REE) patterns (Fig. 8af).
The Mashavera Suite low Y-Zr rhyolite samples are distinctly enriched
in large ion lithophile elements (LILE: Cs, Rb, Ba, K), Th and U, and de-
pleted in some high eld strength elements (HFSE: Hf, Zr, Ti), Y and
middle to heavy REE (Nd to Lu) with respect to the Mashavera Suite
low Y-Zr rhyolite samples (Fig. 8gh). In addition, the Mashavera
Suite low Y-Zr rhyolite has a characteristic U-shaped middle to heavy
REE pattern, which is distinct with respect to the at REE pattern
of the Mashavera Suite high Y-Zr rhyolite (Fig. 8h). The Tandzia,
Shorsholeti and Eocene intrusion samples have spider and REE diagram
trends distinct with respect to those of the Mashavera and Gasandami
Suites (Fig. 8in). The Eocene intrusion samples are enriched in Sr and
depleted in Ta, Nb and REE with respect to the Late Cretaceous rocks
(Fig. 8mn), and they also display a U-shaped middle REE pattern
(Fig. 8n).
4.3. Strontium and neodymium whole-rock isotope geochemistry
The analytical procedures are described in the electronic Appendix
A. The whole-rock radiogenic isotope data are presented in the elec-
tronic Appendix C. Late Cretaceous rocks from the Bolnisi study area
have
143
Nd/
144
Nd ratios between 0.51260 and0.51278, and
87
Sr/
86
Sr ra-
tios between 0.70375 and 0.70629 (Fig. 9a). The Tandzia samples fall on
the mantle array, and the Shorsholeti Suite samples fall along or close to
Fig. 7. Major element (af) andtrace element diagrams (gl) vs. SiO
2
(wt%) for the magmatic rocks from the Bolnisi district. Greyshaded area in each diagram:compositional gap of SiO
2
concentrations (51.7 to 66.3 wt%) between mac rocks (Tandzia Suite) and felsic rocks (Gasandami and Mashavera Suite) of this study. High-silica (HSR) and low-silica rhyolite (LSR)
boundary from Gualda and Ghiorso (2013).
R. Moritz, N. Popkhadze, M. Hässig et al. Lithos xxx (2020) xxx
9
Fig. 8. Primitive mantle-normalized trace element spider diagrams (a, c, e, g, i, k, and m; normalization with respect to Taylor and McLennan, 1985) and rare earth element-normalized
diagrams(b, d, f, h, j, l, and n; normalizationwith respect to Sun and McDonough, 1989)for rock formations of the Bolnisi district(see Fig. 3). Grey shaded areain c to n: compositionaleld
of the Mashavera Suite high Y-Zr rhyolite data depicted in a and b.
R. Moritz, N. Popkhadze, M. Hässig et al. Lithos xxx (2020) xxx
10
the mantle array (Fig. 9a). By contrast, the Mashavera and Gansadami
samples have distinctly higher
87
Sr/
86
Sr ratios and plot mostly to the
right of the mantle array (Fig. 9a). The Gasandami samples have the
highest
143
Nd/
144
Nd ratios between 0.51269 and 0.51278 (Fig. 9a).
The Eocene intrusion sample studied by Hässig et al. (2020) falls along
the mantle array with an elevated
143
Nd/
144
Nd ratio and a low
87
Sr/
86
Sr ratio (Fig. 9b).
Two trends can be recognized in
87
Sr/
86
Sr and
143
Nd/
144
Nd vs. SiO
2
variations diagrams (Fig. 9cd): relatively constant isotopic composi-
tions with variable SiO
2
concentrations typical of magmatic fraction-
ation trends, and oblique trends towards higher
87
Sr/
86
Sr and lower
143
Nd/
144
Nd ratios with increasing SiO
2
concentrations, which are typi-
cal of crustal assimilation.
4.4. U-Pb dating of magmatic zircons
The U-Pb zircon age data of six samples are presented in Fig. 10,in-
cluding cathodoluminescence images of zircons (Fig. 10ac). The analyt-
ical data can be found in the electronic Appendix D, and the analytical
methodology in the electronic Appendix A. The sample locations are
shown in Fig. 2, and the data are summarized in Fig. 3, together with pre-
viously published ages (Hässig et al., 2020). Zircons dated in this study
were separated from volcanic rocks and dikes of the Mashavera Suite.
No zircons could be separated from samples of the Gasandami, Tandzia
and Shorsholeti Suites and tuff interbedded with sedimentary rocks con-
taining the radiolarian fauna at the Madneuli mine (Figs. 2 and 4b). Three
samples were analyzed by LA-ICP-MS and ID-TIMS, one sample by
LA-ICP-MS only, and two samples by ID-TIMS only (Fig. 10dp). Within
analytical error, core and rim analyzed in a single zircon grain yielded
overlapping Late Cretaceous ages (Fig. 10ac).
Sample SA-18-05 is a low Y-Zr rhyolite of the Mashavera Suite
cropping out SW of the Sakdrisi deposit (location 1 in Fig. 2), next to a
high Y-Zr rhyolite of the same suite (Fig. 4c). LA-ICP-MS dating yielded
aweighted
206
Pb/
238
U mean age of 83.3 ± 0.6 Ma (n = 14; Fig. 10de).
Sample BO-10-09 is a rhyolitic dike, affected by strong K alteration
and total depletion of Na linked to the Sakdrisi deposit (location 2 in
Fig. 2). Based on its trace element lithogeochemistry it belongs to the
low Y-Zr Mashavera Suite (electronic Appendix B). LA-ICP-MS dating
yielded a weighted
206
Pb/
238
U mean age of 84.9 ± 0.9 Ma (n = 12;
Fig. 10fg). Eleven single zircon grains were also dated by ID-TIMS
and yielded
206
Pb/
238
U ages between 85.75 ± 0.11 and 86.65 ±
0.09 Ma (Fig. 10h). The ve youngest zircons yield a weighted
206
Pb/
238
U mean age of 85.70 ± 0.05 Ma (Fig. 10h), which is considered
as the best approximation of the end of zircon crystallization. It overlaps
within error with the 84.9 ± 0.9 Ma LA-ICP-MS weighted
206
Pb/
238
U
mean age. The low and anomalous
206
Pb/
238
UagesobtainedbyLA-
ICP-MS dating at 82.6 ± 1.9 and 82.8 ± 1.6 Ma (Fig. 10f; electronic Ap-
pendix D2) are attributed to Pb loss following zircon precipitation.
Sample BO-09-14 is a dacitic dike of the high Y-Zr Mashavera Suite,
sampled west of Madneuli (location 5 in Fig. 2). LA-ICP-MS dating
yielded a weighted
206
Pb/
238
U mean age of 87.0 ± 1.1 Ma (n = 7;
Fig. 10ij). Six zircon grains yielded
206
Pb/
238
U ages by ID-TIMS between
86.60 ± 0.05 and 87.08 ± 0.09 Ma, and one outlier has a
206
Pb/
238
Uage
of 88.08 ± 0.15 Ma (Fig. 10k; electronic Appendix D1). The youngest
Fig. 9. Initial strontium and neodymium isotopic compositions of magmatic rocks from the Bolnisi district; a: Late Cretaceous rocks compared with magmatic rocks from the Eastern
Pontides, Turkey; b: Cenozoic rocks compared with magmatic rocks from the Eastern Pontides, Turkey; c: variation of initial strontium isotopic compositions of magmatic rocks from
the Bolnisi district with respect to SiO
2
concentrations; d: variation of initial neodymium isotopic compositionsof magmatic rocks from the Bolnisi district with respect to SiO
2
concen-
trations. Mantle array from Faure (1986), and CHUR and bulkEarth UR calculated according to Faure (1986) at 85 Ma and 52.5 Ma, respectively, in a and b.
R. Moritz, N. Popkhadze, M. Hässig et al. Lithos xxx (2020) xxx
11
Fig. 10. LA-ICP-MS and TIMSU-Pb ages of magmaticrocks from the Bolnisi district.a to c: representativescanning electron microscopyphotos of dated zircons. d and e: LA-ICP-MS zircon
206
Pb/
238
U weighted average plots and Concordia diagram of dated zircons of sample SA-18-05 (location 1 in Fig. 2). f and g: LA-ICP-MS zircon
206
Pb/
238
U weighted average plots and
Concordia diagram of datedzircons of sample BO-10-09(location 2 in Fig. 2). h: ID-TIMS zircon
206
Pb/
238
U ages of sampleBO-10-09. i and j: LA-ICP-MSzircon
206
Pb/
238
U weighte d average
plots and Concordia diagram of dated zircons of sample BO-09-14 (location 5 in Fig. 2). k: ID-TIMS zircon
206
Pb/
238
U ages of sample BO-09-14. l and m: LA-ICP-MS zircon
206
Pb/
238
U
weighted average plots and Concordia diagram of dated zircons of sample BO-07-18 (location 6 in Fig. 2). n: ID-TIMS zircon
206
Pb/
238
U ages of sample BO-07-18. o: ID-TIMS zircon
206
Pb/
238
U ages of sample BO-10-05A. p: ID-TIMS zircon
206
Pb/
238
U ages of sample BO-07-33B.
R. Moritz, N. Popkhadze, M. Hässig et al. Lithos xxx (2020) xxx
12
86.60 ± 0.05 Ma ID-TIMS age is considered as the best approximation of
the end of zircon crystallization, it overlaps within error with the 87.0 ±
1.1 Ma LA-ICP-MS weighted
206
Pb/
238
Umeanage.
Sample BO-07-18 is a rhyolitic dike from the high Y-Zr Mashavera
Suite next to the Madneuli deposit (location 6 in Fig. 2). It yielded a
weighted
206
Pb/
238
U mean age of 85.9 ± 1.2 Ma (n = 10) by LA-ICP-
MS dating (Fig. 10lm). Seven zircon grains were analyzed by ID-TIMS
and yielded
206
Pb/
238
U ages between 86.96 ± 0.21 and 88.70 ± 0.13
Ma. The four youngest zircons are concordant and yield a weighted
206
Pb/
238
U mean age of 87.13 ± 0.12 Ma (Fig. 10n), which is considered
as the best approximation of nal zircon crystallization.The three grains
with the oldest ID-TIMS ages are interpreted as inherited zircons
(Fig. 10n). Zircon z108 from sample BO-07-18, which yielded low,
anomalous LA-ICP-MS
206
Pb/
238
U ages of 83.2 ± 1.8 and 83.8 ±
2.0 Ma (Fig. 10a and l; Appendix D2) was subsequently dated by ID-
TIMS and yielded a clearly older
206
Pb/
238
U age of 87.14 ± 0.16 Ma.
Since the chemical abrasion technique used in this study is eliminating
any lead loss effect very efciently, we attribute the discrepancy be-
tween the young
206
Pb/
238
U ages obtained by LA-ICP-MS and the older
ones obtained by ID-TIMS to Pb loss following zircon precipitation.
Sample BO-10-05A is a rhyolitic dike of the high Y-Zr Mashavera
Suite from the upper part of the Madneuli open pit (location 7 in
Fig. 2). Five zircon grains yielded ID-TIMS
206
Pb/
238
U ages between
87.02 ± 0.16 and 87.66 ± 0.15 Ma (Fig. 10o). The youngest age of
87.02 ± 0.16 Ma is considered as the best approximation of the end of
zircon crystallization.
BO-07-33B is a dacitic pyroclastic ow of the high Y-Zr Mashavera
Suite (location 15 in Fig. 2). Seven zircon grains yielded ID-TIMS
206
Pb/
238
U ages between 86.61 ± 0.31 and 86.81 ± 0.24 Ma
(Fig. 10p). The youngest age of 86.61 ± 0.31 Ma is considered as the
best approximation of the end of zircon crystallization.
5. Discussion
5.1. Age constraints of the Mashavera and Gasandami Suites
The U-Pb zircon data of the Mashavera and Gasandami Suites fall
between 87.14 ± 0.16 and 83.90 ± 2.40 Ma, with one outlier at
81.64 ± 0.94 Ma (Fig. 3). This brackets the age of magmatic activity
to the late Coniacian, Santonian and the very early Campanian,
within a tight duration of 6.6 m.y. (Fig. 3). The radioisotope ages
are consistent with the Coniacian-Santonian radiolaria age for sedi-
mentary rocks interlayered with tuff of the Mashavera Suite
(Figs. 4band5). Within analytical error, core and rim analyzed in a
single zircon grain yielded overlapping Late Cretaceous ages
(Fig. 10ac). This excludes inheritance of zircon cores from signi-
cantly older rocks, such as the Late Proterozoic-Paleozoic and
Jurassic-Early Cretaceous rock units of the Khrami and Loki massifs
(Fig. 2). Our data support the Coniacian to Santonian stratigraphic
ages of Gambashidze and Nadareishvili (1980, 1987),Gambashidze
(1984),Apkhazava (1988),andVashakidze and Gugushvili (2006)
(Fig. 3).
Both rhyolite and dacite of the Mashavera high Y-Zr Suite yield over-
lapping ages and cover the full range of U-Pb zircon dates between
87.14 ± 0.16 and 81.64 ± 0.94 Ma (Fig. 3). By contrast, the Mashavera
Suite low Y-Zr rhyolite is restricted to the younger age range between
85.70 ± 0.05 and 83.3 ± 0.6 Ma (Fig. 3). We conclude, that magmatism
of the Mashavera Suite started during the Coniacian with thedeposition
of high Y-Zr rhyolite and dacite, and that low Y-Zr rhyolite of the
Mashavera Suite was deposited during the Santonian, coeval with the
wanning stages of the Mashavera high Y-Zr magmatism. Contempora-
neity of the high and low Y-Zr rhyolite sequences of the Mashavera
Suite is supported by their wavy contact at the outcrop scale (Fig. 4c),
which argues for a hot and plastic state during deposition of both rhyo-
lite types next to each other at 83.3 ± 0.6 Ma (location in Fig. 2;Sample
SA-18-05 in Fig. 10de, electronic Appendix D). Only one single age of
83.6 ± 1.3 Ma has been obtained for the Gasandami Suite high Y-Zr rhy-
olite by Hässig et al. (2020; location 3 south of Beqtakariin Fig. 2), which
agrees with its younger stratigraphic age interpretation with respect to
the Mashavera Suite (Fig. 3).
5.2. Late Cretaceous tectonic setting, bimodal magmatism and silicic
magmatic are-up
The negative Nb, Ta and Ti anomaliesin the mantle-normalized trace
element spiderdiagrams of the Gasandami and the Mashavera Suites are
typical for subduction-related magmas (Fig. 8a, b, e and g). This is con-
sistent with the subduction setting of the Bolnisi district during the Late
Cretaceous (Rolland et al., 2011;Sosson et al., 2010). Furthermore, the
chondrite-normalized REE patterns of rhyolite from the Gasandami
and the Mashavera Suites, with normalized La and Lu ratios of, respec-
tively, ~100 and ~10, only weak Eu negative anomalies (Fig. 8b, f and
h), and pronounced U-shaped middle to heavy REE patterns of the
Mashavera low Y-Zr Suite (Fig. 8h), are characteristic of cold-wet arc
rhyolites erupted in convergent margin environments (Bachmann and
Bergantz, 2008).
Rhyolite of the Late Cretaceous Mashavera and Gasandami Suites con-
stitutes the volumetrically most abundant rock type in the Bolnisi district,
and is accompanied by subsidiary dacite (Fig. 6a). Mac rocks of the
Tandzia Suite are a volumetrically minor rock type (Figs. 6a), which oc-
curs as dikes crosscutting both the Mashavera and Gasandami Suites
(Figs. 2 and 4gh), and as local lava breccia (Fig. 4f). There is a distinct
compositional gap of SiO
2
concentrations with Tandzia Suite samples fall-
ing below 51.7 wt% and Mashavera and Gasandami Suite samples above
66.3 wt% (Figs. 6aand7). The absence of intermediate rock types within
the concentration range from 51.7 to 66.3 wt% SiO
2
supports bimodal-
type magmatism (Meade et al., 2014;Melekhova et al., 2013)inthe
Bolnisi district. The alkaline Shorsholeti Suite is not considered here and
will be discussed later, because it is a stratigraphically younger rock se-
quence (Fig. 3), which crops out as a separate entity in the northwestern
part of the district (Fig. 2).
The overlapping U-Pb ages, trace element compositions, spider-
diagrams, REE-normalized patterns and radiogenic compositions of
the Mashavera Suite high Y-Zr rhyolite and dacite indicate a common
petrogenetic evolution (Figs. 3, 7gl, 8adand9a). The dacite and
low- to high-silica rhyolite (<75 wt% SiO
2
and >75 wt% SiO
2
, respec-
tively, Fig. 6a) association of the Bolnisi district is typical for volcanic
products erupted from zoned magma chambers in silicic magmatic
provinces (Graham et al., 1995;Streck and Grunder, 1997;Watts
et al., 2016). The Mashavera low Y-Zr rhyolite, with only high-silica rhy-
olite (>75 wt% SiO
2
,Fig. 6a) and its U-shaped middle to heavy REE
patterns (Fig. 8h), is attributed to a more pronounced magmatic dif-
ferentiation (Bachmann and Bergantz, 2008;Deering et al., 2008)at
the end of the volcanic evolution of the Mashavera Suite, between
85.70 ± 0.05 Ma and 83.3 ± 0.6 Ma (Fig. 3).
5.3. Chemical variations of Mashavera and Gasandami rhyolite and dacite:
coeval Late Cretaceous eruption from multiple isolated magma batches
Chemical variationsamong coeval rhyolite types within the same si-
licic magmatic province can be attributed to either in situ fractionation
of one single, large and layered magma chamber or to discrete, small
and coeval magma chambers having distinct chemical signatures
(e.g., Bégué et al., 2014). The wavy contact at the outcrop scale of the
high and low Y-Zr rhyolite sequences of the Mashavera Suite (Fig. 4c)
supports contemporaneity of both rhyolite types at a given time during
the deposition of the Mashavera Suite. It is support for the coexistence
of discreteand isolated magma batches in the crust, which have fed dis-
tinct, coeval silicic volcanic centres. For instance, such a magmatic set-
ting has been described in the Taupo volcanic zone, New Zealand
(Bégué et al., 2014;Charlier et al., 2005;Smith et al., 2005;Sutton
et al., 1995).
R. Moritz, N. Popkhadze, M. Hässig et al. Lithos xxx (2020) xxx
13
Major and trace element compositions can allow to distinguish coe-
val rhyolite types fed by distinct storage magma chambers with differ-
ent crystallization histories (Deering et al., 2008;Smith et al., 2005). In
the Bolnisi district, the Gasandami high Y-Zr rhyolite and the Mashavera
high Y-Zr rhyolite and dacite share similar trace and major element
compositions, which are distinct from those of the Mashavera low Y-
Zr rhyolite (Figs. 7a, c, gi, kl; 11ad). The ferromagnesian mineralogy
is an important control on the chemical composition of rhyolite and
dacite (Deering et al., 2008;Smith et al., 2005), and volcanic rocks
in general (Pearce and Norry, 1979). Pressure, temperature, fO
2
and
fH
2
O conditions control the stability of ferromagnesian minerals,
which fractionate in distinct magma chambers at different crustal
depths. In the Taupo volcanic zone, New Zealand, Smith et al. (2005)
and Deering et al. (2008) have shown that in shallow magmatic cham-
bers, low pressure and temperature, and high fH
2
Oandoxidizingcondi-
tions promote fractionation of hornblende and other hydrous phases
such as biotite, and suppress fractionation of plagioclase. This results
in depletion of middle REE, Y, Zr and Fe, and U-shaped middle REE pat-
terns. By contrast, in deep magmatic chambers, high pressure and tem-
perature, and low fH
2
O and reducing conditions promote fractionation
of pyroxene, resulting in an enrichment of middle REE, Y, Zr and Fe.
Despite thereplacement of the original magmatic assemblage by re-
gional metamorphic minerals in the Bolnisi district, the distinct chemi-
cal differences of rhyolite and dacite still record eruption from distinct
magma chambers located at different crustal depths. Indeed, the higher
TiO
2
,Fe
2
O
3
, Zr, Y and middle REE concentrations of the high Y-Zr rhyo-
lite and dacite of the Mashavera and Gasandami Suites, compared to the
Mashavera Suite low Y-Zr rhyolite (Figs. 7c, hi, 8b, d, f, h, and 11ac),
support eruption from deeper magma storage chambers, in which py-
roxene was part of the original assemblage (Deering et al., 2008;
Smith et al., 2005). The U-shaped middle to heavy REE patterns
(Fig. 8h) and the data trend in the Sr/Y vs. SiO
2
diagram (Fig. 11e) are
evidence for fractionation of hornblende and suppression of plagioclase
fractionation during petrogenesis of the Mashavera Suite low Y-Zr rhy-
olite, which reveals petrogenesis in shallow magma chambers (Deering
et al., 2008;Smith et al., 2005). The elevated K
2
O and Rb concentrations
of the Mashavera Suite low Y-Zr rhyolite are also in line with an ad-
vanced stage of magmatic fractionation and crystallisation of biotite in
shallow magma chambers (Figs. 6c, 7g, and 11d). The high SiO
2
concen-
trationsabove 75 wt% of the Mashavera Suite low Y-Zr rhyolite (Fig. 6a,
c) is further evidencefor the shallow depth of the magmastorage cham-
bers from which they have been erupted, since high-silica rhyolite
(SiO
2
> 75 wt%) is typical of low pressure environments, i.e. shallow
crust (Gualda and Ghiorso, 2013).
5.4. Tandzia suite mac rocks: high-alumina basalts co-genetic with
rhyolitic-dacitic magmatism
The mac rocks of the Tandzia Suite qualify as high-alumina basalts
with SiO
2
< 54 wt%, Al
2
O
3
>16.5wt%andMgO<7wt%(Crawford et al.,
1987;Figs. 6aand7b,d). They are characterized by gentle REE patterns
with only slight light element enrichment (La
N
/Yb
N
=39), nearly at
heavy element patterns (Tb
N
/Yb
N
=1.21.9) and devoid of a Eu anom-
aly (Fig. 8j), which are typical of high-alumina basalts from bimodal
magmatic provinces (Graham et al., 1995;Shuto et al., 2006;Zellmer
et al., 2020). The primitive nature of the high-alumina basalts at Bolnisi
is supported by their high Ti concentrations, low Zr concentrations,
MORB-type Zr/Y ratios (Fig. 11a, c), and their primitive mantle-
normalized spider diagrams devoid of Sr and Ti anomalies (Fig. 8i).
The trace element systematics of the mac Tandzia Suite samples
falls along similar trends as the ones of the high Y-Zr rhyolite and dacite
samples of the Mashavera and Gasandami Suite (Fig. 12af). This sup-
ports a petrogenetic link between the mac and felsic end-members
of the bimodal magmatism in the Bolnisi district. Although they are gen-
erally volumetrically minor, such mac rocks are a common component
in silicic provinces linked to bimodal magmatism (Graham et al., 1995;
Shuto et al., 2006;Smith et al., 2005;Zellmer et al., 2020). The Tandzia
Suite dikes crosscuting the Mashavera and Gasandami Suites (Fig. 5g
h), and their predominant northeast and subsidiary eastwest orienta-
tion are evidence for a structural control on mac magma ascent during
Late Cretaceous extension. Such structural control on mac magma as-
cent has also been reported in the Taupo volcanic zone, New Zealand
(Gamble et al., 1990;Graham et al., 1995). The lava ows of the Tandzia
Suite (Fig. 5f) are evidence of sporadic mac magmatism reaching the
surface. It suggests that ssure eruptions of the Tandzia Suite were coe-
val with rhyolitic and dacitic volcanism of the Mashavera and
Gasandami Suites, during bimodal magmatism in the Bolnisi district in
an extensional tectonic setting (Fig. 13ab).
5.5. Origin of magmas: evidence from radiogenic isotopes
The mac rocks of the Tandzia Suite have crystallized from mantle-
derived, primary magmas as documented by their juvenile Sr and Nd
isotopic compositions falling along the mantle array (Fig. 9a), and
their lack of Eu anomaly (Fig. 8j), suggesting no or only negligeable pla-
gioclase crystal fractionation or uptake (Graham et al., 1995;Zellmer
et al., 2020). Compared to the Tandzia Suite mac rocks, the Sr isotopic
compositions of most of the Mashavera and Gasandami Suite rocks are
shifted to higher
87
Sr/
86
Sr ratios, to the right of the mantle array
(Fig. 9a). The Nd isotopic compositions of the Mashavera Suite samples
scatter towards lower
143
Nd/
144
Nd ratios with respect to the Tandzia
Suite samples, whereas the ones of the Gasandami Suite are shifted to-
wards more elevated
143
Nd/
144
Nd ratios (Fig. 9a).
In
87
Sr/
86
Sr and
143
Nd/
144
Nd vs. SiO
2
variations diagrams (Fig. 9cd),
a limited number of Mashavera Suite samples have isotopic com-
positions overlapping with those of the Tandzia Suite samples. This
relatively constant isotopic composition despite variable SiO
2
concen-
trations is explained by extreme magmatic differentiation that has pro-
duced the rhyolitic and dacitic magmas from a common macreservoir
(see horizontal trends in Fig. 9cd). However, most Mashavera Suite
rhyolite and dacite samples display a concomitant shift towards higher
87
Sr/
86
Sr and lower
143
Nd/
144
Nd ratios with respect to the macrocks
(oblique trends in Fig. 9cd). This reects interaction with or assimila-
tion of continental crustal rocks by the rhyolitic and dacitic magmas
during their ascent and ponding in the crust (Fig. 13a). Such an evolu-
tion identied by radiogenic isotopes, with combined or successive
magmatic crystal fractionation and crustal interaction/assimilation is
typical during rhyolite and dacite petrogenesis in bimodal magmatic
districts (Graham et al., 1995;Sutton et al., 1995;Wilson et al., 2006).
The
87
Sr/
86
Sr and
143
Nd/
144
Nd vs. SiO
2
variations diagrams of the
Gasandami Suite samples are also consistentwith interaction or assim-
ilation of continental crustal rocks by rhyolitic melts (oblique trends in
Fig. 9cd). However, their elevated
143
Nd/
144
Nd ratios in comparison
to those of the Tandzia Suite mac rocks indicate that the Gasandami
Suite rhyolite is linked to a more juvenile mantle component than the
one that has generated the Tandzia and Mashavera Suite rocks. Al-
though the Gasandami Suite rhyolite has major and trace element con-
centrations and trends mostly overlapping with those of the Mashavera
Suite rhyolite (Figs. 6ab, 7ae, hi, and 11ac, e), it is characterized by
distinctly higher Na
2
OconcentrationsandlowerK
2
O, Rb, Nb, Ba and Th
concentrations (Figs. 6c, 7fg, jl, and 11d). The lower concentrations of
the light ion lithophile elements (LILE: Rb, Ba, Ba) and Th is also
expressed in the primitive mantle-normalized spider diagram, in
which the Gasandami samples are conned to the lower range of the
Mashavera Suite samples (right hand side of Fig. 8e). Due to their higher
Na
2
OandlowerK
2
O concentrations, the Gasandami Suite rhyolite qual-
ies mostly as low K or tholeiitic rocks (Fig. 6c). We attribute the more
pronounced isotopic mantle signature, higher Na
2
O concentrations and
lower LILE concentrations of the Gasandami Suite rhyolite to the pres-
ence of a more juvenile mantle component in the mantle wedge. The
more juvenile mantle component is attributed to asthenospheric man-
tle upwelling and/or tapping of a less metasomatized mantle (Fig. 13b).
R. Moritz, N. Popkhadze, M. Hässig et al. Lithos xxx (2020) xxx
14
5.6. Shorsholeti suite: high-K magmatism during Late Cretaceous slab roll-
back
The Shorsholeti Suite is a separate and stratigraphically younger
unit (Fig. 3), cropping out to the west of the Mashavera and
Gasandami units (Fig. 2), with a distinct high-K calc-alkaline to
shoshonitic composition (Fig. 6cd). The Shorsholeti Suite samples
have negative Nb, Ta and Ti anomalies, which are typical of arc
magmas (Fig. 8k). They have high incompatible element concentra-
tions, including Rb, Zr, Nb, Ba, Th, U, Sr and light REE, especially
when compared to the Tandzia Suite macrocks(Figs. 7g, il, 8l,
and 11ad). The composition of high-K calc-alkaline and shoshonitic
rocks and their strong enrichment in incompatible elements is gen-
erally interpreted to reect their mantle sources, and is attributed
to low-degree partial melting of lithospheric mantle and/or mantle
metasomatised by uids released from the oceanic crust and melting
of subducted sediments (Fig. 13c; Planck, 2005;Behn et al., 2011;
Kirchenbaur et al., 2012;Rezeau et al., 2017).
Subducted sediments are the main repositories of Th, U, Rb, Sr, Ba
and light REE (Kessel et al., 2005). Therefore, enrichment of the latter
in the Shorsholeti Suite (Figs. 7g, kl, 8kl, and 11e) is attributed to
their scavenging from subducted sediments present in the
metasomatized mantle wedge (Kessel et al., 2005). The presence of a
sedimentary component in the metasomatized mantle source is also
consistent with their high
87
Sr/
86
Sr and low
143
Nd/
144
Nd ratios, when
compared tothe Tandzia Suite samples, whichwere sourced by juvenile
mantle, not or less affected by metasomatism (Figs. 9aand14;
e.g., Kirchenbaur et al., 2012). The high Th/Yb and La/Yb ratios of the
Shorsholeti Suite samples also support a more important sedimentary
component in the melts sourced by the mantle wedge, in contrast to
Fig. 11. Trace and major element variation diagrams of magmatic rocks from the Bolnisi district documenting chemical compositional variations of samples from the different rock
formations. Compositional eldsof basalts in a: A = within-plate basalt, B = MORB and within-platebasalt, C = MORB, D = MORB andvolcanic-arc basalt, CAB = continental arc basalt,
OAB = oceanic arc basalt(from Pearce and Norry, 1979;Pearce, 1983). Compositional eld of basalts in c: MORB = middle oceanic ridge basalt (from Pearce, 1982).
R. Moritz, N. Popkhadze, M. Hässig et al. Lithos xxx (2020) xxx
15
Fig. 12. Geochemical composition of magmatic rocks from Late Cretaceous rocks of the Bolnisi district, and comparison with respect to Late Cretaceous rocks of the Eastern Pontides,
Turkey. a: Th/Yb vs. Ta/Yb tectonic discrimination and subduction component diagram (Aydin et al., 2020;Pearce, 1982), MORB: middle ocean ridge basalt, WPB: within plate basalt,
OIB: ocean island basalt; b: Ba/Yb vs. Ta/Yb discrimination diagram (Pearce et al., 2005); b: Nb/Yb vs. Ta/Yb discrimination diagram (Pearce et al., 2005); d: La/Sm vs. Sm/Yb diagram
(Kay and Mpodozis, 2001;Mamani et al., 2010;Shaei et al., 2009), with source enrichment and increasing pressure trends (Shaei et al., 2009); e: Ba/La vs. Th/Yb diagram with slab-
derived uid and sediment or sediment melts enrichment trends (Woodhead et al., 2001); f: Ba/Th vs. La/Sm diagram with uid-related and melt-related enrichment trends (Aydin
et al., 2020); g: La/Yb vs. SiO
2
(wt%) discrimination diagram, with adakite-eld (Richards and Kerrich, 2007), and monazite-allanite fractionation trend (Miller and Mittlefehldt, 1982).
R. Moritz, N. Popkhadze, M. Hässig et al. Lithos xxx (2020) xxx
16
R. Moritz, N. Popkhadze, M. Hässig et al. Lithos xxx (2020) xxx
17
the juvenile mantle source of the Tandzia Suite macrocks(Fig. 12e, g;
e.g., Kirchenbaur et al., 2012).
The Zr and Nb enrichment of the Shorsholeti Suite samples with
respect to the other Late Cretaceous magmatic rocks of the Bolnisi
district (Fig. 7ij) can be explained by high temperature and high pres-
sure (i.e., deep) conditions of mantle melting during petrogenesis
(Fig. 13c). Indeed, the degree of incompatibility of Zr and Nb becomes
more pronounced with increasing temperature and pressure conditions
of mantle melting at deeper settings (Kessel et al., 2005;Kirchenbaur
and Münker, 2015). Deeper pressure conditions during mantle melting,
which has sourced the Shorsholeti Suite rocks, is also supported by
higher Sm/Yb ratios with respect to the other Late Cretaceous magmatic
rocks, in particular the Tandzia Suite (Fig. 12d). Indeed, progressively
higher Sm/Yb ratios of magmatic rocks are typically correlated with
Fig. 13. Late Cretaceous to early Eocene geodynamic and magmatic evolution of the Bolnisi district in its regional context during convergence and subsequent collision of the South
Armenian block with the southern Eurasian margin. To the west, the Adjara-Trialeti belt merges with the Eastern Black Sea basin, the Bolnisi district and the Somkheto-Karabagh belt
with the Eastern Pontides, the Amasia-Sevan-Akera suture zone with the Izmir-Ankara-Erzincan suture zone, and the South Armenian block with the Tauride-Anatolide platform (see
Fig. 1). Horizontal and vertical dimensions of the cross-sections are not to scale, e.g., the width of the Bolnisi district is exaggerated with respect to the other tectonic zones.
Fig. 14. Comparison of the geochemical composition of magmatic rocks from Late Cretaceousrock formations of the Bolnisi district with respect to Late Cretaceous rockformations of the
Eastern Pontides, Turkey; a, c, e and g: primitive mantle-normalized trace element spider diagrams (normalization with respect to Taylor and McLennan, 1985); b, d, f and h: rare earth
element-normalized diagrams (normalization with respect to Sun and McDonough, 1989).
R. Moritz, N. Popkhadze, M. Hässig et al. Lithos xxx (2020) xxx
18
R. Moritz, N. Popkhadze, M. Hässig et al. Lithos xxx (2020) xxx
19
deeper mantle melting environments (e.g., Kay and Mpodozis, 2001;
Mamani et al., 2010;Shaei et al., 2009).
Deeper mantle melting conditions during petrogenesis of the
Shorsholeti Suite high-K magmas can be explained by slab roll-back.
High-K (shoshonitic) rocks occur in two distinct settings. In zoned vol-
canic arcs with a constant subduction angle, calc-alkaline rocks are
proximal with respect to the associated subduction zone and high-K
(shoshonitic) rocks are located in a distal setting, above the deeper
part of the subduction zone. By contrast, other volcanic arcs are devoid
of such a zonation, and successively more K-enriched magmatic rocks
are produced in a nearly stationary setting above a progressively steep-
ening subduction zone (Morrison, 1980). In the tectonic context of
the Bolnisi region, with a north-verging subduction and a roughly
eastwest-oriented magmatic arc during the Late Cretaceous (Hässig
et al., 2016;Rolland et al., 2011;Sosson et al., 2016), steepening of the
subduction zone (Fig. 13c) satisfactorily explains along-arc juxtaposi-
tion of the high-K Shorsholeti Suite and the low-K to calc-alkaline
Mashavera and Gasandami Suites (Fig. 2).
5.7. Coeval Late Cretaceous magmatic and tectonic evolution of the Bolnisi
district and the Eastern Pontides
The late Conacian to earliest Campanian bimodal magmatism of the
Bolnisi district coincides with the development of extensive rhyolitic to
basaltic magmatism in the Eastern Pontides (Fig. 3). Extensive strati-
graphic, petrographic and geochemical data have been published on
Late Cretaceous magmatism on different study areas of the Eastern
Pontides (location 1 to 4 in Fig. 1). The Late Cretaceous magmatism of
the Eastern Pontides is bimodal (Aydin et al., 2020), like in the Bolnisi
district (Figs. 6a, c, 7), and the magmatic evolution in both areas is strik-
ingly similar (Fig. 12). The Mashavera and Gasandami high Y-Zr rhyolite
and dacite, and the Tandzia Suite mac rocks have geochemical trends
(Figs. 12ad, 14ab, ef), which overlap with those of the calc-alkaline
Çatak, Kızılkaya and Çağlayan Formations studied in the Tirebolu and
the Artvin areas of the Eastern Pontides (locations 2 and 4, respectively,
in Fig. 1;Aydin et al., 2020 ;Eyüboğlu et al., 2014). The Mashavera and
Gasandami high Y-Zr rhyolite and dacite, and the Tandzia Suite rocks
yield large variations of Ba/La and Ba/Th ratios overlapping with those
of the Çatak, Kızılkaya and Çağlayan Formations (Fig. 12ef). There is
also a compositional gap between the mac-intermediate rocks of the
Çatak and Çağlayan Formations and the felsic-intermediate rocks of
the Kızılkaya Formation, which is comparable to the bimodal volcanism
of the Bolnisi district (Fig. 12g).
Younger felsic rock units of the Tirebolu and ÇayırbağFormations in
the Eastern Pontides, sitting stratigraphically above the Çatak, Kızılkaya
and Çağlayan Formations (Eyüboğlu et al., 2014;Aydin et al., 2020), dis-
play the same shift towards higher Th/Yb, Ta/Yb, Ba/Yb, Nb/Yb, La/Sm
and La/Yb ratios as the Mashavera Suite low Y-Zr rhyolite (Fig. 12ad,
g). The Mashavera Suite low Y-Zr rhyolite has primitive mantle-
normalized and REE patterns (Fig. 14cd), which mimic those of rhyo-
lite from the Ordu area (location 1 in Fig. 1;Özdamar, 2016) and the
ÇayırbağFormation of the Tirebolu area (location 2 in Fig. 1;Eyüboğlu
et al., 2014) This supports a similar petrogenetic evolution with time
towards shallower magmatic chambers that have fed rhyolitic mag-
matism in both the Eastern Pontides and the Bolnisi district.
The Eastern Pontides and the Bolnisi district host high-K magmatic
rocks, which are stratigraphically younger than the calc-alkaline
volcanic rocks of the Çatak, Kızılkaya, Çağlayan, Tirebolu and Çayırbağ
Formations in Turkey (Eyüboğlu, 2010), and the Mashavera, Gasandami
and Tandzia Suites in the Bolnisidistrict (Fig. 3). High-K magmatism has
been dated as Campanian in the Eastern Pontides (Aydin et al., 2020;
Eyüboğlu, 2010;Özdamar, 2016), and has been interpreted as latest
Santonian to Campanian in the Bolnisi district, Georgia (Fig. 3). The
high-K magmatic rocks of the Shorsholeti Suite, Bolnisi district and of
the Bayburt-Maden area, Eastern Pontides (location 3 in Fig. 1)display
similar geochemical trends, with high Th/Yb, Ta/Yb, Nb/Yb, Sm/Yb and
La/Yb ratios compared to older mac rocks in each area, respectively,
the Tandzia Suite, and the Çatak and Çağlayan Formations (Fig. 12ae,
g). Furthermore, the primitive mantle-normalized and REE patterns of
the Shorsholeti Suite from the Bolnisi district overlap with those of the
Bayburt-Maden area, Eastern Pontides (location 3 in Fig. 1), in particular
trachyandesite of cycle I (Eyüboğlu, 2010;Fig 14gh). Kandemir et al.
(2019) and Aydin et al. (2020) invoke a slab roll-back scenario and an
extensional tectonic setting during petrogenesis of the high-K mag-
matic rocks in the Eastern Pontides, whichis in line with our interpreta-
tion for the Shorsholeti Suite in Bolnisi.
The Late Cretaceous magmatic rocks of the Bolnisi district have gen-
erally distinctly higher
143
Nd/
144
Nd ratios than the rocks from the East-
ern Pontides (Fig. 9a), which documents a higher mantle component in
the magmas in Georgia. This can be attributed to a thinner crust, or al-
ternatively less assimilation of crustal material during petrogenesis of
the Late Cretaceous magmatic rocks in the Bolnisi district. Akin to the
Bolnisi rocks, the majority of the Late Cretaceous rocks of the Eastern
Pontides have
87
Sr/
86
Sr ratios falling to the right of the mantle array
(Fig. 9a), which is also attributed to interaction/assimilation of crustal
rocks by magmas during their ascent or ponding in the crust.
In conclusion, the Eastern Pontides and the Bolnisi district belong to
a common east-west-oriented, late Turonian to Campanian magmatic
belt. The bimodal volcanism with predominant silicic magmatism
(e.g., Aydin et al., 2020; this study) took place in an extensional tectonic
setting (Kandemir et al., 2019), during the wanning subduction stages
of the northern branch of the Neotethys (Fig. 13ab). The Turonian to
earliest Campanian silicic magmatic are-up propagated eastwards
from the Eastern Pontides to the Bolnisi district (Fig. 3), in parallel
with the Late Cretaceous west to east opening of the Black Sea basin
(Hippolyte et al., 2017;Nikishin et al., 2011;Sosson et al., 2016),
which merges with the Adjara-Trialeti belt in Georgia (Fig. 1;Adamia
et al., 2010;Yilmaz et al., 2014). The Turonian to earliest Campanian si-
licic magmatic are-up was followed by Campanian high-K magmatism
during slab roll-back along the entire belt (Fig. 13c; Eyüboğlu, 2010;
Özdamar, 2016;Aydin et al., 2020;thisstudy).
Mesozoic magmatism along the southern Eurasian convergent margin
has been interrupted by a ~40 m.y.-long magmatic lull at the Early to Late
Cretaceous transition, between ~130 and ~90 Ma (Hässig et al., 2020). The
tectonic and magmatic quiescence along the Lesser Caucasus at the Early
to Late Cretaceous transition, and Albian compressional tectonics and uplift
of the Pontides belt are attributed to accretion of oceanic seamounts and
plateaus, which impeded subduction along the Eurasian margin (Hässig
et al., 2016;Okay et al., 2006, 2013;Rolland et al., 2009b, 2011). North-
verging subduction of the northern Neotethys branch only resumed at
~90 Ma (Hässig et al., 2016, 2020;Kandemir et al., 2019;Okay et al.,
2006, 2013;Rice et al., 2006). Thus, the Late Cretaceous bimodal, domi-
nantly silicic magmatic are-up, and the subsequent high-K volcanism
are major markers of the geological evolution of the Eastern Pontides
Fig. 15.Comparison of the geochemical composition of Eocenemagmatic rocks fromthe Bolnisi districtand the Eastern Pontides, Turkey;a: Sr/Y vs. age (Ma); b: La/Yb vs.age (Ma) (ages of
the Bolnisi intrusions: Hässig et al., 2020); cand d: primitive mantle-normalized trace element spider diagrams (normalization with respect to Taylor and McLennan, 1985) with a com-
parisonof the data of the earlyEocene Bolnisi intrusions, respectively,with respect to earlyEocene adakite-like and middleEocene non-adakitic rocks ofthe Eastern Pontides; e: rare earth
element-normalized diagrams (normalization with respect to Sun and McDonough, 1989); f: Dy/Yb vs. SiO
2
(wt%) diagram with amphibole and garnet fractionation trends (Davidson
et al., 2007); g: Sr/Y vs. SiO
2
(wt%) discrimination diagram; h: La/Yb vs. SiO
2
(wt%) discrimination diagram; i: Al
2
O
3
(wt%) vs. SiO
2
(wt%); j: MgO (wt%) vs. SiO
2
(wt%) with high- and
low-silica adakites elds (Martin et al., 2005). Adakite eld in a, b, g, and h according to Richards and Kerrich (2007), with Sr/Y threshold at 20 from Richards and Kerrich (2007),
and at 40 from Castillo et al. (1999). Early Eocene adakite-like magmatic rocks of the Eastern Pontides from: Topuz et al. (2005, 2011),Karsli et al. (2010, 2011),Eyüboğlu et al. (2011)
and Dokuz et al. (2013). Middle Eocene non-adakitic magmatic rocks of the Eastern Pontides from: Aydınçakır(2014),Kaygusuz and Öztürk (2015) and Dokuz et al. (2019).
R. Moritz, N. Popkhadze, M. Hässig et al. Lithos xxx (2020) xxx
20
and the Lesser Caucasus. First, they record reactivation of magmatism
along the southern Eurasian convergent margin after a magmatic lull of
~40 m.y. Secondly, the eastwest-oriented Late Cretecaous extensional tec-
tonics and silicic magmatism were coeval with oblique rifting of the
northwest-oriented Jurassic-Early Cretaceous subduction-related mag-
matic belt during opening of the Black Sea basin, with the Somkheto-
Karabagh segment remaining to the south of the Bolnisi district (Fig. 1),
and displacing the western Greater Caucasus segment to the north, in
the Sochi-Ritsa/Bechasyn regions in Russia (Hässig et al., 2020).
5.8. Early EEEocene postcollisional evolution of the Eastern Pontides and the
Bolnisi district
The Eastern Pontides have been affected by Eocene postcollisional
magmatism, which has been subdivided into early Eocene adakite-like
magmatism (Dokuz et al., 2013;Eyüboğlu et al., 2011;Karsli et al.,
2010, 2011;Topuz et al., 2005, 2011) and middle Eocene non-adakitic
magmatism (Aydınçakır, 2014;Dokuz et al., 2019;Kaygusuz and
Öztürk, 2015). Uranium-lead zircon ages show that the Eocene granodi-
orite and granite of the Bolnisi district (Hässig et al., 2020;Fig. 2) are co-
eval with early Eocene adakite-likemagmatism of the Eastern Pontides
(Figs. 3, 15ab). Thus, the Eastern Pontides and the Bolnisi district share
a common early Eocene postcollisional magmatic evolution.
The early Eocene magmatic rocks in both areas display similar com-
positions and geochemical trends (Fig. 15). The primitive mantle-
normalized patterns ofthe early Eocene Bolnisi granodiorite and granite
mimic those of the early Eocene adakite-like rocks of the Eastern
Pontides (Fig. 15c), but contrast with the ones of the non-adakitic mid-
dle Ecoene rocks (Fig. 15d). The early Eocene intrusions of the Bolnisi
district have U-shaped middle REE and garnet fractionation patterns
similar to the ones of the adakite-like rocks of the Eastern Pontides
(Fig. 15ef). Based on the Sr/Y ratio, the early Eocene intrusions of the
Bolnisi district qualify as adakite-like rocks (Fig. 15a, g), but are only
transitional to the adakite eld according to the La/Yb ratio (Fig. 15b, h).
Petrogenesis of the early Eocene adakite-like magmatism in the
Eastern Pontides has been attributed to asthenospheric upwelling and
partial melting of subducted Late Cretaceous oceanic crust (Dokuz al.,
2019). A similar model may apply to the early Eocene adakite-like
magmatism in the Bolnisi district (Fig. 13d). However, in addition to
mantle-related processes and slab melting, we cannot discount mag-
matic differentiation dominated by amphibole in a thickened lower
crust (Fig. 13d), that could also have generated the adakite-like high
Sr/Y ratios and U-shaped middle REE patterns (Chiaradia, 2015;
Mamani et al., 2010;Richards and Kerrich, 2007) of the early Eocene
Bolnisi intrusions (Fig. 15a, e, g).
6. Conclusions
This study has allowed us to clarify the Late Cretaceous and early Eo-
cene evolution of the Georgian Bolnisi district of the Lesser Caucasus,
and its link with the Turkish Eastern Pontides. The Late Cretaceous evo-
lution of the Bolnisi district was dominated by voluminous silicic
magmatism and subsidiary mac magmatism in an extensional tectonic
setting during the wanning subduction stage of the northern Neotethys.
The magmatic evolution, setting and duration of the Georgian Bolnisi
district is comparable to major silicic magmatic provinces characterized
by bimodal magmatism, also known as ignimbrite are-ups, in New
Zealand, Mexico, Namibia or the southwestern U.S.A.
The Coniacian to earliest Campanian silicic and bimodal magmatism
of the Bolnisi district, its subsequent Campanian high-K magmatism
during slab steepening, and postcollisional early Eocene adakite-like
magmatism are comtemporaneous and comparable with the magmatic
evolution of the Turkish Eastern Pontides. Our study supports the exis-
tence of a regional silicic-dominant andbimodal magmatic belt extend-
ing from eastern Turkey to southern Georgia, which was formed
concomitantly with the opening of the Black Sea and Adjara-Trialeti
basins, in a back-arc setting with respect to the north-verging subduc-
tion of the northern Neotethys. The Late Cretaceous silicic magmatic
are-up of the Bolnisi district and the Eastern Pontides took place
after a ~40 m.y.-long magmatic lull at the Early to Late Cretaceous tran-
sition along the Eurasian convergent margin. The Late Cretaceous silicic
magmatic are-up contributed to the oblique rifting of the northwest-
oriented Jurassic-Early Cretaceous calc-alkaline magmatic arc, with the
Somkheto-Karabagh arc remaining in the south and the western ex-
tremity of the Greater Caucasus being translated to the north.
Declaration of Competing Interest
The authors declare that they have no known competing nancial
interests or personal relationships that could have appeared to inu-
ence the work reported in this paper.
Acknowledgements
The research was supported by the Swiss National Science Founda-
tion (grants 200020-121510, 200020-138130 and 200020-155928)
and the SCOPES Joint Research Projects (IB7620-118901 and IZ73Z0-
128324). Titouan Golay and Jonathan Lavoie were supported by grants
of the Augustin Lombard Foundation (Geneva Société de Physique et
dHistoire Naturelle), the Ernst and Lucie Schmidheiny Foundation,
and the Society of Economic Geologists (McKinstry Fund). The authors
would like to thank the staff of the Madneuli, Sakdrisi and Bektaqari
mines and the Rich Metal Group for access to open pits and prospects,
and for logistical support. We would like to thank the continuing sup-
port and interest of Malkhaz Natsvlishvili throughout the years. We also
thank Tamara Beridze and Sophio Khutsishvili for eldwork support,
and discussions with Shota Adamia and Ramaz Migineishvili. We are
grateful to the technical help by Jean-Marie Boccard, Fabio Capponi
and Antoine de Haller during thin section preparation, and XRF ana-
lyses. We are grateful to Aral Okay and Yann Rolland for their critical
reviews and their comments.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.lithos.2020.105872.
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