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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Article

Reference

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

MORITZ, Robert, et al .

Abstract

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 [...]

MORITZ, Robert, et al . 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. Lithos , 2020, vol. 378-379, no. 105872, p. 105872

DOI : 10.1016/j.lithos.2020.105872

Available at:

http://archive-ouverte.unige.ch/unige:145012

Disclaimer: layout of this document may differ from the published version.

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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

aDepartment of Earth Sciences, University of Geneva, Rue des Maraîchers 13, 1205 Geneva, Switzerland

bAl. Janelidze Institut of Geology, I. Javakhishvili Tbilisi State University, 0186 Tbilisi, Georgia

cInstitut of Earth Sciences, University of Lausanne, Géopolis, 1015 Lausanne, Switzerland

a b s t r a c t a r t i c l e i n f o

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 magmaticﬂare-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, 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 magmaticﬂare-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.

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 subduction environment to a collision and post-collision setting during the latest Cretaceous and Cenozoic (Okay andŞahintürk, 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 subsidiary maﬁc 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 district 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:[email protected](R. Moritz).

LITHOS-105872; No of Pages 23

https://doi.org/10.1016/j.lithos.2020.105872

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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

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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 signiﬁcance of the Bolnisi district, as a link between the Lesser Caucasus and the Eastern Pontides.

Voluminous silicic magmatism occurs in various tectonic settings related 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 ignimbriteﬂare-ups, is an outstanding geological event during an 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 contemporaneous 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 interpretation and understanding with respect to the Eastern Pontides. In this contribution, following an overview of its geological setting, we

present new lithogeochemical and 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 sedimentary 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 inFig. 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 (afterAdamia and Gujabidze, 2004;Hässig et al., 2013;Mederer et al., 2014;Delibaşet al., 2016;

Kandemir et al., 2019). Late Cretaceous areas of the Eastern Pontides referred to in this study: 1 -Özdamar (2016), 2 -Eyüboğlu et al. (2014), 3 -Eyüboğlu (2010), 4–Aydin et al. (2020).

R. Moritz, N. Popkhadze, M. Hässig et al. Lithos xxx (2020) xxx

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Fig. 2.Geology of the Bolnisi district (afterVashakidze, 2001;Vashakidze and Gugushvili, 2006), and location of samples dated by U-Pb geochronology (this study andHässig et al., 2020).

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the suture zone between the Somkheto-Karabakh belt and the South Armenian block (ASASZ inFig. 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 (~73–71 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 andŞahintürk, 1997;Robertson et al., 2013;Şengör and Yilmaz, 1981; Topuz et al., 2005, 2011), althoughRice 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). The Late Cretaceous rocks are dominated by 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 88–89 Ma (Gugushvili and Omiadze, 1988; Rubinstein et al., 1983). Rhyolitic domes at Madneuli have whole-rock K-Ar ages of 84–85 Ma, and 72–71 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 intoﬁve 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 andesitic to rhyolitic tuff and andesite and basalt lavaﬂows 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), lavaﬂows with local columnar jointing textures, abundant pyroclastic rocks, including surge and fall deposits, tuff (Fig. 4b), and ignimbrite (Fig. 4c–d). Hyaloclastite with pillow-like shapes are evidence 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 inFig. 2.

The Tandzia Suite includes aphyric to porphyritic basalt, andesitic basalt and trachybasalt lavaﬂows (Fig. 4f), tuff, and dikes (Fig. 4g–h), 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 intrusions, 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 predominate 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 fine-grained quartz and feldspar matrix, and feldspar phenocrysts with remnants of polysynthetic or Carlsbad twinning. Feldspar is replaced by muscovite-sericite, carbonate, epidote, zoïsite and prehnite- pumpellyite. In intermediate to mafic rocks, ferromagnesian minerals are replaced by chlorite, epidote and carbonate. In pyroclastic rocks, pumice andfiame (Fig. 4d) are replaced by chlorite, iron oxydes, quartz, carbonate, and prehnite-pumpellyite. The Bolnisi district is a major min- ing district (Fig. 2;Migineishvili, 2005;Gugushvili, 2004;Moritz et al., 2016a). Therefore, several locations have also been affected by intense hydrothermal alteration, including silicification, 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 Maastrachtian Tetritskaro 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 devoid of volcanic rocks, therefore it is concluded that magmatic arc activity 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 intrusions, 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, radiolarian fauna was extracted from beds interlayered with volcaniclastic

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mudstone,ﬁne-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 (O’Dogherty, 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 genusAlievium, 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 forAlievium praegallowayiPessagno in the Coniacian-early Santonian, and to completely conical forAlievium gallowayiPessagno in the Santonian-late Campanian.

The radiolarian fauna collected at Madneuli (Figs. 2 and 4b) contains onlyAlievium praegallowayi(Fig. 5a). NeitherAlievium superbumnor Alievium gallowayiare present. Therefore, this fauna places the sample from the Mashavera Suite within the Coniacian-early Santonian Alievium praegallowayiradiolarian zone, and excludes a Campanian age. The Coniacian age is also supported by the presence ofDictyomitra formosaSquinabol sensu Pessagno 1976 (Fig. 5b),Archaeodictyomitra Fig. 3.Comparison of U-Pb zircon ages of magmatic rocks of the Bolnisi district with U-Pb and⁴⁰Ar/³⁹Ar ages of magmatic rocks from the Eastern Pontides, and stratigraphic age interpretations of the Bolnisi Group. Numerical ages (Ma) of the stratigraphic stages are from theInternational Chronostratigraphic Chart (2016). Late Cretaceous volcanic rocks from the Eastern Pontides: a =Kandemir et al. (2019), 92.1 Ma and 88.8 Ma (⁴⁰Ar/³⁹Ar), location 4 inFig. 1; b =Eyüboğlu et al. (2014), 91.1 and 83.1 Ma (SHRIMP U-Pb), location 2 inFig. 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 (SHRIMP U-Pb), location 4 inFig. 1; e =Özdamar (2016), 81.3 Ma (LA-ICP-MS U-Pb), and 86.0 and 75.3 Ma (⁴⁰Ar/³⁹Ar), location 1 inFig. 1; f =Eyüboğlu (2010), 80.9 Ma (⁴⁰Ar/³⁹Ar), location 3 inFig. 1. Early Eocene adakite-like rocks from the Eastern Pontides: g =Dokuz et al. (2013), 54.4 Ma (⁴⁰Ar/³⁹Ar); h =Karsli et al. (2011), 53.6 and 51.3 Ma (⁴⁰Ar/³⁹Ar); i =Eyüboğlu et al. (2011), 53.2 Ma (LA-ICP-MS U-Pb); j =Topuz et al. (2005), 52.1 and 51.8 Ma (⁴⁰Ar/³⁹Ar);

k =Topuz et al. (2011), 51.5 and 51.3 Ma (⁴⁰Ar/³⁹Ar), and 51.1 Ma (LA-ICP-MS U-Pb); l =Karsli et al. (2010), 50.3 and 47.4 Ma (⁴⁰Ar/³⁹Ar). Abbreviations: Fm = formation, volc = volcanic rocks.

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squinaboliPessagno (Fig. 5c),Dictyomitra napaensisPessagno (Fig. 5d), Pseudoaulophacus praeﬂoresensisPessagno (Fig. 5e), andPseudoau- lophacus circularisBragina (Fig. 5f). These species also occur in the Coniacian of the Deva Beds, Romania (P. Dumitrica, unpublished data).

Identiﬁcation ofCrucella irwiniPessagno at Madneuli (Fig. 5g) supports a middle-late Turonian to Coniacian age (Pessagno Jr., 1976).

The presence ofPseudodictyomitra nakasekoiTaketani (Fig. 5h) and Pseudodictyomitrasp. 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 inFig. 2, and their major and trace element 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- niﬁcance, 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 rhyolitefield in the total alkali- silica diagram, and a few Mashavera Suite samples plot in the dacite and trachydacitefields (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 trachyandesiticfields (Fig. 6a). The Eocene intrusive rocks are grano- dioritic to granitic in composition (Fig. 6a). Based on the immobile- element classification diagram, Gasandami and Mashavera Suite samples are mostly rhyodacitic/dacitic, except a few andesite and rhyolite samples, and a larger group plotting in the trachyandesiticfield (Fig. 6b). The Shorsholeti Suite and Eocene intrusive samples are

Fig. 4.a: Late Cretaceous rhyolitic subvolcanic intrusions of the Mashavera Suite and early Eocene intrusion with an adakitic-like composition (sample BO-07-08), north of Qvemo Bolnisi village (Fig. 2), inset: texture of the early Eocene intrusion with feldspar and ferromagnesian phenocrysts in afine-grained matrix; b:fine-grained and pumice- to crystal-rich tuff of the Mashavera Suite interbedded with volcaniclastic mudstone and sandstone, and radiolaria-bearing beds, southeastern Madneuli open pit (Fig. 2); c: Mashavera Suite outcrop with juxta- posed high Y-Zr rhyolite (dark-coloured, sample SA-18-06) and low Y-Zr rhyolite (light-coloured) dated at 83.3 Ma (sample SA-18-05, location 1 inFig. 2, southwest of Sakdrisi), the contact 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 withfiame texture (sample BO-07-33A, location 15 inFig. 2at Fakhalo); e: hyaloclastite with pillow-like shapes in the southeastern part of the Madneuli open pit (Fig. 2); f: basalt brecciaflow be- longing to the Tandzia Suite (sample BO-07-26, north of Tandzia village); g: mafic dike of the Tandzia Suite (sample BO-09-15A) crosscutting volcano-sedimentary rocks of the Mashavera Formation (southwest of location 3 inFig. 2); h: mafic dike of the Tandzia Suite (sample SA-18-02A) crosscutting pumice tuff of the Gasandami Suite (Sakdrisi open pit IV,Fig. 2); i: basaltic trachyandesite of the Shorsholeti Suite (sample BO-07-39, NNW of Dmanisi village,Fig. 2), inset: feldspar phenocrysts in afine-grained matrix of the basaltic trachyandesite.

Fig. 5.Radiolaria fauna sampled in the Madneuli mine (location inFig. 2), see radiolarian-bearing beds hosted by tuff (Fig. 4b). The white horizontal scale bar is 50μm long.

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trachyandesite, and the Tandzia Suite samples plot in the andesitic/basaltic and sub-alkaline basalticﬁelds (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 fields (Fig. 6c). A dominantly calc-alkaline and felsic to intermediate composition 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 mafic rock field, and the Shorsholeti samples, which are high-K calc-alkaline and shoshonitic. Tandzia and Shorholeti Suite samples have the highest TiO2, Al2O3, Fe2O3, MgO and CaO concentrations, which is consistent with their mafic to intermediate composition (Fig. 7a–e). The Gasandami Suite and Eocene intrusions have the highest Na2O concentrations (Fig. 7f), whichfit with their predominant low-K composition (Fig. 6c).

Two distinct groups of Mashavera Suite rhyolite are identiﬁed based on different trace element compositions. One group has

distinctly higher Rb and lower Y and Zr concentrations (Fig. 7g–i), and partly lower Nb, and higher Ba and Th concentrations than the other one (Fig. 7j–l). Therefore, in the remaining part of this contribution, we will distinguish high Y-Zr and low Y-Zr rhyolite types for the Mashavera Suite (Figs. 2, 6–9), which are also clearly visible in outcrops (Fig. 4c). In the immobile-element classiﬁcation diagram (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 rhyolite has SiO2concentrations above 75 wt% (Fig. 6a, c), therefore it qualiﬁes as high-silica rhyolite (Gualda and Ghiorso, 2013). By contrast, the Mashavera high Y-Zr rhyolite falls in a broader range of SiO2concentrations from 71 to 78 wt%, and consists of both low- and high-silica rhyolite (Fig. 6a, c). The Mashavera dacite and the Gasandami rhyolite samples have trace element concentrations overlapping 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, 6–9).

Fig. 6.Geochemical classification diagrams for magmatic rocks from the Bolnisi district; a: TAS classification (Le Maître, 2002), with equivalent names of coarse-grained intrusive rocks in brackets (Middlemost, 1994); b: classification based on immobile elements (Winchester and Floyd, 1977); c: K2O (wt%) vs. SiO2(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 fromGualda and Ghiorso (2013).

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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. 8a–f).

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 depleted in some highﬁeld 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. 8g–h). 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 theﬂat 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. 8i–n). The Eocene intrusion samples are enriched in Sr and depleted in Ta, Nb and REE with respect to the Late Cretaceous rocks (Fig. 8m–n), 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 electronic Appendix C. Late Cretaceous rocks from the Bolnisi study area have¹⁴³Nd/¹⁴⁴Nd ratios between 0.51260 and 0.51278, and⁸⁷Sr/⁸⁶Sr ratios 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 (a–f) and trace element diagrams (g–l) vs. SiO2(wt%) for the magmatic rocks from the Bolnisi district. Grey shaded area in each diagram: compositional gap of SiO2

concentrations (51.7 to 66.3 wt%) between maﬁc rocks (Tandzia Suite) and felsic rocks (Gasandami and Mashavera Suite) of this study. High-silica (HSR) and low-silica rhyolite (LSR) boundary fromGualda and Ghiorso (2013).

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Fig. 8.Primitive mantle-normalized trace element spider diagrams (a, c, e, g, i, k, and m; normalization with respect toTaylor and McLennan, 1985) and rare earth element-normalized diagrams (b, d, f, h, j, l, and n; normalization with respect toSun and McDonough, 1989) for rock formations of the Bolnisi district (seeFig. 3). Grey shaded area in c to n: compositionalﬁeld of the Mashavera Suite high Y-Zr rhyolite data depicted in a and b.

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the mantle array (Fig. 9a). By contrast, the Mashavera and Gansadami samples have distinctly higher⁸⁷Sr/⁸⁶Sr ratios and plot mostly to the right of the mantle array (Fig. 9a). The Gasandami samples have the highest¹⁴³Nd/¹⁴⁴Nd ratios between 0.51269 and 0.51278 (Fig. 9a).

The Eocene intrusion sample studied byHässig et al. (2020)falls along the mantle array with an elevated ¹⁴³Nd/¹⁴⁴Nd ratio and a low

87Sr/⁸⁶Sr ratio (Fig. 9b).

Two trends can be recognized in⁸⁷Sr/⁸⁶Sr and¹⁴³Nd/¹⁴⁴Nd vs. SiO2

variations diagrams (Fig. 9c–d): relatively constant isotopic compositions with variable SiO2concentrations typical of magmatic fractionation trends, and oblique trends towards higher⁸⁷Sr/⁸⁶Sr and lower

143Nd/¹⁴⁴Nd ratios with increasing SiO2concentrations, which are typical of crustal assimilation.

4.4. U-Pb dating of magmatic zircons

The U-Pb zircon age data of six samples are presented inFig. 10, including cathodoluminescence images of zircons (Fig. 10a–c). The analytical data can be found in the electronic Appendix D, and the analytical methodology in the electronic Appendix A. The sample locations are shown inFig. 2, and the data are summarized inFig. 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. 10d–p). Within analytical error, core and rim analyzed in a single zircon grain yielded overlapping Late Cretaceous ages (Fig. 10a–c).

Sample SA-18-05 is a low Y-Zr rhyolite of the Mashavera Suite cropping out SW of the Sakdrisi deposit (location 1 inFig. 2), next to a high Y-Zr rhyolite of the same suite (Fig. 4c). LA-ICP-MS dating yielded a weighted²⁰⁶Pb/²³⁸U mean age of 83.3 ± 0.6 Ma (n = 14;Fig. 10d–e).

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 weighted²⁰⁶Pb/²³⁸U mean age of 84.9 ± 0.9 Ma (n = 12;

Fig. 10f–g). Eleven single zircon grains were also dated by ID-TIMS and yielded ²⁰⁶Pb/²³⁸U ages between 85.75 ± 0.11 and 86.65 ± 0.09 Ma (Fig. 10h). The ﬁve youngest zircons yield a weighted

206Pb/²³⁸U 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 weighted²⁰⁶Pb/²³⁸U mean age. The low and anomalous²⁰⁶Pb/²³⁸U ages obtained by LA- 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 inFig. 2). LA-ICP-MS dating yielded a weighted²⁰⁶Pb/²³⁸U mean age of 87.0 ± 1.1 Ma (n = 7;

Fig. 10i–j). Six zircon grains yielded²⁰⁶Pb/²³⁸U ages by ID-TIMS between 86.60 ± 0.05 and 87.08 ± 0.09 Ma, and one outlier has a²⁰⁶Pb/²³⁸U age 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 SiO2concentrations; d: variation of initial neodymium isotopic compositions of magmatic rocks from the Bolnisi district with respect to SiO2concen- trations. Mantle array fromFaure (1986), and CHUR and bulk Earth UR calculated according toFaure (1986)at 85 Ma and 52.5 Ma, respectively, in a and b.

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Fig. 10.LA-ICP-MS and TIMS U-Pb ages of magmatic rocks from the Bolnisi district. a to c: representative scanning electron microscopy photos of dated zircons. d and e: LA-ICP-MS zircon

206Pb/²³⁸U weighted average plots and Concordia diagram of dated zircons of sample SA-18-05 (location 1 inFig. 2). f and g: LA-ICP-MS zircon²⁰⁶Pb/²³⁸U weighted average plots and Concordia diagram of dated zircons of sample BO-10-09 (location 2 inFig. 2). h: ID-TIMS zircon²⁰⁶Pb/²³⁸U ages of sample BO-10-09. i and j: LA-ICP-MS zircon²⁰⁶Pb/²³⁸U weighted average plots and Concordia diagram of dated zircons of sample BO-09-14 (location 5 inFig. 2). k: ID-TIMS zircon²⁰⁶Pb/²³⁸U ages of sample BO-09-14. l and m: LA-ICP-MS zircon²⁰⁶Pb/²³⁸U weighted average plots and Concordia diagram of dated zircons of sample BO-07-18 (location 6 inFig. 2). n: ID-TIMS zircon²⁰⁶Pb/²³⁸U ages of sample BO-07-18. o: ID-TIMS zircon

206Pb/²³⁸U ages of sample BO-10-05A. p: ID-TIMS zircon²⁰⁶Pb/²³⁸U ages of sample BO-07-33B.

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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 weighted²⁰⁶Pb/²³⁸U mean age.

Sample BO-07-18 is a rhyolitic dike from the high Y-Zr Mashavera Suite next to the Madneuli deposit (location 6 inFig. 2). It yielded a weighted²⁰⁶Pb/²³⁸U mean age of 85.9 ± 1.2 Ma (n = 10) by LA-ICP- MS dating (Fig. 10l–m). Seven zircon grains were analyzed by ID-TIMS and yielded²⁰⁶Pb/²³⁸U ages between 86.96 ± 0.21 and 88.70 ± 0.13 Ma. The four youngest zircons are concordant and yield a weighted

206Pb/²³⁸U mean age of 87.13 ± 0.12 Ma (Fig. 10n), which is considered as the best approximation ofﬁnal 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 ²⁰⁶Pb/²³⁸U 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 older²⁰⁶Pb/²³⁸U age of 87.14 ± 0.16 Ma.

Since the chemical abrasion technique used in this study is eliminating any lead loss effect very efﬁciently, we attribute the discrepancy between the young²⁰⁶Pb/²³⁸U 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-TIMS²⁰⁶Pb/²³⁸U 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 pyroclasticﬂow of the high Y-Zr Mashavera Suite (location 15 in Fig. 2). Seven zircon grains yielded ID-TIMS

206Pb/²³⁸U 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 sedimentary rocks interlayered with tuff of the Mashavera Suite (Figs. 4b and5). Within analytical error, core and rim analyzed in a single zircon grain yielded overlapping Late Cretaceous ages (Fig. 10a–c). This excludes inheritance of zircon cores from signiﬁ- 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 ofGambashidze 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 overlapping 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 the deposition 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 rhyolite types next to each other at 83.3 ± 0.6 Ma (location inFig. 2; Sample SA-18-05 inFig. 10d–e, electronic Appendix D). Only one single age of

83.6 ± 1.3 Ma has been obtained for the Gasandami Suite high Y-Zr rhyolite byHässig et al. (2020; location 3 south of Beqtakari in 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 magmaticﬂare-up

The negative Nb, Ta and Ti anomalies in 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 consistent 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, respectively, ~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). Maﬁc rocks of the Tandzia Suite are a volumetrically minor rock type (Figs. 6a), which occurs as dikes crosscutting both the Mashavera and Gasandami Suites (Figs. 2 and 4g–h), and as local lava breccia (Fig. 4f). There is a distinct compositional gap of SiO2concentrations with Tandzia Suite samples falling below 51.7 wt% and Mashavera and Gasandami Suite samples above 66.3 wt% (Figs. 6a and7). The absence of intermediate rock types within the concentration range from 51.7 to 66.3 wt% SiO2supports bimodal- type magmatism (Meade et al., 2014;Melekhova et al., 2013) in the Bolnisi district. The alkaline Shorsholeti Suite is not considered here and will be discussed later, because it is a stratigraphically younger rock sequence (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, spiderdiagrams, REE-normalized patterns and radiogenic compositions of the Mashavera Suite high Y-Zr rhyolite and dacite indicate a common petrogenetic evolution (Figs. 3, 7g–l,8a–d and9a). The dacite and low- to high-silica rhyolite (<75 wt% SiO2and >75 wt% SiO2, respectively,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 rhyolite (>75 wt% SiO2,Fig. 6a) and its U-shaped middle to heavy REE patterns (Fig. 8h), is attributed to a more pronounced magmatic differentiation (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 variations among coeval rhyolite types within the same silicic magmatic province can be attributed to eitherin situfractionation 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 discrete and isolated magma batches in the crust, which have fed distinct, coeval silicic volcanic centres. For instance, such a magmatic setting 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).

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Major and trace element compositions can allow to distinguish coeval rhyolite types fed by distinct storage magma chambers with different 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, g–i, k–l; 11a–d). 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,fO2and fH2O 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) andDeering et al. (2008)have shown that in shallow magmatic chambers, low pressure and temperature, and highfH2O and oxidizing conditions 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 patterns. By contrast, in deep magmatic chambers, high pressure and temperature, and lowfH2O and reducing conditions promote fractionation of pyroxene, resulting in an enrichment of middle REE, Y, Zr and Fe.

Despite the replacement of the original magmatic assemblage by regional metamorphic minerals in the Bolnisi district, the distinct chemical differences of rhyolite and dacite still record eruption from distinct magma chambers located at different crustal depths. Indeed, the higher TiO2, Fe2O3, Zr, Y and middle REE concentrations of the high Y-Zr rhyolite and dacite of the Mashavera and Gasandami Suites, compared to the Mashavera Suite low Y-Zr rhyolite (Figs. 7c, h–i,8b, d, f, h, and11a–c), support eruption from deeper magma storage chambers, in which pyroxene 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. SiO₂diagram (Fig. 11e) are evidence for fractionation of hornblende and suppression of plagioclase fractionation during petrogenesis of the Mashavera Suite low Y-Zr rhyolite, which reveals petrogenesis in shallow magma chambers (Deering et al., 2008;Smith et al., 2005). The elevated K2O 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, and11d). The high SiO2concen- trations above 75 wt% of the Mashavera Suite low Y-Zr rhyolite (Fig. 6a, c) is further evidence for the shallow depth of the magma storage chambers from which they have been erupted, since high-silica rhyolite (SiO2> 75 wt%) is typical of low pressure environments, i.e. shallow crust (Gualda and Ghiorso, 2013).

5.4. Tandzia suite maﬁc rocks: high-alumina basalts co-genetic with rhyolitic-dacitic magmatism

The maﬁc rocks of the Tandzia Suite qualify as high-alumina basalts with SiO2< 54 wt%, Al2O3> 16.5 wt% and MgO < 7wt% (Crawford et al., 1987;Figs. 6a and7b,d). They are characterized by gentle REE patterns with only slight light element enrichment (LaN/YbN= 3–9), nearlyﬂat heavy element patterns (TbN/YbN= 1.2–1.9) and devoid of a Eu anomaly (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 mafic 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. 12a–f). This supports a petrogenetic link between the mafic and felsic end-members of the bimodal magmatism in the Bolnisi district. Although they are generally volumetrically minor, such mafic 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 mafic magma ascent during Late Cretaceous extension. Such structural control on mafic magma ascent has also been reported in the Taupo volcanic zone, New Zealand (Gamble et al., 1990;Graham et al., 1995). The lavaflows of the Tandzia Suite (Fig. 5f) are evidence of sporadic mafic magmatism reaching the surface. It suggests thatfissure eruptions of the Tandzia Suite were coeval with rhyolitic and dacitic volcanism of the Mashavera and Gasandami Suites, during bimodal magmatism in the Bolnisi district in an extensional tectonic setting (Fig. 13a–b).

5.5. Origin of magmas: evidence from radiogenic isotopes

The maﬁc 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 plagioclase crystal fractionation or uptake (Graham et al., 1995;Zellmer et al., 2020). Compared to the Tandzia Suite maﬁc rocks, the Sr isotopic compositions of most of the Mashavera and Gasandami Suite rocks are shifted to higher ⁸⁷Sr/⁸⁶Sr ratios, to the right of the mantle array (Fig. 9a). The Nd isotopic compositions of the Mashavera Suite samples scatter towards lower¹⁴³Nd/¹⁴⁴Nd ratios with respect to the Tandzia Suite samples, whereas the ones of the Gasandami Suite are shifted towards more elevated¹⁴³Nd/¹⁴⁴Nd ratios (Fig. 9a).

In⁸⁷Sr/⁸⁶Sr and¹⁴³Nd/¹⁴⁴Nd vs. SiO2variations diagrams (Fig. 9c–d), a limited number of Mashavera Suite samples have isotopic compositions overlapping with those of the Tandzia Suite samples. This relatively constant isotopic composition despite variable SiO2concen- trations is explained by extreme magmatic differentiation that has produced the rhyolitic and dacitic magmas from a common maﬁc reservoir (see horizontal trends inFig. 9c–d). However, most Mashavera Suite rhyolite and dacite samples display a concomitant shift towards higher

87Sr/⁸⁶Sr and lower¹⁴³Nd/¹⁴⁴Nd ratios with respect to the mafic rocks (oblique trends inFig. 9c–d). This reflects interaction with or assimilation of continental crustal rocks by the rhyolitic and dacitic magmas during their ascent and ponding in the crust (Fig. 13a). Such an evolution identified 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).

The⁸⁷Sr/⁸⁶Sr and¹⁴³Nd/¹⁴⁴Nd vs. SiO2variations diagrams of the Gasandami Suite samples are also consistent with interaction or assimilation of continental crustal rocks by rhyolitic melts (oblique trends in Fig. 9c–d). However, their elevated¹⁴³Nd/¹⁴⁴Nd ratios in comparison to those of the Tandzia Suite mafic 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 concentrations and trends mostly overlapping with those of the Mashavera Suite rhyolite (Figs. 6a–b,7a–e, h–i, and11a–c, e), it is characterized by distinctly higher Na2O concentrations and lower K2O, Rb, Nb, Ba and Th concentrations (Figs. 6c,7f–g, j–l, and11d). 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 confined to the lower range of the Mashavera Suite samples (right hand side ofFig. 8e). Due to their higher Na2O and lower K2O concentrations, the Gasandami Suite rhyolite qual- ifies mostly as low K or tholeiitic rocks (Fig. 6c). We attribute the more pronounced isotopic mantle signature, higher Na2O concentrations and lower LILE concentrations of the Gasandami Suite rhyolite to the presence of a more juvenile mantle component in the mantle wedge. The more juvenile mantle component is attributed to asthenospheric mantle upwelling and/or tapping of a less metasomatized mantle (Fig. 13b).

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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. 6c–d). The Shorsholeti Suite samples have negative Nb, Ta and Ti anomalies, which are typical of arc magmas (Fig. 8k). They have high incompatible element concentrations, including Rb, Zr, Nb, Ba, Th, U, Sr and light REE, especially when compared to the Tandzia Suite maﬁc rocks (Figs. 7g, i–l,8l, and11a–d). The composition of high-K calc-alkaline and shoshonitic rocks and their strong enrichment in incompatible elements is generally interpreted to reﬂect their mantle sources, and is attributed to low-degree partial melting of lithospheric mantle and/or mantle

metasomatised byﬂuids 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, k–l,8k–l, and11e) 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 high⁸⁷Sr/⁸⁶Sr and low¹⁴³Nd/¹⁴⁴Nd ratios, when compared to the Tandzia Suite samples, which were sourced by juvenile mantle, not or less affected by metasomatism (Figs. 9a and 14;

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. Compositionalﬁelds of basalts in a: A = within-plate basalt, B = MORB and within-plate basalt, C = MORB, D = MORB and volcanic-arc basalt, CAB = continental arc basalt, OAB = oceanic arc basalt (fromPearce and Norry, 1979;Pearce, 1983). Compositionalﬁeld of basalts in c: MORB = middle oceanic ridge basalt (fromPearce, 1982).

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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;Shafiei et al., 2009), with source enrichment and increasing pressure trends (Shafiei et al., 2009); e: Ba/La vs. Th/Yb diagram with slab- derivedfluid and sediment or sediment melts enrichment trends (Woodhead et al., 2001); f: Ba/Th vs. La/Sm diagram withfluid-related and melt-related enrichment trends (Aydin et al., 2020); g: La/Yb vs. SiO2(wt%) discrimination diagram, with adakite-field (Richards and Kerrich, 2007), and monazite-allanite fractionation trend (Miller and Mittlefehldt, 1982).

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the juvenile mantle source of the Tandzia Suite maﬁc rocks (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. 7i–j) can be explained by high temperature and high pressure (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 Cretaceous rock formations of the Bolnisi district with respect to Late Cretaceous rock formations of the Eastern Pontides, Turkey; a, c, e and g: primitive mantle-normalized trace element spider diagrams (normalization with respect toTaylor and McLennan, 1985); b, d, f and h: rare earth element-normalized diagrams (normalization with respect toSun and McDonough, 1989).

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