Late Cretaceous to Early Eocene Magmatic and Geodynamic Evolution of the Bolnisi District, Georgia

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

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 [...]

Reference

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

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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 article info abstract

Article history: The Bolnisi district is a distinct tectonic zone of the Lesser Caucasus, which is considered to represent the eastern Received 19 August 2020 extremity of the Turkish Eastern Pontides. Late Cretaceous, low-K, calc-alkaline to high-K rhyolite of the Received in revised form 26 October 2020 Mashavera and Gasandami Suites is the predominant rock type of the district, and is accompanied by subsidiary Accepted 9 November 2020 dacite, and rare high-alumina basalt and trachyandesite of the Tandzia Suite. The Mashavera and Gasandami rhy- Available online xxxx 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-Santonian ages of radiolarian fauna of the Mashavera Suite. The felsic rocks of Keywords: fl Northern Neotethys the Mashavera and Gasandami Suites were deposited during a ~6.6 m.y.-long silicic magmatic are-up event, Lesser Caucasus which together with the Tandzia Suite mafic rocks, documents Late Cretaceous bimodal magmatism in an exten- Silicic magmatic flare-up sional tectonic setting. Trace element data indicate that high Y-Zr, low- to high-silica rhyolite and dacite, and low Bimodal and high-K magmatism Y-Zr high-silica rhyolite have been erupted, respectively, from coeval deep and shallow crustal reservoirs. The Eastern Pontides 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. © 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 is part of the Lesser Caucasus, where it sits at the northern tip of the Jurassic-Cretaceous sedimentary-volcanic The Bolnisi district in southern Georgia is a distinct tectonic zone Somkheto-Karabagh belt (Fig. 1). Because of its particular geological along the Tethyan orogenic belt, which is located at the crossroads of setup consisting of voluminous Late Cretaceous silicic and subsidi- the Lesser Caucasus and the Eastern Pontides (Fig. 1). Both belts belong ary maﬁc magmatic rocks in a horst and graben setting (Adamia to the southern Eurasian margin, which evolved from a Mesozoic sub- et al., 2011; Apkhazava, 1988; Gugushvili, 2015; Gugushvili, 2018; duction environment to a collision and post-collision setting during Gugushvili and Omiadze, 1988; Popkhadze et al., 2014), the Bolnisi dis- the latest Cretaceous and Cenozoic (Okay and Şahintürk, 1997; Sosson trict is singled out as a separate tectonic zone within the Lesser Caucasus. et al., 2010; Adamia et al., 2011; Rolland et al., 2011; Okay et al., 2013). Furthermore, the Late Cretaceous and the post-collisional Eocene magmatism of the Bolnisi district (Fig. 2) have been correlated with

⁎ Corresponding author. E-mail address: [email protected] (R. Moritz).

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/).

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 R. Moritz, N. Popkhadze, M. Hässig et al. Lithos xxx (2020) xxx the magmatic environment of the Eastern Pontides, along a trend ex- present new lithogeochemical and radiogenic isotope data of Late Creta- tending towards Artvin in northeastern Turkey, but which is concealed ceous and Eocene magmatic rocks of the Bolnisi district, complemented by Oligocene to Quaternary rocks (Fig. 1; Yilmaz et al., 2000, 2014; by U-Pb zircon age data of the magmatism, and fauna ages of sedimen- Adamia et al., 2010, 2011; Hässig et al., 2020). This underlines the special tary and volcano-sedimentary host rocks. Together with published data, tectonic signiﬁcance of the Bolnisi district, as a link between the Lesser the new geochemical and radiometric age data allow us to reconstruct Caucasus and the Eastern Pontides. the Late Cretaceous to Eocene magmatic and geodynamic evolution Voluminous silicic magmatism occurs in various tectonic settings re- of the Bolnisi district, and discuss its link with the Turkish Eastern lated to subduction, continental break-up or continental hot spots Pontides. (e.g., Barker et al., 2020; Deering et al., 2008; Smith et al., 2005). Such voluminous silicic magmatism, also referred to as ignimbrite ﬂare-ups, 2. Regional geological setting is an outstanding geological event during an orogenic evolution, mainly developed in extensional or rift settings and characterized by bimodal The Lesser Caucasus consists of three main tectonic zones (Fig. 1; magmatism, where hot upwelling mantle triggers crustal melting Sosson et al., 2010; Adamia et al., 2011): (1) the Somkheto-Karabakh (Gravley et al., 2016). Therefore, we consider that the voluminous and belt and its southern extension, the Kapan block, belong to a silicic nature of the magmatism of the Bolnisi district records a distinct northwest-oriented Jurassic-Cretaceous magmatic arc, related to the geological event, which must have profoundly affected the northern subduction of the northern Neotethys along the Eurasian margin part of the Lesser Caucasus during Late Cretaceous subduction of the (Fig. 1; Kazmin et al., 1986; Lordkipanidze et al., 1989; Rolland et al., northern Neotethys, and which can be correlated with contemporane- 2011; Mederer et al., 2013); the Bolnisi district under study is located ous events in the Eastern Pontides, and subsequent Cenozoic collisional at the northernmost extremity of the Somkheto-Karabakh belt and post-collisional evolution of the southern Eurasian margin. (Fig. 1); (2) in the southwest, the Gondwana-derived South Armenian While numerous studies have been carried out on the Late Creta- block consists of Proterozoic metamorphic basement rocks and Devo- ceous and Cenozoic magmatism evolution of the Eastern Pontides, and nian to Paleocene sedimentary and volcanic cover rocks (SAB in its relationship with respect to the Black Sea (e.g., Eyüboğlu et al., Fig. 1), which is interpreted as the northeastern extension of the 2011, Eyüboğlu et al., 2014; Özdamar, 2016; Dokuz et al., 2019; Tauride-Anatolide platform (TAP in Fig. 1; Barrier and Vrielynck, 2008; Kandemir et al., 2019; Aydin et al., 2020), there is a paucity of knowl- Sosson et al., 2010; Robertson et al., 2013; Meijers et al., 2015), and edge about the Georgian Bolnisi district, which hinders any regional in- which has been affected by extensive Cenozoic magmatism (Kazmin terpretation and understanding with respect to the Eastern Pontides. In et al., 1986; Moritz et al., 2016b; Rezeau et al., 2016, 2017); and this contribution, following an overview of its geological setting, we (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; Hässig et al., 2013; Mederer et al., 2014; Delibaş et 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).

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

3 R. Moritz, N. Popkhadze, M. Hässig et al. Lithos xxx (2020) xxx the suture zone between the Somkheto-Karabakh belt and the South Mashavera Suite consist of subvolcanic magmatic bodies, domes/extru- Armenian block (ASASZ in Fig. 1; Rolland et al., 2009a, 2009b), and is sions (Fig. 4a), lava flows with local columnar jointing textures, abun- correlated with the Izmir-Ankara-Erzincan suture zone (IAESZ in dant pyroclastic rocks, including surge and fall deposits, tuff (Fig. 4b), Fig. 1), located between the Eastern Pontides and the Tauride- and ignimbrite (Fig. 4c–d). Hyaloclastite with pillow-like shapes are ev- Anatolide platform (Hässig et al., 2013). idence for a subaqueous environment (Fig. 4e), which is also supported Closure of the northern Neotethys and collision of the Gondwana- by interlayered siltstone, limestone, radiolaria-bearing beds (Fig. 4b) derived South Armenian block and Tauride-Anatolide platform with and turbidite sequences. Locally, phreatomagmatic breccia crosscut the Eurasian margin was diachronous. It is interpreted as late Campa- the volcanic and sedimentary rocks (Popkhadze et al., 2014, 2017, nian to early Maastrichtian (~73–71 Ma) in the Lesser Caucasus 2019). (Meijers et al., 2015; Rolland et al., 2009a, 2009b), whereas the The upper part of the Bolnisi Group consists of the Santonian Tauride-Eastern Pontides collision is generally interpreted as Paleocene to Campanian Tandzia, Gasandami and Shorsholeti Suites (Fig. 3; to early Eocene (Hippolyte et al., 2017; Kandemir et al., 2019; Karsli Gambashidze and Nadareishvili, 1980, 1987; Gambashidze, 1984; et al., 2011; Okay and Şahintürk, 1997; Robertson et al., 2013; Şengör Apkhazava, 1988; Gugushvili, 2015, Gugushvili, 2018). The Tandzia and Yilmaz, 1981; Topuz et al., 2005, 2011), although Rice et al. Suite is absent in some stratigraphic reconstructions (Fig. 3,see (2006) suggest collision as early as the Campanian-Maastrichtian, and Vashakidze, 2001), and it is not reported as a separate unit in Fig. 2. Dokuz et al. (2019) during the early Maastrichtian. This evolution was The Tandzia Suite includes aphyric to porphyritic basalt, andesitic ba- coeval with progressive opening of the Black Sea, beginning during salt and trachybasalt lava flows (Fig. 4f), tuff, and dikes (Fig. 4g–h), the Late Cretaceous in the west and propagating eastward with time with sedimentary rock interlayers. It crops out in the northwestern (Hippolyte et al., 2017; Nikishin et al., 2011; Sosson et al., 2016). part of the study area, southwest of Beqtakari (Fig. 2). The Gasandami These interpretations are consistent with northward subduction of the Suite is made up of rhyolitic and rhyodacitic lava, subvolcanic intru- northern Neotethys (Adamia et al., 2011; Aydin et al., 2020; Hässig sions, dikes, pyroclastic rocks (Fig. 4h), and limestone and volcano- et al., 2013, 2020; Karsli et al., 2010; Özdamar, 2016; Rolland et al., sedimentary rocks in its upper part. The uppermost Shorsholeti Suite 2011; Sosson et al., 2010; Yilmaz et al., 2000), although other authors consists of porphyritic trachyandesite and basaltic trachyandesite lava advocate a south-verging Mesozoic subduction (Eyüboğlu, 2010; (Fig. 4i), interlayered with pyroclastic rocks and limestone. It crops Eyüboğlu et al., 2011, 2014). out in the western part of the study area, south of the Khrami massif (Fig. 2). 3. Geological setting of the Bolnisi district The rhyolite and dacite of the Mashavera and Gasandami Suites predominate in the Bolnisi district (Fig. 2). The original mineral assemblage Late Cretaceous rocks predominate in the Bolnisi district. They crop of the Late Cretaceous magmatic rocks has been replaced by regional out between horsts of the Transcaucasian crystalline basement, named prehnite-pumpellyite to low grade greenschist facies metamorphic Khrami and Loki massifs (Fig. 2; Zakariadze et al., 2007), which belong and propyllitic alteration minerals. Felsic rocks generally contain a to the Variscan belt of the Black Sea region (Okay and Topuz, 2017). fine-grained quartz and feldspar matrix, and feldspar phenocrysts Both massifs consist of Late Proterozoic and Paleozoic metamorphic with remnants of polysynthetic or Carlsbad twinning. Feldspar is re- and sedimentary rocks, crosscut by Neoproterozoic to Late Carbonifer- placed by muscovite-sericite, carbonate, epidote, zoïsite and prehnite- ous, Jurassic and Cretaceous magmatic intrusions. Early Jurassic pumpellyite. In intermediate to mafic rocks, ferromagnesian minerals volcaniclastic rocks, and Late Jurassic to Early Cretaceous limestone are replaced by chlorite, epidote and carbonate. In pyroclastic rocks, and volcanoclastic rocks overlay unconformably the basement rocks pumice and fiame (Fig. 4d) are replaced by chlorite, iron oxydes, quartz, (Adamia et al., 2011; Zakariadze et al., 2007). carbonate, and prehnite-pumpellyite. The Bolnisi district is a major min- The Late Cretaceous rock sequence is known as the Bolnisi Group, and ing district (Fig. 2; Migineishvili, 2005; Gugushvili, 2004; Moritz et al., unconformably overlies the Jurassic and older crystalline basement rock 2016a). Therefore, several locations have also been affected by intense units (Adamia et al., 2011).TheLateCretaceousrocksaredominatedby hydrothermal alteration, including silicification, and variable potassic calc-alkaline rhyolitic to rhyodacitic volcanic and sub-volcanic rocks, and (muscovite or K-feldspar), carbonate, argillic and epidote-zoïsite are accompanied by subsidiary dacitic, andesitic and basaltic rocks, and alteration. sedimentary rocks (Adamia et al., 2011; Gugushvili, 2015; Gugushvili, Shallow-marine to hemipelagic limestone, marl and conglomerate 2018; Kazmin et al., 1986; Lordkipanidze et al., 1989). At Madneuli of the Campanian to Maastrachtian Tetritskaro Suite (Fig. 3), and Paleo- (Fig. 2), a porphyritic granodiorite-quartz diorite crosscut by drilling cene marl, sandstone, and turbiditic terrigenous clastic rocks cover the yielded a whole-rock K-Ar age of 88–89 Ma (Gugushvili and Omiadze, Bolnisi Group in the northern part of the study area (Fig. 2). They are de- 1988; Rubinstein et al., 1983). Rhyolitic domes at Madneuli have void of volcanic rocks, therefore it is concluded that magmatic arc activ- whole-rock K-Ar ages of 84–85 Ma, and 72–71 Ma at Sakdrisi and ity had ceased by the Maastrichtian (Adamia et al., 2011; Yilmaz et al., Beqtakari. Pyroclastic rocks from Sakdrisi yielded K-Ar ages of 77.6 Ma 2000). To the west and north, the Late Cretaceous rocks of the Bolnisi (Fig. 2; Gugushvili, 2004). Group are covered by Eocene volcanic rocks (Fig. 2), and in the northern The Bolnisi Group has been subdivided into five to seven rock suites part by late Eocene shallow marine, turbiditic sedimentary rocks of the with debated stratigraphic age interpretations (Fig. 3; Gambashidze and Adjara-Trialeti belt related to rifting of the Black Sea (Adamia et al., Nadareishvili, 1980, 1987; Gambashidze, 1984; Apkhazava, 1988; 2010; Lordkipanidze et al., 1979). Eocene rhyolitic subvolcanic intru- Vashakidze, 2001; Gugushvili, 2015, Gugushvili, 2018). The lower sions, granodiorite and plagiogranite crosscut the Late Cretaceous lithostratigraphic sequences are predominantly exposed next to the rocks in the central part of the study area (Figs. 2 and 4a). The youngest Khrami and Loki massifs (Fig. 2), and consist of sedimentary rocks and units consist of Oligo-Miocene euxinic sedimentary rocks and Miocene rhyolitic tuff of the Cenomanian Ophreti and Tserakvi Suites, and an- to Quaternary molasse and basaltic to rhyolitic volcanic rocks (Adamia desitic to rhyolitic tuff and andesite and basalt lava flows interlayered et al., 2011). with limestone and marl of the early Turonian Digverdi Suite. The over- lying Mashavera Suite is interpreted as late Turonian-Coniacian 4. Results (Vashakidze, 2001) or late Turonian-Santonian (Fig. 3; Gambashidze, 1984; Gambashidze and Nadareishvili, 1987; Apkhazava, 1988; 4.1. Radiolaria determinations and age constraints of the Mashavera Suite Gugushvili, 2015, Gugushvili, 2018). Migineishvili and Gavtadze (2010) reinterpreted the Mashavera Suite as Campanian based on For stratigraphic age determination of the Mashavera Suite, radiolar- nannofossil determinations (Fig. 2). The main lithologies of the ian fauna was extracted from beds interlayered with volcaniclastic

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Fig. 3. Comparison of U-Pb zircon ages of magmatic rocks of the Bolnisi district with U-Pb and 40Ar/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 the International 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 in Fig. 1;b=Eyüboğlu et al. (2014), 91.1 and 83.1 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 (SHRIMP U-Pb), location 4 in Fig. 1;e=Özdamar (2016), 81.3 Ma (LA-ICP-MS U-Pb), and 86.0 and 75.3 Ma (40Ar/39Ar), location 1 in Fig. 1;f=Eyüboğlu (2010),80.9Ma(40Ar/39Ar), location 3 in Fig. 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.

mudstone, ﬁne-grained and pumice- to crystal rich tuff (Fig. 4b; see Ap- bladed morphology for Alievium superbum (Squinabol) in the pendix A), cropping out in the eastern part of the Madneuli mine Turonian-lowermost Coniacian, to three-bladed proximally and conical (Fig. 2). The stratigraphic age of the radiolarian fauna was determined distally for Alievium praegallowayi Pessagno in the Coniacian-early based on zonations for the Cenomanian-Maastrichtian (Pessagno Jr., Santonian, and to completely conical for Alievium gallowayi Pessagno 1976), the late Barremian-early Turonian (O’Dogherty, 1994), and the in the Santonian-late Campanian. Cenomanian-Campanian (Bragina, 2004). Our determinations were The radiolarian fauna collected at Madneuli (Figs. 2 and 4b) contains compared to Coniacian fauna from Deva Beds, Romania, and only Alievium praegallowayi (Fig. 5a). Neither Alievium superbum nor Coniacian-Santonian fauna from Masirah Island, Oman (P. Dumitrica, Alievium gallowayi are present. Therefore, this fauna places the sample unpublished data). The most compelling age constraint is based on the from the Mashavera Suite within the Coniacian-early Santonian genus Alievium, which is characterized by an evolutionary trend of the Alievium praegallowayi radiolarian zone, and excludes a Campanian three main spines of the shell from the Turonian to Campanian age. The Coniacian age is also supported by the presence of Dictyomitra (Pessagno Jr., 1976). The spines changed from a completely three- formosa Squinabol sensu Pessagno 1976 (Fig. 5b), Archaeodictyomitra

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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. squinaboli Pessagno (Fig. 5c), Dictyomitra napaensis Pessagno (Fig. 5d), samples for petrogenetic interpretations were collected outside of min- Pseudoaulophacus praefloresensis Pessagno (Fig. 5e), and Pseudoau- eralized areas. Therefore, data trends in diagrams have petrogenetic sig- lophacus circularis Bragina (Fig. 5f). These species also occur in the nificance, and are devoid of hydrothermal alteration effects. Major oxide Coniacian of the Deva Beds, Romania (P. Dumitrica, unpublished data). data were normalized to a 100% volatile-free basis. Identification of Crucella irwini Pessagno at Madneuli (Fig. 5g) sup- Samples from the Gasandami Suite and most of those of the ports a middle-late Turonian to Coniacian age (Pessagno Jr., 1976). Mashavera Suite samples plot in the rhyolite field in the total alkali- The presence of Pseudodictyomitra nakasekoi Taketani (Fig. 5h) and silica diagram, and a few Mashavera Suite samples plot in the dacite Pseudodictyomitra sp. A (Fig. 5i) at Madneuli is consistent with a and trachydacite fields (Fig. 6a). The Tandzia Suite samples are basaltic, Turonian-Coniacian age, as illustrated by the Parapedhi Formation of trachybasaltic and basaltic trachyandesitic in composition, and the Cyprus (Bragina and Bragin, 2006). Shorsholeti Suite samples plot across the trachyandesitic and basaltic trachyandesitic fields (Fig. 6a). The Eocene intrusive rocks are grano- 4.2. Whole-rock major and trace element geochemistry dioritic to granitic in composition (Fig. 6a). Based on the immobile- element classification diagram, Gasandami and Mashavera Suite The analytical procedures are described in the electronic Appendix samples are mostly rhyodacitic/dacitic, except a few andesite and rhyo- A. The sample locations are shown in Fig. 2, and their major and trace el- lite samples, and a larger group plotting in the trachyandesitic field ement compositions are presented in the electronic Appendix B. Rock (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 a fine-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 in Fig. 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 with fiame 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 flow 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 in Fig. 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 a fine-grained matrix of the basaltic trachyandesite.

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

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 from Gualda and Ghiorso (2013).

trachyandesite, and the Tandzia Suite samples plot in the andesitic/ba- distinctly higher Rb and lower Y and Zr concentrations (Fig. 7g–i), saltic and sub-alkaline basaltic fields (Fig. 6b). and partly lower Nb, and higher Ba and Th concentrations than the The Marshavera Suite, Tandzia Suite and Eocene intrusions belong other one (Fig. 7j–l). Therefore, in the remaining part of this contri- mostly to the calc-alkaline series and partly to the low-K/tholeiite series, bution, we will distinguish high Y-Zr and low Y-Zr rhyolite types and a few Marshavera dacite samples are high-K calc-alkaline (Fig. 6c). for the Mashavera Suite (Figs.2,6–9), which are also clearly visible The Gasandami Suite has a dominantly low-K/tholeiitic composition in outcrops (Fig. 4c). In the immobile-element classification dia- (Fig. 6c). The trachyandesitic Marshavera Suite and the Shorsholeti gram (Fig. 6b), the Mashavera low Y-Zr rhyolite samples plot mostly Suite samples fall in the high-K calc-alkaline to shoshonite fields as trachyandesite, and the high Y-Zr rhyolite samples plot as (Fig. 6c). A dominantly calc-alkaline and felsic to intermediate composi- rhyodacite/dacite. The high Rb concentration of the Mashavera low tion of the samples of this study is supported by the Th vs. Co diagram Y-Zr rhyolite (Fig. 7g) is consistent with their high-K calc-alkaline (Fig. 6d), except for the Tandzia samples, which plot in the maficrock and shoshonitic composition (Fig. 6c). The Mashavera low Y-Zr rhyo-

ﬁeld, and the Shorsholeti samples, which are high-K calc-alkaline and lite has SiO2 concentrations above 75 wt% (Fig. 6a, c), therefore it shoshonitic. Tandzia and Shorholeti Suite samples have the highest qualiﬁes as high-silica rhyolite (Gualda and Ghiorso, 2013). By con-

TiO2,Al2O3,Fe2O3, MgO and CaO concentrations, which is consistent trast, the Mashavera high Y-Zr rhyolite falls in a broader range of with their mafic to intermediate composition (Fig. 7a–e). The SiO2 concentrations from 71 to 78 wt%, and consists of both low- Gasandami Suite and Eocene intrusions have the highest Na2Oconcen- and high-silica rhyolite (Fig. 6a, c). The Mashavera dacite and the trations (Fig. 7f), which fit with their predominant low-K composition Gasandami rhyolite samples have trace element concentrations over- (Fig. 6c). lapping with those of the Mashavera Suite high Y-Zr rhyolite (Fig. 7g– Two distinct groups of Mashavera Suite rhyolite are identified l), and therefore are also labeled as Mashavera Suite high Y-Zr dacite based on different trace element compositions. One group has and Gasandami Suite high Y-Zr rhyolite (Figs. 2, 6–9).

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

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 from Gualda and Ghiorso (2013).

Mashavera Suite high Y-Zr rhyolite and dacite and the Gasandami Suites (Fig. 8i–n). The Eocene intrusion samples are enriched in Sr and Suite have similar primitive, mantle-normalized trace element spider depleted in Ta, Nb and REE with respect to the Late Cretaceous rocks and chondrite-normalized rare earth element (REE) patterns (Fig. 8a–f). (Fig. 8m–n), and they also display a U-shaped middle REE pattern The Mashavera Suite low Y-Zr rhyolite samples are distinctly enriched (Fig. 8n). in large ion lithophile elements (LILE: Cs, Rb, Ba, K), Th and U, and depleted in some high ﬁeld strength elements (HFSE: Hf, Zr, Ti), Y and 4.3. Strontium and neodymium whole-rock isotope geochemistry 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 The analytical procedures are described in the electronic Appendix Suite low Y-Zr rhyolite has a characteristic U-shaped middle to heavy A. The whole-rock radiogenic isotope data are presented in the elec- REE pattern, which is distinct with respect to the ﬂat REE pattern tronic Appendix C. Late Cretaceous rocks from the Bolnisi study area of the Mashavera Suite high Y-Zr rhyolite (Fig. 8h). The Tandzia, have 143Nd/144Nd ratios between 0.51260 and 0.51278, and 87Sr/86Sr ra- Shorsholeti and Eocene intrusion samples have spider and REE diagram tios between 0.70375 and 0.70629 (Fig. 9a). The Tandzia samples fall on trends distinct with respect to those of the Mashavera and Gasandami the mantle array, and the Shorsholeti Suite samples fall along or close to

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

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; normalization with respect to Sun and McDonough, 1989) for rock formations of the Bolnisi district (see Fig. 3). Grey shaded area in c to n: compositional ﬁeld of the Mashavera Suite high Y-Zr rhyolite data depicted in a and b.

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

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 SiO2 concentrations; d: variation of initial neodymium isotopic compositions of magmatic rocks from the Bolnisi district with respect to SiO2 concentrations. Mantle array from Faure (1986), and CHUR and bulk Earth UR calculated according to Faure (1986) at 85 Ma and 52.5 Ma, respectively, in a and b.

the mantle array (Fig. 9a). By contrast, the Mashavera and Gansadami LA-ICP-MS only, and two samples by ID-TIMS only (Fig. 10d–p). Within samples have distinctly higher 87Sr/86Sr ratios and plot mostly to the analytical error, core and rim analyzed in a single zircon grain yielded right of the mantle array (Fig. 9a). The Gasandami samples have the overlapping Late Cretaceous ages (Fig. 10a–c). highest 143Nd/144Nd ratios between 0.51269 and 0.51278 (Fig. 9a). Sample SA-18-05 is a low Y-Zr rhyolite of the Mashavera Suite The Eocene intrusion sample studied by Hässig et al. (2020) falls along cropping out SW of the Sakdrisi deposit (location 1 in Fig. 2), next to a the mantle array with an elevated 143Nd/144Nd ratio and a low high Y-Zr rhyolite of the same suite (Fig. 4c). LA-ICP-MS dating yielded 87Sr/86Sr ratio (Fig. 9b). aweighted206Pb/238U mean age of 83.3 ± 0.6 Ma (n = 14; Fig. 10d–e). 87 86 143 144 Two trends can be recognized in Sr/ Sr and Nd/ Nd vs. SiO2 Sample BO-10-09 is a rhyolitic dike, affected by strong K alteration variations diagrams (Fig. 9c–d): relatively constant isotopic composi- and total depletion of Na linked to the Sakdrisi deposit (location 2 in tions with variable SiO2 concentrations typical of magmatic fraction- Fig. 2). Based on its trace element lithogeochemistry it belongs to the ation trends, and oblique trends towards higher 87Sr/86Sr and lower low Y-Zr Mashavera Suite (electronic Appendix B). LA-ICP-MS dating 143 144 206 238 Nd/ Nd ratios with increasing SiO2 concentrations, which are typi- yielded a weighted Pb/ U mean age of 84.9 ± 0.9 Ma (n = 12; cal of crustal assimilation. 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 ± 4.4. U-Pb dating of magmatic zircons 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 The U-Pb zircon age data of six samples are presented in Fig. 10,in- as the best approximation of the end of zircon crystallization. It overlaps cluding cathodoluminescence images of zircons (Fig. 10a–c). The analyt- within error with the 84.9 ± 0.9 Ma LA-ICP-MS weighted 206Pb/238U ical data can be found in the electronic Appendix D, and the analytical mean age. The low and anomalous 206Pb/238UagesobtainedbyLA- methodology in the electronic Appendix A. The sample locations are ICP-MS dating at 82.6 ± 1.9 and 82.8 ± 1.6 Ma (Fig. 10f; electronic Ap- shown in Fig. 2, and the data are summarized in Fig. 3, together with pre- pendix D2) are attributed to Pb loss following zircon precipitation. viously published ages (Hässig et al., 2020). Zircons dated in this study Sample BO-09-14 is a dacitic dike of the high Y-Zr Mashavera Suite, were separated from volcanic rocks and dikes of the Mashavera Suite. sampled west of Madneuli (location 5 in Fig. 2). LA-ICP-MS dating No zircons could be separated from samples of the Gasandami, Tandzia yielded a weighted 206Pb/238U mean age of 87.0 ± 1.1 Ma (n = 7; and Shorsholeti Suites and tuff interbedded with sedimentary rocks con- Fig. 10i–j). Six zircon grains yielded 206Pb/238U ages by ID-TIMS between taining the radiolarian fauna at the Madneuli mine (Figs. 2 and 4b). Three 86.60 ± 0.05 and 87.08 ± 0.09 Ma, and one outlier has a 206Pb/238Uage samples were analyzed by LA-ICP-MS and ID-TIMS, one sample by of 88.08 ± 0.15 Ma (Fig. 10k; electronic Appendix D1). The youngest

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

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 in Fig. 2). f and g: LA-ICP-MS zircon 206Pb/238U weighted average plots and Concordia diagram of dated zircons of sample BO-10-09 (location 2 in Fig. 2). h: ID-TIMS zircon 206Pb/238U ages of sample BO-10-09. i and j: LA-ICP-MS zircon 206Pb/238U weighted average plots and Concordia diagram of dated zircons of sample BO-09-14 (location 5 in Fig. 2). k: ID-TIMS zircon 206Pb/238U ages of sample BO-09-14. l and m: LA-ICP-MS zircon 206Pb/238U weighted average plots and Concordia diagram of dated zircons of sample BO-07-18 (location 6 in Fig. 2). n: ID-TIMS zircon 206Pb/238U ages of sample BO-07-18. o: ID-TIMS zircon 206Pb/238U ages of sample BO-10-05A. p: ID-TIMS zircon 206Pb/238U ages of sample BO-07-33B.

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

86.60 ± 0.05 Ma ID-TIMS age is considered as the best approximation of 83.6 ± 1.3 Ma has been obtained for the Gasandami Suite high Y-Zr rhy- the end of zircon crystallization, it overlaps within error with the 87.0 ± olite by Hässig et al. (2020; location 3 south of Beqtakari in Fig. 2), which 1.1 Ma LA-ICP-MS weighted 206Pb/238Umeanage. agrees with its younger stratigraphic age interpretation with respect to Sample BO-07-18 is a rhyolitic dike from the high Y-Zr Mashavera the Mashavera Suite (Fig. 3). Suite next to the Madneuli deposit (location 6 in Fig. 2). It yielded a weighted 206Pb/238U mean age of 85.9 ± 1.2 Ma (n = 10) by LA-ICP- 5.2. Late Cretaceous tectonic setting, bimodal magmatism and silicic MS dating (Fig. 10l–m). Seven zircon grains were analyzed by ID-TIMS magmatic flare-up and yielded 206Pb/238U ages between 86.96 ± 0.21 and 88.70 ± 0.13 Ma. The four youngest zircons are concordant and yield a weighted The negative Nb, Ta and Ti anomalies in the mantle-normalized trace 206Pb/238U mean age of 87.13 ± 0.12 Ma (Fig. 10n), which is considered element spiderdiagrams of the Gasandami and the Mashavera Suites are as the best approximation of final zircon crystallization. The three grains typical for subduction-related magmas (Fig. 8a, b, e and g). This is con- with the oldest ID-TIMS ages are interpreted as inherited zircons sistent with the subduction setting of the Bolnisi district during the Late (Fig. 10n). Zircon z108 from sample BO-07-18, which yielded low, Cretaceous (Rolland et al., 2011; Sosson et al., 2010). Furthermore, the anomalous LA-ICP-MS 206Pb/238U ages of 83.2 ± 1.8 and 83.8 ± chondrite-normalized REE patterns of rhyolite from the Gasandami 2.0 Ma (Fig. 10a and l; Appendix D2) was subsequently dated by ID- and the Mashavera Suites, with normalized La and Lu ratios of, respec- TIMS and yielded a clearly older 206Pb/238U age of 87.14 ± 0.16 Ma. tively, ~100 and ~10, only weak Eu negative anomalies (Fig. 8b, f and Since the chemical abrasion technique used in this study is eliminating h), and pronounced U-shaped middle to heavy REE patterns of the any lead loss effect very efficiently, we attribute the discrepancy be- Mashavera low Y-Zr Suite (Fig. 8h), are characteristic of cold-wet arc tween the young 206Pb/238U ages obtained by LA-ICP-MS and the older rhyolites erupted in convergent margin environments (Bachmann and ones obtained by ID-TIMS to Pb loss following zircon precipitation. Bergantz, 2008). Sample BO-10-05A is a rhyolitic dike of the high Y-Zr Mashavera Rhyolite of the Late Cretaceous Mashavera and Gasandami Suites con- Suite from the upper part of the Madneuli open pit (location 7 in stitutes the volumetrically most abundant rock type in the Bolnisi district, Fig. 2). Five zircon grains yielded ID-TIMS 206Pb/238U ages between and is accompanied by subsidiary dacite (Fig. 6a). Mafic rocks of the 87.02 ± 0.16 and 87.66 ± 0.15 Ma (Fig. 10o). The youngest age of Tandzia Suite are a volumetrically minor rock type (Figs. 6a), which oc- 87.02 ± 0.16 Ma is considered as the best approximation of the end of curs as dikes crosscutting both the Mashavera and Gasandami Suites zircon crystallization. (Figs. 2 and 4g–h), and as local lava breccia (Fig. 4f). There is a distinct

BO-07-33B is a dacitic pyroclastic ﬂow of the high Y-Zr Mashavera compositional gap of SiO2 concentrations with Tandzia Suite samples fall- Suite (location 15 in Fig. 2). Seven zircon grains yielded ID-TIMS ing below 51.7 wt% and Mashavera and Gasandami Suite samples above 206Pb/238U ages between 86.61 ± 0.31 and 86.81 ± 0.24 Ma 66.3 wt% (Figs. 6aand7). The absence of intermediate rock types within

(Fig. 10p). The youngest age of 86.61 ± 0.31 Ma is considered as the the concentration range from 51.7 to 66.3 wt% SiO2 supports bimodal- best approximation of the end of zircon crystallization. type magmatism (Meade et al., 2014; Melekhova et al., 2013)inthe Bolnisi district. The alkaline Shorsholeti Suite is not considered here and 5. Discussion will be discussed later, because it is a stratigraphically younger rock sequence (Fig. 3), which crops out as a separate entity in the northwestern 5.1. Age constraints of the Mashavera and Gasandami Suites part of the district (Fig. 2). The overlapping U-Pb ages, trace element compositions, spider- The U-Pb zircon data of the Mashavera and Gasandami Suites fall diagrams, REE-normalized patterns and radiogenic compositions of between 87.14 ± 0.16 and 83.90 ± 2.40 Ma, with one outlier at the Mashavera Suite high Y-Zr rhyolite and dacite indicate a common 81.64 ± 0.94 Ma (Fig. 3). This brackets the age of magmatic activity petrogenetic evolution (Figs. 3, 7g–l, 8a–dand9a). The dacite and to the late Coniacian, Santonian and the very early Campanian, low- to high-silica rhyolite (<75 wt% SiO2 and >75 wt% SiO2, respec- within a tight duration of 6.6 m.y. (Fig. 3). The radioisotope ages tively, Fig. 6a) association of the Bolnisi district is typical for volcanic are consistent with the Coniacian-Santonian radiolaria age for sedi- products erupted from zoned magma chambers in silicic magmatic mentary rocks interlayered with tuff of the Mashavera Suite provinces (Graham et al., 1995; Streck and Grunder, 1997; Watts (Figs. 4band5). Within analytical error, core and rim analyzed in a et al., 2016). The Mashavera low Y-Zr rhyolite, with only high-silica rhy- single zircon grain yielded overlapping Late Cretaceous ages olite (>75 wt% SiO2, Fig. 6a) and its U-shaped middle to heavy REE (Fig. 10a–c). This excludes inheritance of zircon cores from signiﬁ- patterns (Fig. 8h), is attributed to a more pronounced magmatic dif- cantly older rocks, such as the Late Proterozoic-Paleozoic and ferentiation (Bachmann and Bergantz, 2008; Deering et al., 2008)at Jurassic-Early Cretaceous rock units of the Khrami and Loki massifs the end of the volcanic evolution of the Mashavera Suite, between (Fig. 2). Our data support the Coniacian to Santonian stratigraphic 85.70 ± 0.05 Ma and 83.3 ± 0.6 Ma (Fig. 3). ages of Gambashidze and Nadareishvili (1980, 1987), Gambashidze (1984), Apkhazava (1988),andVashakidze and Gugushvili (2006) 5.3. Chemical variations of Mashavera and Gasandami rhyolite and dacite: (Fig. 3). coeval Late Cretaceous eruption from multiple isolated magma batches 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 Chemical variations among coeval rhyolite types within the same si- 87.14 ± 0.16 and 81.64 ± 0.94 Ma (Fig. 3). By contrast, the Mashavera licic magmatic province can be attributed to either in situ fractionation Suite low Y-Zr rhyolite is restricted to the younger age range between of one single, large and layered magma chamber or to discrete, small 85.70 ± 0.05 and 83.3 ± 0.6 Ma (Fig. 3). We conclude, that magmatism and coeval magma chambers having distinct chemical signatures of the Mashavera Suite started during the Coniacian with the deposition (e.g., Bégué et al., 2014). The wavy contact at the outcrop scale of the of high Y-Zr rhyolite and dacite, and that low Y-Zr rhyolite of the high and low Y-Zr rhyolite sequences of the Mashavera Suite (Fig. 4c) Mashavera Suite was deposited during the Santonian, coeval with the supports contemporaneity of both rhyolite types at a given time during wanning stages of the Mashavera high Y-Zr magmatism. Contempora- the deposition of the Mashavera Suite. It is support for the coexistence neity of the high and low Y-Zr rhyolite sequences of the Mashavera of discrete and isolated magma batches in the crust, which have fed dis- Suite is supported by their wavy contact at the outcrop scale (Fig. 4c), tinct, coeval silicic volcanic centres. For instance, such a magmatic set- which argues for a hot and plastic state during deposition of both rhyo- ting has been described in the Taupo volcanic zone, New Zealand lite types next to each other at 83.3 ± 0.6 Ma (location in Fig. 2;Sample (Bégué et al., 2014; Charlier et al., 2005; Smith et al., 2005; Sutton SA-18-05 in Fig. 10d–e, electronic Appendix D). Only one single age of et al., 1995).

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

Major and trace element compositions can allow to distinguish coe- Shuto et al., 2006; Smith et al., 2005; Zellmer et al., 2020). The Tandzia val rhyolite types fed by distinct storage magma chambers with differ- Suite dikes crosscuting the Mashavera and Gasandami Suites (Fig. 5g– ent crystallization histories (Deering et al., 2008; Smith et al., 2005). In h), and their predominant northeast and subsidiary eastwest orienta- the Bolnisi district, the Gasandami high Y-Zr rhyolite and the Mashavera tion are evidence for a structural control on mafic magma ascent during high Y-Zr rhyolite and dacite share similar trace and major element Late Cretaceous extension. Such structural control on mafic magma as- compositions, which are distinct from those of the Mashavera low Y- cent has also been reported in the Taupo volcanic zone, New Zealand Zr rhyolite (Figs. 7a, c, g–i, k–l; 11a–d). The ferromagnesian mineralogy (Gamble et al., 1990; Graham et al., 1995). The lava flows of the Tandzia is an important control on the chemical composition of rhyolite and Suite (Fig. 5f) are evidence of sporadic mafic magmatism reaching the dacite (Deering et al., 2008; Smith et al., 2005), and volcanic rocks surface. It suggests that fissure eruptions of the Tandzia Suite were coe- in general (Pearce and Norry, 1979). Pressure, temperature, f O2 and val with rhyolitic and dacitic volcanism of the Mashavera and f H2O conditions control the stability of ferromagnesian minerals, Gasandami Suites, during bimodal magmatism in the Bolnisi district in which fractionate in distinct magma chambers at different crustal an extensional tectonic setting (Fig. 13a–b). depths. In the Taupo volcanic zone, New Zealand, Smith et al. (2005) and Deering et al. (2008) have shown that in shallow magmatic cham- 5.5. Origin of magmas: evidence from radiogenic isotopes bers, low pressure and temperature, and high fH2Oandoxidizingcondi- tions promote fractionation of hornblende and other hydrous phases The mafic rocks of the Tandzia Suite have crystallized from mantle- such as biotite, and suppress fractionation of plagioclase. This results derived, primary magmas as documented by their juvenile Sr and Nd in depletion of middle REE, Y, Zr and Fe, and U-shaped middle REE pat- isotopic compositions falling along the mantle array (Fig. 9a), and terns. By contrast, in deep magmatic chambers, high pressure and tem- their lack of Eu anomaly (Fig. 8j), suggesting no or only negligeable pla- perature, and low fH2O and reducing conditions promote fractionation gioclase crystal fractionation or uptake (Graham et al., 1995; Zellmer of pyroxene, resulting in an enrichment of middle REE, Y, Zr and Fe. et al., 2020). Compared to the Tandzia Suite mafic rocks, the Sr isotopic Despite the replacement of the original magmatic assemblage by re- compositions of most of the Mashavera and Gasandami Suite rocks are gional metamorphic minerals in the Bolnisi district, the distinct chemi- shifted to higher 87Sr/86Sr ratios, to the right of the mantle array cal differences of rhyolite and dacite still record eruption from distinct (Fig. 9a). The Nd isotopic compositions of the Mashavera Suite samples magma chambers located at different crustal depths. Indeed, the higher scatter towards lower 143Nd/144Nd ratios with respect to the Tandzia

TiO2,Fe2O3, Zr, Y and middle REE concentrations of the high Y-Zr rhyo- Suite samples, whereas the ones of the Gasandami Suite are shifted to- lite and dacite of the Mashavera and Gasandami Suites, compared to the wards more elevated 143Nd/144Nd ratios (Fig. 9a). 87 86 143 144 Mashavera Suite low Y-Zr rhyolite (Figs. 7c, h–i, 8b, d, f, h, and 11a–c), In Sr/ Sr and Nd/ Nd vs. SiO2 variations diagrams (Fig. 9c–d), support eruption from deeper magma storage chambers, in which py- a limited number of Mashavera Suite samples have isotopic com- roxene was part of the original assemblage (Deering et al., 2008; positions overlapping with those of the Tandzia Suite samples. This

Smith et al., 2005). The U-shaped middle to heavy REE patterns relatively constant isotopic composition despite variable SiO2 concen- (Fig. 8h) and the data trend in the Sr/Y vs. SiO2 diagram (Fig. 11e) are trations is explained by extreme magmatic differentiation that has pro- evidence for fractionation of hornblende and suppression of plagioclase duced the rhyolitic and dacitic magmas from a common maficreservoir fractionation during petrogenesis of the Mashavera Suite low Y-Zr rhy- (see horizontal trends in Fig. 9c–d). However, most Mashavera Suite olite, which reveals petrogenesis in shallow magma chambers (Deering rhyolite and dacite samples display a concomitant shift towards higher 87 86 143 144 et al., 2008; Smith et al., 2005). The elevated K2O and Rb concentrations Sr/ Sr and lower Nd/ Nd ratios with respect to the maficrocks of the Mashavera Suite low Y-Zr rhyolite are also in line with an ad- (oblique trends in Fig. 9c–d). This reflects interaction with or assimila- vanced stage of magmatic fractionation and crystallisation of biotite in tion of continental crustal rocks by the rhyolitic and dacitic magmas shallow magma chambers (Figs. 6c, 7g, and 11d). The high SiO2 concen- during their ascent and ponding in the crust (Fig. 13a). Such an evolu- trations above 75 wt% of the Mashavera Suite low Y-Zr rhyolite (Fig. 6a, tion identified by radiogenic isotopes, with combined or successive c) is further evidence for the shallow depth of the magma storage cham- magmatic crystal fractionation and crustal interaction/assimilation is bers from which they have been erupted, since high-silica rhyolite typical during rhyolite and dacite petrogenesis in bimodal magmatic

(SiO2 > 75 wt%) is typical of low pressure environments, i.e. shallow districts (Graham et al., 1995; Sutton et al., 1995; Wilson et al., 2006). 87 86 143 144 crust (Gualda and Ghiorso, 2013). The Sr/ Sr and Nd/ Nd vs. SiO2 variations diagrams of the Gasandami Suite samples are also consistent with interaction or assim- 5.4. Tandzia suite mafic rocks: high-alumina basalts co-genetic with ilation of continental crustal rocks by rhyolitic melts (oblique trends in rhyolitic-dacitic magmatism Fig. 9c–d). However, their elevated 143Nd/144Nd ratios in comparison to those of the Tandzia Suite mafic rocks indicate that the Gasandami The mafic rocks of the Tandzia Suite qualify as high-alumina basalts Suite rhyolite is linked to a more juvenile mantle component than the with SiO2 < 54 wt%, Al2O3 >16.5wt%andMgO<7wt%(Crawford et al., one that has generated the Tandzia and Mashavera Suite rocks. Al- 1987; Figs. 6aand7b,d). They are characterized by gentle REE patterns though the Gasandami Suite rhyolite has major and trace element con- with only slight light element enrichment (LaN/YbN =3–9), nearly flat centrations and trends mostly overlapping with those of the Mashavera heavy element patterns (TbN/YbN =1.2–1.9) and devoid of a Eu anom- Suite rhyolite (Figs. 6a–b, 7a–e, h–i, and 11a–c, e), it is characterized by aly (Fig. 8j), which are typical of high-alumina basalts from bimodal distinctly higher Na2OconcentrationsandlowerK2O, Rb, Nb, Ba and Th magmatic provinces (Graham et al., 1995; Shuto et al., 2006; Zellmer concentrations (Figs. 6c, 7f–g, j–l, and 11d). The lower concentrations of et al., 2020). The primitive nature of the high-alumina basalts at Bolnisi the light ion lithophile elements (LILE: Rb, Ba, Ba) and Th is also is supported by their high Ti concentrations, low Zr concentrations, expressed in the primitive mantle-normalized spider diagram, in MORB-type Zr/Y ratios (Fig. 11a, c), and their primitive mantle- which the Gasandami samples are confined to the lower range of the normalized spider diagrams devoid of Sr and Ti anomalies (Fig. 8i). Mashavera Suite samples (right hand side of Fig. 8e). Due to their higher

The trace element systematics of the mafic Tandzia Suite samples Na2OandlowerK2O concentrations, the Gasandami Suite rhyolite qual- falls along similar trends as the ones of the high Y-Zr rhyolite and dacite ifies mostly as low K or tholeiitic rocks (Fig. 6c). We attribute the more samples of the Mashavera and Gasandami Suite (Fig. 12a–f). This sup- pronounced isotopic mantle signature, higher Na2O concentrations and ports a petrogenetic link between the mafic and felsic end-members lower LILE concentrations of the Gasandami Suite rhyolite to the pres- of the bimodal magmatism in the Bolnisi district. Although they are gen- ence of a more juvenile mantle component in the mantle wedge. The erally volumetrically minor, such mafic rocks are a common component more juvenile mantle component is attributed to asthenospheric man- in silicic provinces linked to bimodal magmatism (Graham et al., 1995; tle upwelling and/or tapping of a less metasomatized mantle (Fig. 13b).

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

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 ﬁelds 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 (from Pearce and Norry, 1979; Pearce, 1983). Compositional ﬁeld of basalts in c: MORB = middle oceanic ridge basalt (from Pearce, 1982).

5.6. Shorsholeti suite: high-K magmatism during Late Cretaceous slab roll- metasomatised by fluids released from the oceanic crust and melting back of subducted sediments (Fig. 13c; Planck, 2005; Behn et al., 2011; Kirchenbaur et al., 2012; Rezeau et al., 2017). The Shorsholeti Suite is a separate and stratigraphically younger Subducted sediments are the main repositories of Th, U, Rb, Sr, Ba unit (Fig. 3), cropping out to the west of the Mashavera and and light REE (Kessel et al., 2005). Therefore, enrichment of the latter Gasandami units (Fig. 2), with a distinct high-K calc-alkaline to in the Shorsholeti Suite (Figs. 7g, k–l, 8k–l, and 11e) is attributed to shoshonitic composition (Fig. 6c–d). The Shorsholeti Suite samples their scavenging from subducted sediments present in the have negative Nb, Ta and Ti anomalies, which are typical of arc metasomatized mantle wedge (Kessel et al., 2005). The presence of a magmas (Fig. 8k). They have high incompatible element concentra- sedimentary component in the metasomatized mantle source is also tions, including Rb, Zr, Nb, Ba, Th, U, Sr and light REE, especially consistent with their high 87Sr/86Sr and low 143Nd/144Nd ratios, when when compared to the Tandzia Suite maficrocks(Figs. 7g, i–l, 8l, compared to the Tandzia Suite samples, which were sourced by juvenile and 11a–d). The composition of high-K calc-alkaline and shoshonitic mantle, not or less affected by metasomatism (Figs. 9aand14; rocks and their strong enrichment in incompatible elements is gen- e.g., Kirchenbaur et al., 2012). The high Th/Yb and La/Yb ratios of the erally interpreted to reflect their mantle sources, and is attributed Shorsholeti Suite samples also support a more important sedimentary to low-degree partial melting of lithospheric mantle and/or mantle component in the melts sourced by the mantle wedge, in contrast to

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

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- derived fluid and sediment or sediment melts enrichment trends (Woodhead et al., 2001); f: Ba/Th vs. La/Sm diagram with fluid-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).

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

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

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 to Taylor and McLennan, 1985); b, d, f and h: rare earth element-normalized diagrams (normalization with respect to Sun and McDonough, 1989).

the juvenile mantle source of the Tandzia Suite maﬁcrocks(Fig. 12e, g; more pronounced with increasing temperature and pressure conditions e.g., Kirchenbaur et al., 2012). of mantle melting at deeper settings (Kessel et al., 2005; Kirchenbaur The Zr and Nb enrichment of the Shorsholeti Suite samples with and Münker, 2015). Deeper pressure conditions during mantle melting, respect to the other Late Cretaceous magmatic rocks of the Bolnisi which has sourced the Shorsholeti Suite rocks, is also supported by district (Fig. 7i–j) can be explained by high temperature and high pres- higher Sm/Yb ratios with respect to the other Late Cretaceous magmatic sure (i.e., deep) conditions of mantle melting during petrogenesis rocks, in particular the Tandzia Suite (Fig. 12d). Indeed, progressively (Fig. 13c). Indeed, the degree of incompatibility of Zr and Nb becomes 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.

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

19 R. Moritz, N. Popkhadze, M. Hässig et al. Lithos xxx (2020) xxx deeper mantle melting environments (e.g., Kay and Mpodozis, 2001; volcanic rocks of the Çatak, Kızılkaya, Çağlayan, Tirebolu and Çayırbağ Mamani et al., 2010; Shafiei et al., 2009). Formations in Turkey (Eyüboğlu, 2010), and the Mashavera, Gasandami Deeper mantle melting conditions during petrogenesis of the and Tandzia Suites in the Bolnisi district (Fig. 3). High-K magmatism has Shorsholeti Suite high-K magmas can be explained by slab roll-back. been dated as Campanian in the Eastern Pontides (Aydin et al., 2020; High-K (shoshonitic) rocks occur in two distinct settings. In zoned vol- Eyüboğlu, 2010; Özdamar, 2016), and has been interpreted as latest canic arcs with a constant subduction angle, calc-alkaline rocks are Santonian to Campanian in the Bolnisi district, Georgia (Fig. 3). The proximal with respect to the associated subduction zone and high-K high-K magmatic rocks of the Shorsholeti Suite, Bolnisi district and of (shoshonitic) rocks are located in a distal setting, above the deeper the Bayburt-Maden area, Eastern Pontides (location 3 in Fig. 1)display part of the subduction zone. By contrast, other volcanic arcs are devoid similar geochemical trends, with high Th/Yb, Ta/Yb, Nb/Yb, Sm/Yb and of such a zonation, and successively more K-enriched magmatic rocks La/Yb ratios compared to older mafic rocks in each area, respectively, are produced in a nearly stationary setting above a progressively steep- the Tandzia Suite, and the Çatak and Çağlayan Formations (Fig. 12a–e, ening subduction zone (Morrison, 1980). In the tectonic context of g). Furthermore, the primitive mantle-normalized and REE patterns of the Bolnisi region, with a north-verging subduction and a roughly the Shorsholeti Suite from the Bolnisi district overlap with those of the eastwest-oriented magmatic arc during the Late Cretaceous (Hässig Bayburt-Maden area, Eastern Pontides (location 3 in Fig. 1), in particular et al., 2016; Rolland et al., 2011; Sosson et al., 2016), steepening of the trachyandesite of cycle I (Eyüboğlu, 2010; Fig 14g–h). Kandemir et al. subduction zone (Fig. 13c) satisfactorily explains along-arc juxtaposi- (2019) and Aydin et al. (2020) invoke a slab roll-back scenario and an tion of the high-K Shorsholeti Suite and the low-K to calc-alkaline extensional tectonic setting during petrogenesis of the high-K mag- Mashavera and Gasandami Suites (Fig. 2). matic rocks in the Eastern Pontides, which is in line with our interpretation for the Shorsholeti Suite in Bolnisi. 5.7. Coeval Late Cretaceous magmatic and tectonic evolution of the Bolnisi The Late Cretaceous magmatic rocks of the Bolnisi district have gen- district and the Eastern Pontides erally distinctly higher 143Nd/144Nd ratios than the rocks from the East- ern Pontides (Fig. 9a), which documents a higher mantle component in The late Conacian to earliest Campanian bimodal magmatism of the the magmas in Georgia. This can be attributed to a thinner crust, or al- Bolnisi district coincides with the development of extensive rhyolitic to ternatively less assimilation of crustal material during petrogenesis of basaltic magmatism in the Eastern Pontides (Fig. 3). Extensive strati- the Late Cretaceous magmatic rocks in the Bolnisi district. Akin to the graphic, petrographic and geochemical data have been published on Bolnisi rocks, the majority of the Late Cretaceous rocks of the Eastern Late Cretaceous magmatism on different study areas of the Eastern Pontides have 87Sr/86Sr ratios falling to the right of the mantle array Pontides (location 1 to 4 in Fig. 1). The Late Cretaceous magmatism of (Fig. 9a), which is also attributed to interaction/assimilation of crustal the Eastern Pontides is bimodal (Aydin et al., 2020), like in the Bolnisi rocks by magmas during their ascent or ponding in the crust. district (Figs. 6a, c, 7), and the magmatic evolution in both areas is strik- In conclusion, the Eastern Pontides and the Bolnisi district belong to ingly similar (Fig. 12). The Mashavera and Gasandami high Y-Zr rhyolite a common east-west-oriented, late Turonian to Campanian magmatic and dacite, and the Tandzia Suite mafic rocks have geochemical trends belt. The bimodal volcanism with predominant silicic magmatism (Figs. 12a–d, 14a–b, e–f), which overlap with those of the calc-alkaline (e.g., Aydin et al., 2020; this study) took place in an extensional tectonic Çatak, Kızılkaya and Çağlayan Formations studied in the Tirebolu and setting (Kandemir et al., 2019), during the wanning subduction stages the Artvin areas of the Eastern Pontides (locations 2 and 4, respectively, of the northern branch of the Neotethys (Fig. 13a–b). The Turonian to in Fig. 1;Aydin et al., 2020 ; Eyüboğlu et al., 2014). The Mashavera and earliest Campanian silicic magmatic flare-up propagated eastwards Gasandami high Y-Zr rhyolite and dacite, and the Tandzia Suite rocks from the Eastern Pontides to the Bolnisi district (Fig. 3), in parallel yield large variations of Ba/La and Ba/Th ratios overlapping with those with the Late Cretaceous west to east opening of the Black Sea basin of the Çatak, Kızılkaya and Çağlayan Formations (Fig. 12e–f). There is (Hippolyte et al., 2017; Nikishin et al., 2011; Sosson et al., 2016), also a compositional gap between the mafic-intermediate rocks of the which merges with the Adjara-Trialeti belt in Georgia (Fig. 1; Adamia Çatak and Çağlayan Formations and the felsic-intermediate rocks of et al., 2010; Yilmaz et al., 2014). The Turonian to earliest Campanian si- the Kızılkaya Formation, which is comparable to the bimodal volcanism licic magmatic flare-up was followed by Campanian high-K magmatism of the Bolnisi district (Fig. 12g). during slab roll-back along the entire belt (Fig. 13c; Eyüboğlu, 2010; Younger felsic rock units of the Tirebolu and Çayırbağ Formations in Özdamar, 2016; Aydin et al., 2020;thisstudy). the Eastern Pontides, sitting stratigraphically above the Çatak, Kızılkaya Mesozoic magmatism along the southern Eurasian convergent margin and Çağlayan Formations (Eyüboğlu et al., 2014; Aydin et al., 2020), dis- has been interrupted by a ~40 m.y.-long magmatic lull at the Early to Late play the same shift towards higher Th/Yb, Ta/Yb, Ba/Yb, Nb/Yb, La/Sm Cretaceous transition, between ~130 and ~90 Ma (Hässig et al., 2020). The and La/Yb ratios as the Mashavera Suite low Y-Zr rhyolite (Fig. 12a–d, tectonic and magmatic quiescence along the Lesser Caucasus at the Early g). The Mashavera Suite low Y-Zr rhyolite has primitive mantle- to Late Cretaceous transition, and Albian compressional tectonics and uplift normalized and REE patterns (Fig. 14c–d), which mimic those of rhyo- of the Pontides belt are attributed to accretion of oceanic seamounts and lite from the Ordu area (location 1 in Fig. 1; Özdamar, 2016) and the plateaus, which impeded subduction along the Eurasian margin (Hässig Çayırbağ Formation of the Tirebolu area (location 2 in Fig. 1; Eyüboğlu et al., 2016; Okay et al., 2006, 2013; Rolland et al., 2009b, 2011). North- et al., 2014) This supports a similar petrogenetic evolution with time verging subduction of the northern Neotethys branch only resumed at towards shallower magmatic chambers that have fed rhyolitic mag- ~90 Ma (Hässig et al., 2016, 2020; Kandemir et al., 2019; Okay et al., matism in both the Eastern Pontides and the Bolnisi district. 2006, 2013; Rice et al., 2006). Thus, the Late Cretaceous bimodal, domi- The Eastern Pontides and the Bolnisi district host high-K magmatic nantly silicic magmatic flare-up, and the subsequent high-K volcanism rocks, which are stratigraphically younger than the calc-alkaline are major markers of the geological evolution of the Eastern Pontides

Fig. 15. Comparison of the geochemical composition of Eocene magmatic rocks from the Bolnisi district and 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); c and d: primitive mantle-normalized trace element spider diagrams (normalization with respect to Taylor and McLennan, 1985) with a comparison of the data of the early Eocene Bolnisi intrusions, respectively, with respect to early Eocene adakite-like and middle Eocene non-adakitic rocks of the Eastern Pontides; e: rare earth element-normalized diagrams (normalization with respect to Sun and McDonough, 1989); f: Dy/Yb vs. SiO2 (wt%) diagram with amphibole and garnet fractionation trends (Davidson et al., 2007); g: Sr/Y vs. SiO2 (wt%) discrimination diagram; h: La/Yb vs. SiO2 (wt%) discrimination diagram; i: Al2O3 (wt%) vs. SiO2 (wt%); j: MgO (wt%) vs. SiO2 (wt%) with high- and low-silica adakites ﬁelds (Martin et al., 2005). Adakite ﬁeld 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).

20 R. Moritz, N. Popkhadze, M. Hässig et al. Lithos xxx (2020) xxx and the Lesser Caucasus. First, they record reactivation of magmatism basins, in a back-arc setting with respect to the north-verging subduc- along the southern Eurasian convergent margin after a magmatic lull of tion of the northern Neotethys. The Late Cretaceous silicic magmatic ~40 m.y. Secondly, the eastwest-oriented Late Cretecaous extensional tec- ﬂare-up of the Bolnisi district and the Eastern Pontides took place tonics and silicic magmatism were coeval with oblique rifting of the after a ~40 m.y.-long magmatic lull at the Early to Late Cretaceous tran- northwest-oriented Jurassic-Early Cretaceous subduction-related mag- sition along the Eurasian convergent margin. The Late Cretaceous silicic matic belt during opening of the Black Sea basin, with the Somkheto- magmatic ﬂare-up contributed to the oblique rifting of the northwest- Karabagh segment remaining to the south of the Bolnisi district (Fig. 1), oriented Jurassic-Early Cretaceous calc-alkaline magmatic arc, with the and displacing the western Greater Caucasus segment to the north, in Somkheto-Karabagh arc remaining in the south and the western ex- the Sochi-Ritsa/Bechasyn regions in Russia (Hässig et al., 2020). tremity of the Greater Caucasus being translated to the north.

5.8. Early EEEocene postcollisional evolution of the Eastern Pontides and the Declaration of Competing Interest Bolnisi district The authors declare that they have no known competing financial The Eastern Pontides have been affected by Eocene postcollisional interests or personal relationships that could have appeared to influ- magmatism, which has been subdivided into early Eocene adakite-like ence the work reported in this paper. magmatism (Dokuz et al., 2013; Eyüboğlu et al., 2011; Karsli et al., 2010, 2011; Topuz et al., 2005, 2011) and middle Eocene non-adakitic Acknowledgements magmatism (Aydınçakır, 2014; Dokuz et al., 2019; Kaygusuz and Öztürk, 2015). Uranium-lead zircon ages show that the Eocene granodi- The research was supported by the Swiss National Science Founda- orite and granite of the Bolnisi district (Hässig et al., 2020; Fig. 2) are co- tion (grants 200020-121510, 200020-138130 and 200020-155928) eval with early Eocene adakite-like magmatism of the Eastern Pontides and the SCOPES Joint Research Projects (IB7620-118901 and IZ73Z0- (Figs. 3, 15a–b). Thus, the Eastern Pontides and the Bolnisi district share 128324). Titouan Golay and Jonathan Lavoie were supported by grants a common early Eocene postcollisional magmatic evolution. of the Augustin Lombard Foundation (Geneva Société de Physique et The early Eocene magmatic rocks in both areas display similar com- d’Histoire Naturelle), the Ernst and Lucie Schmidheiny Foundation, positions and geochemical trends (Fig. 15). The primitive mantle- and the Society of Economic Geologists (McKinstry Fund). The authors normalized patterns of the early Eocene Bolnisi granodiorite and granite would like to thank the staff of the Madneuli, Sakdrisi and Bektaqari mimic those of the early Eocene adakite-like rocks of the Eastern mines and the Rich Metal Group for access to open pits and prospects, Pontides (Fig. 15c), but contrast with the ones of the non-adakitic mid- and for logistical support. We would like to thank the continuing sup- dle Ecoene rocks (Fig. 15d). The early Eocene intrusions of the Bolnisi port and interest of Malkhaz Natsvlishvili throughout the years. We also district have U-shaped middle REE and garnet fractionation patterns thank Tamara Beridze and Sophio Khutsishvili for fieldwork support, similar to the ones of the adakite-like rocks of the Eastern Pontides and discussions with Shota Adamia and Ramaz Migineishvili. We are (Fig. 15e–f). Based on the Sr/Y ratio, the early Eocene intrusions of the grateful to the technical help by Jean-Marie Boccard, Fabio Capponi Bolnisi district qualify as adakite-like rocks (Fig. 15a, g), but are only and Antoine de Haller during thin section preparation, and XRF ana- transitional to the adakite field according to the La/Yb ratio (Fig. 15b, h). lyses. We are grateful to Aral Okay and Yann Rolland for their critical Petrogenesis of the early Eocene adakite-like magmatism in the reviews and their comments. Eastern Pontides has been attributed to asthenospheric upwelling and partial melting of subducted Late Cretaceous oceanic crust (Dokuz al., Appendix A. Supplementary data 2019). A similar model may apply to the early Eocene adakite-like magmatism in the Bolnisi district (Fig. 13d). However, in addition to Supplementary data to this article can be found online at https://doi. mantle-related processes and slab melting, we cannot discount mag- org/10.1016/j.lithos.2020.105872. matic differentiation dominated by amphibole in a thickened lower crust (Fig. 13d), that could also have generated the adakite-like high References Sr/Y ratios and U-shaped middle REE patterns (Chiaradia, 2015; Mamani et al., 2010; Richards and Kerrich, 2007) of the early Eocene Adamia, Sh., Gujabidze, G., 2004. Geological map of Georgia 1: 500,000 (on the basis of 1: 200,000 and 1:50,000 scales, State Geological maps of Georgia). Department of Geol- Bolnisi intrusions (Fig. 15a, e, g). ogy, Nodia Institute of Geophysics http://www.ig-geophysics.ge/sakartvelo.html. Adamia, Sh., Alania, V., Chabukiani, A., Chichua, G., Enukidze, O., Sadradze, N., 2010. Evo- 6. Conclusions lution of the late Cenozoic basins of Georgia (SW Caucasus): a review. Geol. Soc. London Spec. Pub. 340, 239–259. 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