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Although Variscan terranes have been documented from the Balkans to the Caucasus, the southeastern portion of the Variscan Belt is not well understood. The Strandja Zone along the border between Bulgaria and Turkey encompasses one such terrane linking the Balkanides and the Pontides. However, the evolution of this terrane, and the Late Carboniferous to Triassic granitoids within it, is poorly resolved. Here we present laser ablation – inductively coupled plasma – mass spectrometry (LA-ICP-MS) U–Pb zircon ages, coupled with petrography and geochemistry from the Izvorovo Pluton within the Sakar Unit (Strandja Zone). This pluton is composed of variably metamorphosed and deformed granites which yield crystallization ages of c. 251–256 Ma. These ages are older than the previously assumed age of the Izvorovo Pluton based on a postulated genetic relationship between the Izvorovo Pluton and Late Jurassic to Early Cretaceous metamorphism. A better understanding of units across the Strandja Zone can now be achieved, revealing two age groups of plutons within it. An extensive magmatic episode occurred c. 312–295 Ma, and a longer-lived episode between c. 275 and 230 Ma. Intrusions associated with both magmatic events were emplaced into pre-Late Carboniferous basement, and were overprinted by Early Alpine metamorphism and deformation. These two stages of magmatism can likely be attributed to changes in tectonic setting in the Strandja Zone. Such a change in tectonic setting is likely related to the collision between Gondwana-derived terranes and Laurussia, followed by either subduction of the Palaeo-Tethys Ocean beneath Laurussia or rifting in the southern margin of Laurussia, with granitoids forming in different tectonic environments. [ABSTRACT FROM AUTHOR]

The Strandja Zone (SE Bulgaria/ NW Turkey) comprises tectonic units affected by Late Jurassic to Early Cretaceous upper greenschist- to amphibolite-facies metamorphism. A contemporaneous tectonothermal event occurred in the Rhodope Metamorphic Complex, which has led to many authors correlating both zones. Three tectonic units can be distinguished in the Bulgarian part of the Strandja Zone: the Sakar, Strandja and Veleka units. This study focuses on albitized rocks of the Sakar Unit that are suspected to be related to this tectonothermal event. Albitized samples were collected from the Late Carboniferous Sakar Batholith, while for comparison country-rock orthogneiss unaffected by albitization was also analyzed. Geochemical and petrological studies indicate that albitization was coupled with the removal of quartz and the growth of rutile-rich aggregates replacing titanite. U Pb rutile dating of an orthogneiss (ca. 154 Ma) confirms Late Jurassic amphibolite-facies conditions, whereas rutile from albitized metagranitoids yields Early Cretaceous ages (ca. 125–116 Ma). Zr-in-rutile thermometry reveals similar crystallization temperatures (∼530-620 °C) for the albitized Sakar granitoids and country rock orthogneiss. U Pb zircon and titanite dating implies a Late Carboniferous crystallization age for various granitoids forming the Sakar Batholith. Our results indicate that at least some parts of the Sakar Unit were affected by thermal events and associated albitization in the Early Cretaceous. However, this did not lead to the formation of a new penetrative fabric, unlike in the Rhodopes, where contemporaneous regional metamorphism is associated with penetrative deformation. • U Pb rutile dating reveals two thermal events in the Strandja Zone. • Early Alpine metamorphism affected the Strandja Zone in the Late Jurassic. • The NW Strandja Zone was also affected by a thermal event in the Early Cretaceous synchronous with albitization. • Zr-in-rutile thermometry showed that these events reached similar temperatures (∼530-620 °C). • Albitization caused the breakdown of titanite to rutile-rich aggregates. [ABSTRACT FROM AUTHOR]

Southeastern Bulgaria is composed of a variety of rocks from pre-Variscan (ca. 0.3 Ga) to pre-Alpine sensu lato (ca. 0.15 Ga) time. The Sakar Unit in this region comprises a series of granitoids and gneisses formed or metamorphosed during these events. It is cut by a series of post-Variscan hydrothermal veins, yet lacks pervasive Alpine deformation. It thus represents a key unit for detecting potential tectonism associated with the enigmatic Cimmerian Orogenic episode, but limited geochronology has been undertaken on this unit. Here we report age constraints on hydrothermal activity in the Sakar Pluton. The investigated veins contain mainly albite–actinolite–chlorite–apatite–titanite–quartz–tourmaline–epidote and accessory minerals. The most common accessory minerals are rutile and molybdenite. Apatite and titanite from the same vein were dated by U–Pb LA–ICP-MS geochronology. These dates are interpreted as crystallization ages and are 149 ± 7 Ma on apatite and 114 ± 1 Ma on titanite, respectively. These crystallization ages are the first to document two stages of hydrothermal activity during the late Jurassic to early Cretaceous, using U–Pb geochronology, and its association with the Cimmerian orogenesis. The Cimmerian tectono-thermal episode is well-documented further to the east in the Eastern Strandja Massif granitoids. However, these are the first documented ages from the western parts of the Strandja Massif, in the Sakar Unit. These ages also temporally overlap with previously published Ar–Ar and K–Ar cooling ages, and firmly establish that the Cimmerian orogeny in the studied area included both tectonic and hydrothermal activity. Such hydrothermal activity likely accounted for the intense albitization found in the Sakar Unit. [ABSTRACT FROM AUTHOR]