New LA-ICP-MS U-Pb zircon dating for Strandja granitoids (SE Bulgaria): evidence for two-stage late Variscan magmatism in the internal Balkanides

Abstract

The Strandja Massif (Sakar-Strandja Zone) forms an important link between the Balkan Zone (external Balkanides) of Bulgaria, which is commonly correlated with the Variscan orogen in Central Europe, and the Western Pontides of Turkey. The Bulgarian part of the massif is composed of a metamorphic basement (various granite gneisses, paragneisses, and schists) traditionally interpreted as having Precambrian age, Triassic-Jurassic metasedimentary cover, and Upper Cretaceous volcanosedimentary sequences. The basement is intruded by large granitic plutons of Variscan age that are widespread mostly across Turkish territory. New LA-ICP-MS data support the suggestion of Variscan granitoid magmatism in the studied area but do not confirm the presence of Precambrian rocks. Furthermore, two stages of magmatism are determined in relation to the Variscan metamorphism and deformation. The first one (301.9 ± 1.1 Ma) is represented by strongly deformed metagranites and thus is interpreted as syntectonic, while the second one is relatively younger (293.5 ± 1.7 Ma) and postmetamorphic.

U-Pb zircon geochronology of intrusive rocks from an exotic block in the Late Cretaceous – Paleocene Taraklı Flysch (northern Turkey): Constraints on the tectonics of the Intrapontide suture zone

“…The late Neoproterozoic granitoids as well as the Paleozoic sedimentary sequence were deformed and metamorphosed during the Carboniferous and were subsequently intruded by syn-and post-tectonic Late Carboniferous and Permian granitoids. These intrusive rocks show a wide range of ages from 309 to 235 Ma similar to the plutonic rocks described in the Strandja massif, Istanbul area and central Pontides (Ustaömer et al, 2005;Sunal et al, 2006;Sahin et al, 2014;Machev et al, 2015).…”

The phanerozoic palaeotectonics of Turkey. Part I: an inventory

“…The oldest 'basement' seen in the Strandja Mountains in Turkey consists of Precambrian and Cambrian granitoids (consisting dominantly of granites with K-feldspar) unconformably covered by Ordovician to Permo-Carboniferous sedimentary rocks in places cut by late Carboniferous granitoids containing granites, granodiorites and tonalites. This complex is exposed in Bulgaria in the Sakar Mountains (Ivanov 1998;Gerdjikov 2005;Machev et al 2015) and the Srednogorie/Balkan sectors (see Velichkova et al 2004;Carrigan et al 2005). What had been called 'basement' in earlier studies (Aydın 1974(Aydın , 1982 Çağlayan and Yurtsever 1998) turned out to be intruded by Permian Kırklareli-type granites.…”

40Ar/39Ar age constraints for an early Alpine metamorphism of the Sakar unit, Sakar–Strandzha zone, Bulgaria

“…Stratigraphic and tectonic relationships within the Sakar-Strandzha zone nappe pile, that is, the N-wards thrusting of the Triassic metasedimentary rocks onto both the Middle Jurassic (Bathonian) metasedimentary rocks and unmetamorphosed Upper Cretaceous (Cenomanian) sedimentary rocks (Chatalov, 1988(Chatalov, , 1990, provided evidence for the Alpine tectonic and metamorphic history of the zone. U-Pb zircon dating of igneous and/or meta-igneous bodies and host metamorphic rocks in the Sakar-Strandzha zone revealed protracted Palaeozoic magmatic crystallization (Okay et al 2001;Sunal et al 2006;Natal'in et al 2012;Georgiev et al 2012;Machev et al 2015;Bonev et al 2019a), and also provided some information about early and late Palaeozoic metamorphic history (Okay et al 2001;Natal'in et al 2016;Bonev et al 2019a), which was overprinted by Alpine metamorphism. The latter, which occured during Bathonian-Cenomanian time according to the stratigraphy, is also indicated by the same temporal spread of the available dates of mid-to low-temperature radioisotopic systems (Fig.…”

“…U-Pb zircon dating in both the metagranites and the host high-grade basement gneisses, together with overlaping late Carboniferousearly Permian ages and common metamorphic fabric, was used as an argument for early Permian upper greenschist to lower amphibolite-facies metamorphism at c. 271 Ma linked to the Variscan orogeny (Okay et al 2001). Lower amphibolite-facies metamorphism at the Carboniferous-Permian boundary was shown as the most obvious from the textures, field and U-Pb zircon geochronologic data for the Sakar-Strandzha zone metagranites and the host basement gneisses (Sunal et al 2011;Machev et al 2015;Bonev et al 2019a). A pre-Middle Ordovician metamorphic event (c. < 484-450 Ma) was also inferred from the crystallization age of a magmatic body (Bonev et al 2019a) and the detrital zircons contained in the high-grade basement and cover metasedimentary rocks (Sunal et al 2008;Bonev et al 2019b), which might extend to the Neoproterozoic-Cambrian (Natal'in et al 2016).…”