An Early Cretaceous thermal event in the Sakar Unit (Strandja Zone, SE Bulgaria/NW Turkey) revealed based on U Pb rutile geochronology and Zr-in-rutile thermometry

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[sbnd]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[sbnd]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.