Two-Stage Late Jurassic to Early Cretaceous Hydrothermal Activity in the Sakar Unit of Southeastern Bulgaria

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.