Abstract
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.
Highlights
The Balkan Peninsula consists of a series of variably metamorphosed terranes that predominantly formed in the Phanerozoic due to the collision of Gondwana against Baltica, Laurussia, and later proto-Africa against proto-Europe during the Caledonian, Variscan, and Alpine orogeny [1]
We provide the first U–Pb LA-ICP-MS dates from apatite and titanite in one vein type, documenting two episodes of hydrothermal activity within the Sakar Unit of the Strandja Massif, which critically escaped significant
The U–Pb ages in this study are interpreted as distinct crystallization episodes, at 149 ± 7 Ma on apatite and 114 ± 1 Ma on titanite, respectively
Summary
The Balkan Peninsula consists of a series of variably metamorphosed terranes that predominantly formed in the Phanerozoic due to the collision of Gondwana against Baltica, Laurussia, and later proto-Africa against proto-Europe during the Caledonian, Variscan, and Alpine orogeny [1]. Southern Bulgaria is divided into the Rhodope and Strandja massifs, with the relationship between them remaining uncertain [2]. These massifs are composed of highly deformed orthogneisses and paragneisses. Cimmerian-aged events preceded the main collision of Africa and Europe during the Alpine Orogeny [4]. The enigmatic Cimmerian Orogeny in Southeastern Europe, is difficult to identify mostly due to the pervasive later Alpine orogenic overprint [8].
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