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.

Here we present the results from several studies confirming the existence of Variscan metamorphic and deformational events in the Sakar Unit of the Strandja Massif. Structural observations and new U-Pb geochronological data from the Mladinovo Complex confirm the existence of migmatisation and high-temperature shearing at 327 ± 2 Ma. The new geochronological data indicate similarities with basement of the Sredna Gora Zone, and it is interpreted that they were both part of the Variscan orogenic belt.

The Puduhush gabbro is located on the western margin of the proto-Kalahari Craton in Southern Africa. This gabbro intrudes the Volop Formation, which conformably overlies the Hartley Formation lava of the late Palaeoproterozoic Olifantshoek Group. Here we report a new U-Pb ID-TIMS baddeleyite age as well as petrographic, whole-rock geochemical and palaeomagnetic results for the Puduhush gabbro. The gabbro shows a well-preserved sub-ophitic texture between clinopyroxene and plagioclase, with minor amounts of amphibole, olivine, biotite and Fe-Ti oxides. The new U-Pb ID-TIMS baddeleyite age of 1 881 ± 1 Ma reported here for the Puduhush gabbro, together with existing ages for the Hartley Formation, define a ca.1 916 to 1 881 Ma age bracket for the Volop Formation. Our 1 881 ± 1 Ma age is also within error of ages reported for the oldest episode (so-called Episode 1) of the ca.1.89 to 1.83 Ga magmatism in the eastern and northern parts of the proto-Kalahari Craton. Our geochemical results also suggest compositional similarities between the Puduhush gabbro and Episode 1 magmatism, particularly the post-Waterberg sills. The virtual geomagnetic pole calculated here for the Puduhush gabbro (VGP: 1.6°N; 352.0°E; A₉₅ = 14.2°) is consistent with the Episode 1 pole. All data are therefore combined to produce a new palaeomagnetic pole (11.7°N; 8.8°E, A₉₅ = 9.3°) for Episode 1 magmatism. The present study provides the first evidence that the ca.1.89 to 1.83 Ga magmatism had a wider footprint that previously thought, extending to the western margin of the proto-Kalahari Craton. This wide-scale magmatism, previously proposed to be related to a back-arc extension setting, is here reinterpreted in the context of a mantle plume. Our results are consistent with the lithostratigraphic-based notion that at least parts of the red-bed successions (i.e., Olifantshoek and Waterberg Groups) that are hosts to the ca.1.89 to 1.83 Ga magmatism could be correlative units, representing an extensive sedimentary sequence that once covered large expanses of the proto-Kalahari Craton.