VOLUME 44 (1-3)/December 2015

Cenomanian sediments, and Albian/Cenomanian boundary strata, are not largely exposed in NW Bulgaria. Many of the boreholes that have penetrated them have poor core recovery for this stratigraphic interval, and this has greatly hampered investigations. Most of the previously published data are based on sporadic, often poorly preserved, macro- and microfossil finds and some biostratigraphic interpretations are outdated. Sometimes, age assumptions have been made only by means of lithologic comparisons and/or stratigraphic position, without fossil evidence. This paper presents the first calcareous nannofossil study on the available (limited and discontinuous) cores from the Upper Cretaceous of the Shishentsi-1 and Makresh-1 boreholes drilled in the Kula tectonic unit (Vidin District, NW Bulgaria). The Albian–Cenomanian sediments of the Rabisha Formation in both boreholes yielded relatively diverse, generally moderately well-preserved nannofloras; samples from higher stratigraphic levels, which have been assigned to the Kula Formation, proved to be barren. As a result, the presence of the uppermost Albian–lower Cenomanian subzone UC0c (in Shishentsi-1 and Makresh-1) and the lower Cenomanian UC2a (in Makresh-1) has been indicated. The consecutive bases of Gartnerago theta and Prediscosphaera cretacea were both noted within UC0c. The former event has been used to divide UC0c into uppermost Albian–lower Cenomanian and lower Cenomanian intervals. The presence of Gartnerago theta from 1529 m to 1555.15 m in the Shishentsi-1 borehole also suggests that the Albian/Cenomanian boundary lies stratigraphically lower than previously thought.

The present study provides data on the high-grade metamorphic basement of the Serbo-Macedonian Massif (SMM) that is exposed in Ograzhden and Belasitsa Mountains, SW Bulgaria. We combined the conventional ID-TIMS with the in-situ LA-ICP-MS U-Pb zircon dating and compiled them with petrological and geochemical studies to better constrain the age and origin of the protoliths and the grade of metamorphic overprint. The orthogneisses are dated in the range of 450–455 Ma, except for the Kolarovo metagranites, which define two clusters of age data at 554.8 ± 3.3 Ma and 510.2 ± 3.5 Ma. We assume sedimentary origin for the plagiogneisses that show zircon population and age distribution similar to the gneiss-schists: main peak at ~600 Ma and minor populations at 700–800 Ma, 1000–1100 Ma and 2400 Ma. The amphibolites yield the youngest precursor ages of 435–440 Ma. The whole succession of rocks is metamorphosed to upper amphibolite facies conditions, with peak temperatures (T_max) between 675–710 °C in Ograzhden Mountain and at ~650 °C in Belasitsa Mountain. Pressure estimates (P_min) range from 6–8 kbar to 12–13 kbar, based on 3T phengite relics and application of different calibrations of phengite barometer. However, higher pressures cannot be excluded due to evidence from the eclogitic metabasites. The metamorphism has been dated in zircon rims of metasediments, vein rocks and amphibolites, as well as in xenotime of newly formed leucosome, at ~330 Ma. The wide range of primary rocks, their major- and trace-element chemistry and Sr-Nd isotope signature, combined with the age data of zircon populations, give evidence for an old crustal fragment with Gondwana affinity that was rejuvenated in the Ordovician–Silurian, before the collision and metamorphic overprint during the Variscan orogeny.