GEOLOGICA BALCANICA, 46 (2), Sofia, Nov. 2017, pp. 143–151. Analysis of the zircon population and Sr-isotope data of the Paleogene igneous rocks from Kyustendil area, SW Bulgaria Valentin Grozdev1, Irena Peytcheva1, 2, Albrecht von Quadt2, Rossitsa Vassileva1, Stoyan Georgiev1 1 Geological Institute, Bulgarian Academy of Sciences, Acad. G. Bonchev, Bl. 24, 1113 Sofia, Bulgaria; e-mail:
[email protected]. 2 Institute of Geochemistry and Petrology, ETH, 8092 Zurich, Switzerland. (Accepted in revised form: November 2017) Abstract. In the present study, we apply a combination of Sr-isotope whole-rock analyses and LA-ICP-MS U-Pb zircon dating to understand the magmatic evolution of the igneous rocks in the Ruen Zone, western Bulgaria, and bracket the time of their formation. This approach reveals a general interval of magmatic ac- tivity from 35.57 ± 0.27 Ма (Pishtilski volcanics) to 28.63 ± 0.58 Ма (cross-cutting dykes in the Osogovo Mountain). The volcanic rocks from the Prekolnitsa Graben are formed with significant (50%) crustal con- tamination/assimilation. Three major sources of crustal rocks can be outlined, with Triassic, Ordovician and Cambrian age, respectively. For the subvolcanic to plutonic magmatic rocks of the Osogovo Mountain, we suggest maturation and zircon crystallization over 2–4 Ma. The magma was generated dominantly in the con- tinental crust and evolved through mainly fractional crystallization (FC) in the upper crustal chamber, where- as the evolution of the Osogovo dykes requires additional involvement of AFC (Assimilation and Fractional Crystallization) and contamination processes. Grozdev, V., Peytcheva, I., von Quadt, A., Vassileva, R., Georgiev, S.