U-Pb monazite and zircon ages of a granite xenolith and its host alkaline basalt from the Byala Reka Dome

The Oligocene Krumovgrad alkaline basalts and lamprophyre (KABL) intruded into the Kesebir-Kardamos and Byala Reka metamorphic domes carry various mantle and crustal xenoliths. This study presents the results from petrological and geochronological investigations of one granitic xenolith entrained in an alkaline (camptonite) dyke intruded into the Byala Reka dome, close to the village of Gorni Yurutsi (SE Bulgaria). The studied xenolith is granite formed at an estimated depth of 6–9 km. During the transport to the surface, it records some host-melt interaction with the penetrating melt, reflected mainly in the reaction with the biotite. In situ U/Pb dating of four zircons and one monazite from the xenolith yield different and slightly discordant to nearly concordant ages. One zircon reveals Ediacaran (606±19 Ma) ²⁰⁶Pb/²³⁸U age of the core and Permian (277±16 Ma) of the rim (all error uncertainties are 2σ). Two zircons at three analytical points overlap in their error uncertainties at 198±10 Ma and 207±8 Ma, and one zircon rim yields a concordant Jurassic age of 157±10 Ma. The age of this zircon should reflect the age of the crystallization of the granitic xenolith, whereas the older zircons are inherited and are typical for the metamorphic and magmatic rocks in the Rhodope Massif. The monazite crystal yields two almost concordant Chattian ages of 28.8±1.3 Ma and 27.6±1.8 Ma. They coincide with previous K/Ar dating of the alkaline basalts, thus confirming their Oligocene (Chattian) age.