Cambrian magmatism, Variscan high-grade metamorphism and imposed greenschist facies shearing in the Central Sredna Gora basement units (Bulgaria)

Gneisses from the deep structural levels of the European Variscan Belt are well exposed in the Central Sredna Gora in Bulgaria. In general, migmatites predominate, but unmigmatized domains (or domains with incipient migmatization) are also documented in this area. This paper presents new structural, petrographic and U-Pb isotope geochronological data from such an unmigmatized part of the Variscan high-grade metamorphic basement (the Koprivshtitsa Unit). A predominant part of this unit represents an alternation of metagranitoids and metabasites. The protolith crystallization age of the metagranitoids is constrained at 491.5 +/- 7.6 Ma by U-Pb laser ablation method on zircons. This age coincides with the previously available Late Cambrian protolith ages of metabasic rocks that crop out within the adjacent migmatitic unit. The Korpivshtitsa Unit comprises also lesser orthogneisses with Late Neoproterozoic protoliths. Based on the available local and regional paleogeographic reconstruction schemes, we suggest that the Upper Cambrian magmatic rocks intruded Upper Neoproterozoic crust during the initial opening stages of the Rheic Ocean or a related basin. Subsequently, both were involved in the Variscan high-grade deformation. The contact of the Koprivshitsa Unit with the migmatitic part of the metamorphic complex coincides with a north-vergent greenschist facies thrust zone - the Chuminska Shear Zone. The exact time of the shearing is still not well constrained but it clearly postdates the Variscan high-temperature metamorphism of the gneisses.