The Kraishte magmato-tectonic zone (Western Bulgaria) — a review

Abstract

The Kraishte magmato-tectonic zone (KMTZ) is the northernmost of the four Nw-SE trending Paleogene linear magmato-tectonic zones cropping out in Western Bulgaria and referred, as second order structures, to the collision-related first order Macedonian-Rhodope-North Aegean magmatic zone (MRNAMZ). The Kraishte volcanics (KV) are quite uniform dacites to rhyodacites of the calc-alkaline series. They were cooled at a shallow subvolcanic level and are affected irregularly by a low temperature hydrothermal alteration. Most of the KV bodies are conformable and concordant because their morphology is controlled mainly by the trust boundary between the Paleozoic rocks of the Penkyovtsi allochthonous unit and the Berriassian-Titonian flysch of the Luzhnitsa-Tran unit. The presence of KV-epiclastics in the Gorna Glogovitsa and Sekirna grabens shows (in agreement with the available magnetometric data) that at least some of the KV-bodies have been exhumed before the Priabonian-Oligocene (?) sedimentation started. The projections of all of the new whole-rock K-Ar ages (47.4-42.2 ± 1.60-1.80 Ma) fall within a Pre-Priabonian (Lutetian — Bartonian) part of the Gradstein, Ogg's (1996) geochronologic scale. The possible alternative suggestions about the age of the KV emplacement could be: a) the whole-rock ages reflect a long-term low temperature hydrothermal alteration of volcanic bodies, intruded before 47.4 Ma; b) the w. r. record is a complex combination of a polystage (?) Lutetian — Bartonian emplacement and of the related hydrothermal alteration processes, which caused a rejuvenation to a different extent. The existing model for the KMTZ geodynamic position has obviously to be changed taking into account that: a) KMTZ differs from the other Paleogene magmato-tectonic zones in Western Bulgaria by the steeper NW-SE trend (150-160°), the calc-alcaline affinity of the KV, the older age and the absence of a base metal metallogenic specialization, typical for the magmatics from the other zones; b) in a regional scale KMTZ acts as a southern end of the more than 250 km long Late Alpine (Late Cretaceous — Early Tertiary) subduction-related magmato-tectonic structure, following the erosional front of Mid Cretaceous thrusts and described in Eastern Serbia as Ridanj-Krepolin belt and further to the North (in SW Romania) — as “Western banatitic zone”; c) the available discrimination diagrams for KV are characteristic for their orogenic (s. 1.) setting and not for a well grounded collisional one. All these data can serve as a basis of the following suggestions: a) KMTZ does not belong to the collision-related MRNAMZ; b) the KMTZ acid (strongly differentiated?) magmas have been generated either during the late stages of the subductional processes responsible for the formation of the Late Cretaceous island-arc associations on the territory of the Balkan Peninsula, or in an environment transitional to a collisional one.