The territory of Bulgaria covers part of the active continental margin of the Eurasian plate. Several first-order tectonic units may be distmguished: Moesian platform, paraautochthonous margin of the platform, zone of Mid-Mesozoic collage units, Late Cretaceous island-arc system and a system of Cenozoic collisional and post-collisional grabens. The Alpine magmatism is genetically related to the evolution of the Eurasian margin and the Tethyan ocean south of it. The earliest Alpine magmatic activity is represented by Triassic, basic and intermediate, strongly altered volcanics (NW Bulgaria, in boreholes). They are related to initial, embryonal rifting of the Moesian platform. The Triassic and Jurassic-Lower Cretaceous small bodies of basic volcamcs in the SE collage units are interpreted as ensimatic. During the Upper Cretaceous an ensialic island-arc system originated. The related intensive magmatism formed the Srednogoric volcano-intrusive zone (SVIZ). The magmatic rocks are products of complex differentiation processes. Their formation was accompanied by deep-water sedimentation. All magmatic groups, according to SiO2 content, occur. By the K2O/SiO2 ratio they belong to the TH, CA, HKCA, SH, HKTR (high-K transitional) and BG (bulgaritic) series. Longitudinal and transversal zonahties may be traced. In a global aspect SVIZ is one of the most ancient segments of the Alpine-Himalayan volcano-intrusive belt. The bulgaritic petrochemical trend is related to the earliest generation of K-cnriched magmas in the Mediterranean region. The products of the collisional magmatism are exposed in the Macedonian-Rhodope-North Aegean volcanic zone (MRNAVZ) located south of the Late Cretaceous island arc. It originated during the Eocene-Oligocene (37-25 Ma) as a result of the collision between Eurasia and the Apulian promontory of Africa. The magmatic rocks belong dominantly to the intermediate and acid groups and to the CA, HKCA and SH series. The distribution of intermediate and acid rocks in this zone is controlled by the thickness of the crust. In the Eastern and Central Rhodopes the K-content increases from south to north. The volcanic activity in the zone occurred in conditions of intensive block orogeny and was accompanied by terrigenous molasse sedimentation. During the Neogene a zone of transversal faulting developed in the collisional orogen (remainding of Himalayan-type orogen) and parts of the Moesian platform. This zone is marked by small bodies of basic and ultrabasic, Na-alkalinc and subalkalinc rocks. The K-content increases in southward direction, toward the collisional front.
Mesta Paleogene volcanic massif (SW Bulgaria) is a complex synsedimentary structure. Its lava rocks form numerous endogenous domes and subvolcanic bodies that are components of two polygonal cauldrons and two linear volcano-tectonic zones. The related pyro- and epiclastics interager laterally with a 1500-2000 m thick terrigenous Upper Eocene-Oligocene section. The volcanics are silicic-rhyodacites and dacites (with very rare transitions to latites). According to Na2O+K2O/SiO2 they are transitional and normal, and according to K2O/SiO2 the rocks belong to the HKCA and SH series. Their structural anisotropy is defined by planar and linear flow structures of diverse morphology and petrography. Flow layers of different colour, texture and composition are widespread. Part of the structures (magmatic boudinage, magmatic folds and magmatic cleavage) resulted from synmagmatic deformations of the already layered but still not solidified melts. Magmatic folds are represented by mesoscopic folds and by crenulations on flow layers surfaces. Magmatic cleavage corresponds to parallel and convergent axial-plane cleavage, the cleavage planes being cemented during the cooling of the melts. Jointing in the magmatic bodies is controlled by their facies. Block, prismatic and platy jointing as well as various transitional types have been observed. A vertical zoning in the distribution of flow structures and jointing types was inferred. The air fall tuffs show parallel bedding while base surge and pyroclastics surge deposits are characterized by cross-bedding. The jointing in pyroclastics is a combination of regional jointing and jointing due to local syn- and post magmatic faulting.
St udia brevioraNet data about the comagmatic character of vole an ic rocks in the Central Rhodope Mountains (South Bulgaria and Northern Greece)Acid volcanics and vo!canoclastics occur in several small bodies in the area of the town of Pefki (Northern Greece) , about 10 km from the Greek-Bulgarian border (S k 1 avo uno s, K ass o I i -
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.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.