General features of the Paleogene orogenic magmatism in Bulgaria

This paper is a review of the available data on the Paleogene magmatism in Bulgaria. On the Balkan Peninsula its products (mainly volcanics) form the arcuate Maccdonian-Rhodope-North Aegean volcanic zone (MRNAVZ). The Bulgarian section has been divided into volcanic areas (Struma – SVA, Central Rhodope – CRVA and East Rhodope –ERVA) which have been further subdivided into volcanic regions. The volcanic activity began during the Late Eocene (Priabonian) and continued during the Early Oligocene (Rupelian). The volcanic rocks associate with contemporaneous, mainly terrigenous sediments accumulated in intramountain grabens of different form and dimensions under condition of intensive block orogenesis (morphogenesis). Sedimentation and volcanism developed together with the block-faulting and gravity thrusting. A number of calderas formed during the volcanic activity.
   Across the section of sedimentary and sedimentary-volcanogenic complexes up to 2000-2500 m thick the lithological bodies are in lateral and superpositional relations.
   The magmatic rocks belong to the Ca-alkaline and transitional (shoshonitic) series. Varieties of the acid group (dacites and trachydacites to rhyolites and trachyrhyolites) with magma chambers at a depth of 14-4 km dominate. The intermediate rocks (basaltic andesites, andesites, latites, shoshonites, trachytes) are in equal amount to the acid rocks only in ERVA where basic rocks (basalts and absarokites) also occur. The depth of their magma chambers is 39-25 km. The available data on the major element composition of the magmatic rocks are discussed in view of their origin (crustal and mantle) and petrochemical zoning.
   The volcanism is polyphase, in some regions with manifestation of hydrovolcanism in the earliest phases. The intermediate (to basic) volcanics and more evolved members of the series form edifices of stratovolcanoe and shield type and subvolcanic bodies. The acid volcanics form polygenous and polyfacial, polychannel central structures, dome-fields, ignimbrite sheets, subvolcanic and hypabyssal bodies, dyke swarms. The rare and small epizonal plutonic bodies have granitic or differentiated (gabro-granodioritic) composition.
   The magma conduits of the Paleogene volcanism form a structural arc which follows the outlines of the arcs of Neogene and Quaternary volcanics in the Aegean region. It is assumed that ENE magma generating structures are also present in ERVA.
   Two-types hydrothermal activity is manifested in MRNAVZ. The early, fumarole-solphataric type took place in subaerial and subaqual conditions. Zeolitites, bentonites, etc. were formed. The later, medium to low temperature (360-60°C) hydrothermal processes occurred in hypabyssal and subvolcanic conditions. Many economic important Pb-Zn deposits are related to them.
   In geodynamic respect the Paleogene magmatism is interpreted as collisional. A two-stage collisional model is proposed on the basis of data from the regional tectonics of Bulgaria.