A geodynamic model of the Alpine magmatism in Bulgaria

Abstract

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.

Genesis of the Kremikovtsi deposit and metallogenic perspectives of the Sredec iron ore region

“…The Triassic volcanic activity described above is the earliest sign of Alpine magmatism in Bulgaria related to the embryonal rifting of the Moesian platform (Rneo, liaxneaa, 1980;Haqeo, 1980;Boyanov et al, 1989;Dabovski et al, 1991). The West Fore-Balkan rift zone is situated along the border of the Moesian platform with the West Balkan accreted block.…”

Kraishtides (Kraištiden) 70 years later: Myth or Reality?

“…The present knowledge about the Alpine igneous activity in SW Bulgaria gives no grounds to relate this activity to a special Kraishtide zone. Harkovska et al (1989Harkovska et al ( , 2003 and Dabovski et al (1991) demonstrated that both the Late Cretaceous and the Palaeogene igneous activities in the Morava-Rhodope zone Zagorchev et al (2006) are related to two large arcs roughly parallel to the boundaries of this zone. Zagorčev, Moorbath (1987) have demonstrated an increasing tendency in the values of the initial strontium isotopic ratio ( 87 Sr/ 86 Sr) 0 from the Srednogorie zone towards South related to the thinning of the crust with crustal necking and formation of magmas in the upper mantle of the Srednogorie, and upwelling of the melting isotherm in the Rhodope (Northern Pirin included) region with formation of anatectic granitoid magmas with crustal signature.…”

“…However, the latest Eocene -Early Oligocene ingression that formed the Padesh-Piyanets shallow gulf is clearly oblique to the "Kraishtides", penetrating from the Southwest (Tikvesh and Ovchepolje) to Northest. Observations over a wider region show that both the Palaeogene (Eocene -Oligocene) sedimentation and volcanism (Harkovska et al, 1989;Dabovski et al, 1991) were located within a large arc South of the Srednogorie zone, and related to the extensional collapse of the Srednogorie Late Cretaceous orogen, and mostly to the collapse of the "plateau" situated to the South (Morava-Rhodope zone - Dabovski et al, 2002).…”

Introduction to the geology of SW Bulgaria

“…Alluvial conglomerates with strongly varying thickness and fragment size have been formed followed by a lake (or even a shallow sea gulf) with periodic influx of coarser and finer-grained material (Souhostrel Formation). The erosion of the Palaeogene lateritic weathering crust and the intensification of the block movements in Late Eocene times Jed to the deposition of the polymictic conglomerates of the Komatinitsa Formation, and especially, to the coarse gravity-induced breccia ("olistostromes" - MocKoB-CKH, 1987, 1991 and fault-bound coarse conglomerate of the Logodash Formation interfingering with sandstone and shale in the deeper lacustrine parts of the Piyanets basin (3aropt.feB, ITonos, 1968). The active synsedimentary normal faulting occurred along the Lisiya fault zone, with a considerable subsidence and deposition of more than 700 m of sediments within the western block.…”