VOLUME 20 (1)/February 1990

The present paper reports for the first time the presence of representatives of the ammonite subfamily Venezuellinae Kvantaliani in the Mediterranean area (in particular Bulgaria). Until now they were considered as endemic and were known only from Aptian sediments in South America (Venezuela). Species of g. g. Venezuella and Renziella were found in three sections on the territory of North Bulgaria. The lithology and the ammonite sequences of the sections are described. The representatives of g. Venezuella and Renziella associate with characteristic, including also zonal Aptian ammonites from A. nolani and H. jacobi zones. The concept of the Aptian Age of the ammonites of this group is advocated. The phyllogenetic concept for the origin and evolution of the subfamily is rejected. Five species of g. Venezuella and 1 species of g. Renziella are described and figured. Three of them – V. bulgarensis sp. n., V. sumerensis sp. n., V. gracilis sp. n., are new for the science.

D. Yosifov, B. Maneva, A. Tsvetkov, D. Tsvetkova, V. Pchelarov. Geotectonic position and structure of Spahievo ore field. Based on analysis of the physical parameters of rocks and the distribution of geophysical fields, many new data on the structure and geotectonic position of Spahievo ore field were obtained. A complex of geological and geophysical information was interpreted using physicogeological modelling. The results show that the tectonic position of the ore field is controlled mainly by the deep-seated Ardino-Spahievo and Novakovo-Pilaševo fault zones and the Momcilgrad-Spahievoregional fault. The tectonic knot, formed at their intersection, is a source structure marked by concentration of intensive magnetic anomalies of different sign. A large part of these anomalies are related to abundant comagmatic bodies both exposed on the surface (Sârnica, Mezarlâksârt, Pilaševo, Karaman) and cryptointrusive (Garno Bryastovo, Susam, Ramadan). Their magnetic properties enable to evaluate the depth, form and sometimes the approximate composition of the bodies.
   The area studied shows a block-disintegrated and faulted pre-Paleogene basement, dipping westsouthwest towards Borovica Depression. Depending on the distribution of the geophysical fields, the structural, magmatic and metallogenic features, three large mesoblocks may be divided: Borovica (I), Pilaševo (II) and Sârnica (Ill). Each mesoblock contains characteristic mineralization – rare-metal, gold-polymetallic and polymetallic, respectively. The 1ead-zinc ore mineralizations are related to an uplifted block of the basement, located in the central part of the eastern segment and formed by the intersection of Ardino-Spahievo and Novakovo-Pilaševo deep-seated fault zones. The knot areas of the regional deep zones control the origin of polyformational mineralization.

Potassic feldspars from the Central Rhodope amphibolite-facies metamorphic rocks between the Rivers of Vâča and Čepelarska have been studied. They comprise orthoclases, intermediate microclines and maximum microclines distributed regularly across the stratigraphic section, as follows: I) orthoclases dominate the metamorphic rocks in the lower part of the section (the Široka Lâka, Varbovo, Ljaskovo and Posestrimo Formations) affected by migmatization processes; 2) orthoclases grading into intermediate microclines prevail in the incumbent Dobralâk Formation, and 3) intermediate and maximum microclines characterize the Bačkovo Formation higher up the section. The potassic feldspar structural changes from the lower to the upper stratigraphic levels with ordering increasing even among the monoclinic feldspars reflect primarily the temperature variations across the section at the time of formation or during later periods of thermal action. The temperature variations derived from the study of potassic feldspar structural states are compatible with estimates based on the garnet-biotite geothermometer: 800-600°C for the metamorphic formations affected by migmatization; 550-500°C for the Dobralâk Formation; and 450-400°C for the Bačkovo Formation.
   The evidence gained in this study on the distribution of Ba, Sr and Rb both in the potassic feldspars and in the host metamorphic rocks reveals it as reflecting largely the primary premetamorphic relations between these elements. The Bačkovo Formation leptitoid gneisses and the constituent potassic feldspars show a well defined fractionation of Rb against Ba and Sr similar to that in acid magmatic differentiates, thus suggesting a magmatic origin.