VOLUME 20 (4)/August 1990

Ortho- and paraamphibolites in the northern part of the Central Rhodope Mts. are metamorphosed into amphibolite facies showing no traces of migmatization. This study is an analysis of the major and trace elements, REE included. Part of the orthoamphibolites represented by amphibolized metabasalts and garnet amphibolites are products of a subvolcanic autochthonous magmatism of tholeiitic affinities. Geochemically, they are comparable to the island-arc igneous rocks. The mrtabasalts show high alumina contents, varying iron and titanium contents and a tholeiite-like REE distribution. The garnet amphibolites are characterized by high iron and titanium contents. The predominating role of garnet in these rocks is reflected in the marked prevalence of HREE among REE and in the negative europium anomaly. Some bedded amphibolites of unknown origin show similar geochemical characteristics combining the features of island-arc and of ocean-floor tholeiites. The paraamphibolites differ sharply from the typical orthoamphibolites in their much lower iron, titanium and vanadium contents and higher calcium, aluminium, zirconium and LREE contents. Fe, Ti, Mg. V, Cr, Ni, Zr, Hf and REE are relatively immobile elements during metamorphism and their ratios can be reliably used to distinguish, characterize and determine the primary features of the amphibolite rock groups.

The reconstruction of the stress fields in Northeast Bulgaria by means of tectonic fracture patterns and physical anisotropy of rock species from the Lower Cretaceous period up to the Upper Pliocene shows that in the investigated area exists a stable trend towards a clockwise rotati on of the maximum and minimum compress ion axes. They are subhorizontal, the compression after the Lower Cretaceous period being in NE-SW direction, while after the Pliocene the compression is already in NW-SE direction. According to the mechanisms of seismic foci in the regi on this direction has been preserved till now. Rotation either of tectonic stress fields in aclockwise manner or the more probable counter-clockwise rotation of this block of the Moezian plate has been suggested.

Metasomatic facies in the zones of acid leaching of the Spahievo ore field are defined in terms of the theory of metasomatic zonality by paragenetic analyses of each zone. The integral metasomatic column derived identifies the metasomatites as belonging to the secondary quartzite formation. The propylitic type of rocks forming aureoles around the secondary quartzites proper do not belong to the propylite formation but are the outer zones of the secondary quartzite forrnation.
   The physico-chemical analysis of mineral parageneses in the secondary quartzites proper determines the equilibria of coexisting quartz, alunite, diaspore, dickite and sericite and of coexisting quartz, alunite, diaspore sericite and pyrophyl!ite as dependent on two of the intensive parameters, namely the chemical potentials of K+ and SO₄^-2 and pH of the mineral-forming environment.
   The physico-chemical conditions of formation of secondary quartzites in the Spahievo ore field are characterized using experimental and thermodynamic data reported in the literature.

No abstract is available for this article.