Результаты рубидиево-стронциевых и калиево-аргоновых радиогеохронологических исследований метаморфических и магматических пород Южной Болгарии

I. Zagorcev, P. Lilov, S. Moorbath. Results of rubidium-strontium and potassium-argon radiogeochronological studies on metamorphic and igneous rocks in South Bulgaria. Rubidium-strontium whole-rock isochron isotopic studies complemented by potassium-argon studies, permitted the solution of some open problems of the geology of South Bulgaria (South of the Balkanides). Rocks of the Ograždenian Supergroup sufferred an intense Cadomian (550-530 Ma) deforma tional and metamorphic event which is parallelized with the greenschist metamorphism of the Diabase-phyllitoid complex, and with Struma diorite formation. Granitoid magmatism in the high-grade Precambrian basement is referred mainly to two tectonomagmatic cycles: Hercynian (340-240 Ma) and Alpine (Late Cretaceous and Paleogene). The first Hercynian granitoid complex of the South Bulgarian granitoids (340-320 Ma; initial ⁸⁷Sr/⁸⁶Sr ratio about 0.706) is probably related to magma of an initial upper mantle (or lower depleted crust) origination, and subsequent contamination with crustal material followed by differentiation and fractionation. These I-granites have been followed by two main phases of S-granites, their magma being formed by crustal anatexis. Late Cretaceous metamorphic and igneous processes deeply affected the whole Rhodope Massif and the Srednogorie Zone. The rocks of the Rhodopian Supergroup have been reworked by deformational and metamorphic events ca. 100 Ma ago which probably reached to anatexis at deeper levels still not exposed at the present erosion level. During the Early Senonian, magmas of primary upper mantle genesis have been extruded and intruded in the Srednogorie Zone; in the same time, the southern parts of the Rhodope Massif have been intruded by magmas of mixed primary origin, and the central and western parts of the massif - by granitoid magmas of crustal, anatectic origin. The new igneous activization in the Paleogene led to extrusion or intrusion of crustal, anatectic magmas, in the southernmost parts of the Rhodope Massif, of possible mixed origin.