VOLUME 17 (5)/October 1987

Z. Cherneva, E. Stancheva, L. Tadjer. Geochemistry of migmatized orthtogneisses from the northern slope of the Shirokta Laka River in Central Rltodope Mls. Major and trace elements (Sr, Ba, Rb, Li, Pb, Cu, Zn, Co, Ni, Cr, V, Mn, Ti, Zr) in migmatized gneisses from the deepest levels of the Central Rhodope metamorphic complex are studied. Different petrographic varieties of gneisses (hornblende-biotite, biotite, two-mica and aplitoide) as well as the present migmatic leucosome are considered.
   Two geochemical trends are distinguished as a result of two different processes: 1 – granite-magma differentiation; and 2 - migmatization. The first one reveals in the range: hornblende-biotite gneisses - biotite gneisses - two-mica gneisses - aplitoide gneisses. The geochemical characteristic of this succession is similar to those of genetically related granitoides obtained through magmatic differentiation. Within the range established the rocks composition changes as follows: SiO₂ % (62.25–70.06–72.86–75.60), K₂O% (3.68–4.10–4.57–4.89), Al₂O₃% (15.99–14.77–13.96–13.92), Fe₂O₃% (5.28–2.56–1.78–0.56), CaO% (4.02–2.05–0.90–0.56), Na₂O% (3.33–3.73–3.62–3.66); the normative mineral proportions (Or-Ab-An-Q) approach the cotectic composition of the granite system: the contents of Ti, Mn, V, Zn, Co and Ni decrease: and Rb shows a considerable concentration (Rb: 137-193-240-320 ppm) regarding to Ba (1272–780–670–51 ppm), Sr (656–348–184–49 ppm) and K.
   On this geochemical basis migmatizalion develops and causes formation of leucosome, consisting mostly of alkaline feldspars and quartz. The leucosome is enriched in K₂O (up to 10-11%) and Ba (2000-3000 ppm) which is concentrated towards Rb. The formation of leucosome is explained with the redistribution of elements during migmatization (possibly partial melting) mainly depending on alkaline feldspar content and composition in the parent al orthogneisses.