Abstract
The Skrut granitoids crop out in the western part of Belasitsa Mountain in SW Bulgaria. They are intruded in biotite gneisses and mica schists, building up together a complex nappe structure in the frame of the Serbo-Macedonian Massif (SMM). Main rock forming minerals in the Skrut granitoids are plagioclase, potassium feldspar, quartz, and biotite and accessories are apatite, allanite, epidote, zircon, titanite and garnet. These granitoids have undergone processes of dynamic metamorphism and non-uniform hydrothermal alteration. In respect to K2O content they belong to the high potassium calc-alkaline magmatic series while according to the ASI coefficient they are related to the weakly per-aluminium granitoids. The magma is enriched in water and REE. Probably, its crystallization has proceeded at pressure of 11 to 11.5 kbar (presence of magmatic epidote) and temperature of 800 to 600°C. The positive ε-Hf values of zircon crystals (from +0.4 to +3.5) propose mixed crust-mantle magma source whereas calc-alkaline characteristics and discrimination diagrams propose a volcanic-arc/continental margin or postcollisional environment of the magma generation. The age of the granitoids is determined by conventional U-Pb method on zircons as 248.85±0.70 Ma. The regression line obtained by the Rb–Sr isotope analyses on whole rock samples yield an age of 167.3±8.1 Ma, which is interpreted as a mixing line, probably influenced also by overprinted events. The new data suggest formation of the Skrut granitoids during the Early Triassic as a result of Late Variscan event in extensional conditions and block dismembering (rifting?). The post-metamorphic intrusion of the granitoids defines the high grade metamorphism in Belasitsa Mountain as pre-Triassic in age.
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