Petrology and geochronology of the Vitosha volcano-plutonic edifice, Western Srednogorie, Bulgaria

The Vitosha volcano-plutonic edifice crops out in the western part of the Srednogorie structural zone. The plutonic body is composed of abyssal gabbros and anorthosites, hypoabyssal monzonites, syenites and late veins of granosyenitic composition, intruded in Late Cretaceous volcano-sedimentary sequence. Volcanic rocks are represented by basaltic andesites and andesites.
   The major rock-forming mineral phases are plagioclase, K-feldspar, amphibole and clinopyroxene. Common accessory minerals include apatite, titanite, magnetite, ilmenite and zircon. Secondary minerals are epidote, tourmaline, chlorite, actinolite, adularia and clay minerals.
   Plagioclases span much of the crystallization history throughout the magmatic series, generally decreasing in anorthite component from basic to acid plutonic varieties. In volcanic rock the plagioclase is in the bytownite–labradorite range.
   Potassium feldspar of orthoclase composition is typical for the monzonite and syenite. It forms large crystals, disposed between plagioclase. The orthoclase is the major carrier for Sr, Ba, Pb, Rb.
   Amphibole is the main mafic mineral in all rock types, with Mg# 58-97. In the classification diagram of Leake et al. (1997), the amphibole from the plutonic rock falls in the field of the magnesio hornblende and ferrohornblende, whereas the amphibole from the volcanic rocks is tschermakite.
   Clinopyroxene is a characteristic mineral for all rock types with Mg# 58-84. It forms deep resorption nuclei or single grains with euhedral contours. Compositionally it is augite and diopside.
   U-Pb single zircon method was used for the precise geochronological dating of the Vitosha volcano-plutonic edifice. Sr and Nd whole rock and Hf–zircon tracing have been used to clarify the origin of the studied rocks.
   Following U-Pb single zircon age data have been obtained for the plutonic rocks: gabbro 81.58±0.23, monzonite 82.45±0.4 and syenite 79.67±0.76. U-Pb data of single zircon grains from an andesite plot on a discordia with a Paleozoic age.
   Strontium isotope data are quite variable, ranging between 0.7044 and 0.7042 in the less evolved gabbro and andesite, through 0.7052 in the monzonite, to 0.7091 in the syenite. Nd (80 Ma) values also show a large variation, from 0.37 to 2.74.
   The new age data reported here provide that the rocks of the Vitosha pluton range between 82.4 and 79.7 Ma. Chemistry of the parental magma suggests similarity with the other plutonic suites from the axial part of the western Srednogorie. Compositional variations of the rock-forming minerals indicate calc-alkaline I-type signature for the Vitosha pluton. Sr and Nd isotope data indicate the presence of depleted mantle source for the parental magma, whereas generation of most evolved magmas requires different degree of crystal contamination. The upper discordia intercept U-Pb zircon of Paleozoic age suggest that the contaminant must have been lithologies from the Variscan basement.