Metagranitoids from the eastern part of the Central Rhodopean Dome (Bulgaria): U-Pb, Rb-Sr and 40Ar/39Ar timing of emplacement and exhumation and isotope-geochemical features

Geochronological data (U-Pb, Rb-Sr and 40Ar/39Ar) are used to unravel the Late Alpine high-grade metamorphism, migmatization and exhumation of Variscan granitoids within the core of the Central Rhodopean dome, Bulgaria. The age of the granitoid protolith is 300 ± 11 Ma, as determined by U-Pb analyses on single zircons selected from the core of the dome structure. Rb-Sr whole rock data define an errorchron with a large scatter of the data points due to the Late Alpine metamorphic overprint. The slope of the reference line indicates a Variscan magmatic event. Strontium characteristics are used to discriminate the samples most influenced by metamorphism from those, which reflect possible differences in the protolith age of the granitoids. Petrological-geochemical data, the initial strontium ratio of 0.708 ± 0.001, and εHf zircon values ranging from -2.58 to -3.82 point to a mixed, but crust-dominated origin of the Variscan magmas; young crustal material and mantle fragments were sources for the I-type metagranitoids. The exhumation of the granitoids from depths greater than 20-25 km to about 5 km below the surface was a rapid geological process. lt started with the formation of granitic eutectic minimum melts at the temperature peak of metamorphism. Monazite crystallization at about 650 oC continued during isothermal decompression to possible depths of about 10-12 km. An age of 35.83 ± 0.40 Ma was determined using conventional U-Pb isotope methods on four multigrain monazite fractions. A maximum average age of 36.6-37.5 Ma (assuming same error uncertainties) for crystallization of the metamorphic monazites was calculated assuming 10 to 20 % monazite resetting during the subsequent Oligocene volcanism and hydrothermal activity in the region of the Central Rhodopean Dome. The rocks were then cooled to about 350-300 oC at 35.35 ± 0.22 Ma according to 40Ar/39Ar ages of biotites and below 300 oC at 35.31 ± 0.25 Ma (Rb-Sr data), as indicated by crystallization of adularia in an open vein subsequent to pegmatite intrusion. A minimum exhumation rate of 3-5 km per 1 million years can therefore be calculated for the exhumation of the metagranitoids during the period from 38-35 Ma. (author)