Gondwana-derived terranes and the Variscan orogeny in Bulgaria: Zircon geochronology and thermobarometry.

Abstract

New geochronology and thermobarometry for the Balkan, Sredna Gora, and Rhodope massifs of Bulgaria shed light on the Proterozoic-Paleozoic plate tectonic development of Eastern Europe. Volcanic and intrusive rocks from the Balkan Terrane island arc yield ages that verify its Cambro-Ordovician age. Magmatism began at ∼530 Ma and ended at least ∼490 Ma. Sedimentary rocks interlayered with the volcanics yield detrital ages of two distinct groups at ∼500--700 Ma and at ∼2.0--2.4 Ga. Samples of metamorphic basement orthogneiss in the Sredna Gora zone yield ages of 616.9 +/- 9.5 Ma and 595 +/- 23 Ma, indistinguishable from Cadomian basement ages in peri-Gondwanan terranes. Zircon from leucosomes in paragneiss consists of metamorphic rims surrounding detrital cores. The metamorphic rims yield an age of 336.5 +/- 5.4 Ma, indistinguishable from ages for Variscan metamorphism across Central Europe. These findings support the notion that the Sredna Gora was derived from Gondwana and became attached to Europe during Variscan orogenesis much like other peri-Gondwanan terranes. Pressure-temperature (P-T) conditions of these Variscan metamorphic rocks indicate peak conditions of ∼7--9 kbar and ∼620--640°C. Both the Balkan and Sredna Gora zones were intruded by granitoids during the late Variscan, with ages of 311.9 +/- 4.1 Ma, 304.6 +/- 4.0 Ma, 304.1 +/- 5.5 Ma, and 303.5 +/- 3.3 Ma for the first suite. The second suite consists of two-mica, leucocratic granites with ages of 312.0 +/- 5.4 Ma and 289.5 +/- 7.8 Ma, similar to the first suite. Eclogites in the Rhodope massif consist of at least two generations. The first yields a prograde path beginning in the epidote-amphibolite facies at ∼570°C and ∼7.5 kbar. Pressure increased dramatically compared to temperature as metamorphism progressed to the eclogite facies at ∼625°C and 12--14 kbar. The retrograde path is likely clockwise, as retrogression produced an assemblage in the amphibolite facies of hornblende, augite, and plagioclase. Kyanite eclogite had distinct different pressure-temperature evolution. Garnet rims contain inclusions of omphacite, quartz, and rutile, with sparse barroisite, epidote, and kyanite. Matrix kyanite also contains abundant inclusions of quartz, epidote, barroisite, omphacite, and rutile. The sparseness of amphibole and epidote inclusions in garnet makes their chemical evolution difficult to determine. Conditions of eclogite facies metamorphism are much higher in T and P at ∼800°C and 17--18 kbar. Retrogression occurred through the granulite facies based upon the presence of sapphirine in symplectites surrounding kyanite. Thermobarometry of this assemblage indicates P of ∼10--13 kbar at T greater than ∼700°C. The distinctiveness of the P-T paths suggests formation in different tectonic settings and that they have different ages.