AN EASTERN EXTENSION OF GONDWANA-DERIVED TERRANES AND THE VARISCAN OROGENY IN BULGARIA

The peri-Gondwanan terranes located along the formerly contiguous Appalachian-Tornquist margin of Laurussia are well documented from Carolina to Newfoundland and from Iberia to the Bohemian Massif. Farther east along the Baltica Craton, however, the histories of accreted terranes are poorly known, in part due to further obscuration by Meso-Cenozoic reworking. New HR-SIMS zircon U-Pb data for the Balkan and Sredna-Strandja massifs shed light on the evolution of this area and have implications for the Proterozoic-Early Paleozoic development of the Gondwanan margin. Basement rocks of the Balkan Terrane consist of a complete Late Proterozoic ophiolite sequence that is overlain and intruded by a Cambro-Ordovician island arc. Ashgillian glacial deposits testify to a Gondwanan heritage, and age patterns of detrital zircons suggest a paleogeographic location adjacent to West Africa in the early Paleozoic. The earliest island arc volcanic rocks are still poorly dated but are likely Early Cambrian (528 ± 17 Ma), whereas late magmatic rocks are well dated at 490.9 ± 6.1 Ma and 493.0 ± 6.6 Ma. An overlying olistostrome, derived endogenously, marks obduction during the Arenigian. In contrast to the intra-oceanic subduction throughout the Cambrian recorded in the Balkan Terrane, many peri-Gondwanan terranes record continental-arc subduction in the Late Proterozoic followed by erosion, sedimentation, and extension during the Cambrian. Subduction at the Gondwanan margin, therefore, appears to have shifted from a continental margin setting in the Late Proterozoic to an intra-oceanic setting in the Early Cambrian. South of the Balkan Terrane, the Sredna-Strandja basement primarily consists of high-grade metapsammites, metapelites, amphibolites, and eclogites. Rare orthogneiss yields ages of 616.9 ± 9.5 Ma and 595 ± 23 Ma, similar to many Cadomian magmatic rocks. Zircon rims from a migmatite leucosome yield an age of 336.5 ± 5.4 Ma, indistinguishable from ages of Variscan metamorphism across Central Europe. Both the Balkan and Sredna-Strandja terranes are intruded by undeformed ~315-290 Ma stitching plutons. These terranes, therefore, have numerous features that signify their Gondwanan affinity and accretion to Baltica during the Variscan Orogeny, and provide a new eastern extent of the Avalonian-Cadomian belt.