Compositional diversity of Eocene–Oligocene basaltic magmatism in the Eastern Rhodopes, SE Bulgaria: implications for genesis and tectonic setting.

Basaltic magmatism occurred only rarely within the extensive Eocene–Oligocene volcanic activity in the Eastern Rhodope Mts., SE Bulgaria. The earliest mafic volcanism started at ca. 34 Ma with K-rich trachybasalts strongly enriched in large ion lithophile elements (LILE), particularly Ba, Sr, Pb, Th, and light rare earth elements (REE) relative to the high field strength elements (HFSE). They have high 87Sr/86Sr ratios (0.70688–0.70756), low 144Nd/144Nd (0.51252–0.51243), and very high 207Pb/204Pb (15.74–15.76) and 208Pb/204Pb (39.07–39.14) at low 206Pb/204Pb (18.72–18.73) ratios, reflecting high degrees of crustal contamination. Shoshonitic basalts and absarokites and calc-alkaline and high-K calc-alkaline magmas, which erupted between 33 and 31 Ma, have decreasing Sr isotope initial ratios from west (0.70825) to east (0.70647) at approximately constant 143Nd/144Nd isotopic compositions (0.51252–0.51243) and slightly decreasing 207Pb/204Pb (15.66–15.72) and 208Pb/204Pb (38.80–38.96) and increasing 206Pb/204Pb (18.73–18.90) in comparison with the trachybasalts. All these rocks are characterized by negative Nb–Ti and Eu anomalies. They resulted from different degrees of partial melting of enriched asthenosphere, and the magmas were later contaminated by the Rhodopian crust. The end of the magmatic activity (28–26 Ma) was marked by emplacement of alkaline dykes, spatially associated with metamorphic core complexes. They are characterized by low 87Sr/86Sr (0.70323–0.70338), high 144Nd/144Nd (0.51290–0.51289), and 206Pb/204Pb (18.91–19.02) at lower 207Pb/204Pb (15.52–15.64) and 208Pb/204Pb (38.59–38.87) ratios, consistent with an origin from a source similar to OIB-like European Asthenospheric reservoir contaminated by depleted mantle lithosphere. The Eastern Rhodope Eo-Oligocene mafic magmatism formed as part of the prolonged extensional tectonics of the whole Rhodope region in Late Cretaceous–Paleogene time, similar to those in the U.S. Cordillera and Menderes Massif (Turkey). Initiation of extension is constrained by the formation of metamorphic core complexes, low-angle detachment faults, and supradetachment Maastrichtian–Paleocene sedimentary basins, intimately associated with 70–42 Ma granitoids and metamorphism which record mantle perturbation. The Eo-Oligocene stage started with block faulting, sedimentary basin formation, and extensive acid-intermediate and basic volcanism over the entire Eastern Rhodope area. The order of emplacement of the basalts from high-Ba trachybasalts through shoshonites, calc-alkaline and high-K calc-alkaline basalts, and finally to purely asthenospheric-derived alkaline basalts, with progressively decreasing amount of crustal component, reflects upwelling asthenospheric mantle. Most of the models proposed in the literature to explain extension and magmagenesis in the Rhodopes and the Mediterranean region cannot be applied directly. Critical evaluation of these models suggest that some form of convective removal of the lithosphere and mantle diapirism provide the most satisfactory explanation for the Paleogene structural, metamorphic, and magmatic evolution of the Rhodopes.