Adakitic magmatism in post-collisional setting: An example from the Early–Middle Eocene Magmatic Belt in Southern Bulgaria and Northern Greece

• We investigate a 250 × 200 km belt of Early–Middle Eocene normal and adakitic rocks.
• Adakitic signature is a result of combined amphibole and accessory fractionation.
• Deep break-off model could explain the large spatial distribution of the rocks.

Post-collisional (56.0–40.4 Ma) adakitic magmatism in the Rhodope Massif and the Kraishte region, including W. Srednogorie, in South Bulgaria followed the collision of the Rhodope and Pelagonian Massifs. It forms a 250 km NW trending belt which continues into the 1000 km long belt of Eocene magmatism in northern Turkey and Iran. The rocks are represented by felsic subvolcanic dykes and sills in the Kraishte and plutons in the Rhodopes. Here, we synthesize new chemical (whole-rock major and trace elements, and Sr and Nd isotopes) and LA–ICP/MS mineral and U–Pb zircon age data along with published similar data in order to constrain the genesis of this magmatism and the early Cenozoic geodynamic evolution of the central Balkan Peninsula. The rocks display typical subduction-related characteristics with enrichment in LILE and LREE and depletion in HFSE (Nb, Ta and Ti). In the Kraishte and western Srednogorie Zones these are calc-alkaline to high-K calc-alkaline rhyolites, displaying a typical adakitic signature, i.e. high La/Yb and Sr/Y ratios. The studied Rhodope Massif rocks are predominantly high-K calc-alkaline and subordinate calc-alkaline granites and granodiorites with a minor amount of tonalites. Petrographically, they are H2O- and accessory-rich (allanite, epidote, titanite, apatite) rocks, showing geochemical affinities from non-adakitic tonalites and mafic granodiorites to adakitic granodiorites and granites. Similarity of Sr and Nd isotopic compositions of the Kraishte subvolcanic and Rhodope intrusive adakitic rocks with the neighboring and coeval NW Anatolian basaltic to dacitic volcanics and plutons suggests that the most likely source for the South Bulgarian adakitic rocks is the subduction-enriched depleted lithospheric mantle. The nearby and contemporaneous East Serbian alkaline basalts are isotopically and compositionally different and, probably, originate from an OIB-like mantle source. Subsequent fractionation within an isotopically similar lower or middle crust in the Kraishte and interaction with the mid- to lower part of collision- and underplating-induced thickened crust in the Rhodopes can explain their isotopic variations. Transition from non-adakitic tonalites and granodiorites into adakitic granodiorites and granites in the Rhodopes was developed in response to amphibole fractionation accompanied by trace-element rich accessory minerals. Data from the literature show that the adakitic signature of the Early–Middle Eocene rocks disappeared in the following Late Eocene–Early Oligocene (35–26 Ma) magmatic episode. Our interpretation is that adakitic magmatism is related to a deep (> 250 km) slab break-off, followed by asthenospheric upwelling, heating, fast exhumation and formation of core complexes in the Rhodopes and Kraishte in the interval 42–35 Ma. The process was followed by thinning of the crust, orogenic collapse, steep faulting and extensional magmatism.