40Ar/39Ar age constraints on the timing of Tertiary crustal extension and its temporal relation to ore-forming and magmatic processes in the Eastern Rhodope Massif, Bulgaria

Abstract We quantify the timing of the Tertiary crustal extension in the eastern Rhodope Massif of south Bulgaria using 40Аr/39Аr geochronology to constrain the temporal evolution of exhumation of the metamorphic domes. 40Аr/39Аr analyses of mineral phases with different closure temperatures extracted from metamorphic rocks collected in the footwall and the hanging wall of the extensional system reveal the low-temperature cooling history towards shallow crustal levels. The results reveal that subsequent to regional amphibolite facies metamorphism (i) the hanging wall gradually cooled between 500 and 300 °C during the Paleocene to Late Eocene (64–34 Ma), starting at a low rate of 25 °C/Ma and increasing to 50–67 °C/Ma during 38–34 Ma, (ii) cooling below 400–300 °C of the footwall beneath the extensional detachments occurred in the Middle–Late Eocene (39–35.5 Ma) at an average rate of 35 °C/Ma. 40Аr/39Аr data from the metamorphic basement has been combined with 40Аr/39Аr data from the volcanic and hydrothermal rocks in the vicinity to the extensional domes, to determine the temporal relationships between extensional tectonics, ore-forming and magmatic processes in the region. Extension of the high-grade basement, the ore-formation and volcanism overlaps within a 5 Ma lasting time interval, especially during the cooling and exhumation of the footwall. The eastern Rhodope Massif records an early stage of Eocene extension within the Aegean extensional province, where distinct crustal-scale processes occurred simultaneously implying the presence of cause and effect processes, and thus represents a key example of coeval continental extension, magmatism and hydrothermal activity in the late-stage evolution of the orogens.