Post-Orogenic Extension and Hydrothermal Ore Formation: High-Precision Geochronology of the Central Rhodopian Metamorphic Core Complex (Bulgaria-Greece)

Abstract

The late Alpine evolution of the Rhodope Massif in southern Bulgaria and northern Greece involved postcollisional extension, which generated detachment faults, syndeformational sedimentary basins, and exhumation of a large metamorphic core complex composed of gneisses and marbles: the Central Rhodopian dome. Closely associated with this complex, subvolcanic rhyolite dikes and extrusive rocks were emplaced, shortly followed by major swarms of epithermal to mesothermal Pb-Zn veins and carbonate replacement orebodies. High-precision geochronology using complementary Ar-Ar, Rb-Sr, and U-Pb dating methods resolves how this process of tectonic denudation from deep crustal metamorphism to near-surface epithermal ore formation occurred within a period of about 12 m.y. After an early Alpine phase of accretion, eclogite-facies metamorphism, and orogenic nappe stacking, the late Alpine postcollisional evolution of the Central Rhodopian dome started with the intrusion of granitic bodies at about 42 to 41 Ma, probably marking the beginning of extension and core complex formation. The early stages of extension were characterized by normal faulting, rotation of fault blocks, and thinning that caused cooling of the hanging wall through ~300°C at about 40 to 38 Ma, as dated by Rb-Sr and Ar-Ar geochronology of metamorphic biotite. The main extensional phase occurred between 38 and 36 Ma and led to horizontal displacements of tens of kilometers in the hanging wall. In the footwall, high metamorphic temperatures and decompression persisted and resulted in partial melting and the formation of migmatites at 37 Ma and vuggy pegmatites at about 36 Ma. Cooling of the footwall below ~300°C occurred between 36 and 34 Ma, followed by emplacement of undeformed rhyolite porphyry dikes and the extrusion of volcanic products deposited onto the surface-exposed center of the dome at about 33 to 30 Ma. The hydrothermal ores were formed ca. 30.5 Ma in the south and ca. 29.3 Ma in the northern part of the dome during the last major event of focused heating to 270° to 330°C of near-surface rocks by hydrothermal fluid advection. Ore formation and localized, later fluid processes caused disturbance and younging of some Rb-Sr ages in the footwall of the dome. Field and geochronologic constraints indicate that the formation of the Pb-Zn deposits (~31–29 Ma) is up to 2 m.y. younger than the local rhyolitic magmatism, which is volumetrically minor in the mineralized core complex. This contrasts with ore formation related to calc-alkaline magmatism in the Eastern Rhodopes, where polymetallic Cu-Au-Ag-Pb-Zn mineralization was found to be coeval with the latest phases of igneous activity (~32 Ma). The chemically simpler but considerably larger metamorphic-hosted Pb-Zn deposits of the Central Rhodopian dome were generated by large-scale hydrothermal fluid circulation, driven by the high heat flow attending core complex formation, exhumation, and final fracturing of a rapidly thinned crust.