Geochronology, geochemistry and isotope tracing of the Oligocene magmatism of the Buchim–Damjan–Borov Dol ore district: Implications for timing, duration and source of the magmatism

Abstract

Timing, source and magmatic evolution of the intrusions in the Buchim–Damjan–Borov Dol ore district of the Former Yugoslav Republic of Macedonia (F.Y.R.O.M.) have been studied. They intrude the Circum Rhodope Unit close to the contact with the Vardar Zone and are a part of the Late Eocene–Oligocene Macedonian Rhodope–North Aegean belt. The magmatism at Buchim–Damjan–Borov Dol occurred between 24.04±0.77 and 24.51±0.89Ma, as indicated by chemical-annealing (CA)–LA ICP-MS zircon dating. Major element, trace and rare earth element analyses have been performed on the various intrusive rocks. All ore bearing magmas were classified as trachyandesitic, except the youngest intrusion which is not associated with mineralization; the Black Hill locality (24.04±0.77Ma) shows a trachytic composition. The distribution of the trace elements, enrichment of large ion lithophile elements (LILE) and depletion in high field strength elements (HFSE), indicates subduction-related magmatism; most of the magmas follow a calc-alkaline fractionation trend with shoshonitic affinities; additionally, Sr/Y (10 to 90) and La/Yb values show some similarities to adakite-like magmas. Sr and Nd isotope ratios (Sri=0.70658 to 0.70740 and Ndi=0.512425–0.512497) show that the magmatic products were slightly contaminated by continental crust material, e.g., the Variscan/Cadomian basement. In the Late Eocene–Oligocene belt the magmatism between 29 and 35Ma is dominated by crustal melting with an increase in the mantle contribution between 20 and 27Ma. We suggest the following scenario for the magmatic history of the Buchim–Damjan–Borov Dol ore district: a slab rollback of an oceanic slab located further to the SW which led to extensional and compressional features in upper levels of the continental crust. In the middle to upper crust three consecutive crystallization stages occurred at variable depths as indicated by amphibole zonation. Mixing of newly formed crust with mantle-like affinities and continental crust material in variable degrees during the ascent of the magma can explain all geochemical characteristics. The magma crystallized as dykes or stocks near the Earth's surface (>1km) after its final emplacement and contemporaneous hydrothermal activity led to different mineralization styles depending on the lithology of the host rocks.