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[Display omitted] • A unique Oligocene dyke swarm is defined in the eastern Sakarya Zone, NE Turkey. • Oligocene magmatism sourced by partial melting of metasomatized lithospheric mantle. • Asthenospheric upwelling triggered by delamination of lower crust. • Dyke compositions record fractional crystallization and crustal assimilation. During the Cenozoic, the eastern Sakarya Zone, northeastern Turkey, has witnessed voluminous Eocene and subsidiary Miocene magmatism. By contrast, except in one previous study, Oligocene magmatism has not been reported. Here, we present new data about an Oligocene dyke swarm from the Artvin district in the easternmost part of the Sakarya Zone. Our study improves our understanding of the Oligocene magmatic and geodynamic evolution of the central Tethyan orogenic belt. The Oligocene dykes consist of two distinct rock types: quartz-bearing trachytic and adakitic trachyandesite/andesite, dated at 31.0 ± 0.2 Ma and 31.1 ± 0.2 Ma to 29.2 ± 0.3 Ma by U-Pb zircon geochronology, respectively. Geochemical analysis reveals enrichment in LREE and MREE for quartz-bearing trachytic dykes, whereas adakitic trachyandesite/andesite dykes show depletion in MREE and HREE. Radiogenic isotopic ratios (87Sr/86Sr (i) and 143Nd/144Nd (i)) and trace element data suggest that the source of Oligocene magmatism was linked to low degree (≈10–15 %) partial melting of a metasomatised lithospheric mantle previously enriched by slab-derived melts and fluids. During the magma ascent in the crust, parental magmas of the trachyandesite/andesite dykes have assimilated crustal rocks and undergone fractional crystallization dominated by amphibole. By contrast, quartz-bearing trachytic dykes have recorded plagioclase fractionation. Local extension, in conjunction with continuous foundering of delaminated thickened lower crust beneath the eastern Sakarya Zone, which likely triggered asthenospheric mantle upwelling and resulted in sporadic Oligocene magmatism. Our study highlights the distinct tectonic and magmatic setting of the rare Oligocene magmatism in the eastern Sakarya Zone compared to Oligocene magmatism in the Tauride-Anatolide and South Armenian blocks. [ABSTRACT FROM AUTHOR]

The South Armenian Block (SAB) belongs to the Lesser Caucasus, and it is located where the Central Tethyan belt bends from an EW orientation in Turkey to the NW-oriented Iranian tectonic zones. Magmatism in these belts is the result of convergence, collision and post-collision evolution of the Arabian plate with the Eurasian margin. An exhaustive study including whole-rock major, trace element, and radiogenic isotope (Sr, Nd, Pb) data, and zircon Hf isotopes, and U-Pb dating has been conducted in three areas of the SAB, including Tejsar, Amulsar and Bargushat. Combined with previous data, this study reconstructs the Cenozoic magmatic and geodynamic evolution of the Turkish-Lesser Caucasian-Iranian magmatic belt, with a particular focus on the SAB. The Cenozoic intrusions in the SAB are dominantly K-rich and characterized by an enrichment in LILE and depletion in Nb, Ta and Ti. They were emplaced in Tejsar and Bargushat between 43.0 and 37.7 Ma, in Amulsar between 35.9 and 32.1 Ma, predating or being contemporaneous with a 37.8 to 28.1 Ma shoshonitic pulse recorded in the southernmost SAB at the Meghri-Ordubad pluton. This K-rich magmatism is characterized by a juvenile mantle dominated composition, evidenced by εHf(i) zircon values higher than 8.1 and εNd(i) whole-rock values higher than 3.1. Modelling of the Nd and Sr isotope data shows that the mid-Eocene to early Oligocene K-rich magmatism of the SAB is the result of low degree partial melting of a metasomatized mantle source (SCLM), mixing with an upwelling asthenospheric mantle. Mid-Eocene-early Oligocene K-rich magmatism in the SAB and NW Iran coincided with a magmatic lull in Turkey at 40–20 Ma. While frontal Arabia-Turkish plate collision totally shut off magmatism, ongoing coeval magmatism in the SAB and NW-Iran is explained by strike-slip fault tectonics during oblique convergence of the Arabian plate along the Iranian and SAB segment. The dominant juvenile mantle signature of the Cenozoic SAB magmatism contrasts with more crustal-enriched sources in the adjacent NW Iranian and eastern Turkish Cenozoic magmatic rocks, revealing a mantle anomaly underneath the SAB. Our study concludes that the compressive to tranpressive tectonic regime in the SAB during Arabia-Eurasia collision resulted in delamination of a metasomatized SCLM, which interacted with a juvenile asthenospheric mantle upwelling underneath the SAB. Low degree partial melting of this mixed mantle reservoir resulted in the formation of the mid-Eocene to Oligocene K-rich magmatic rocks in the SAB. [ABSTRACT FROM AUTHOR]