Arabia-Eurasia convergence and collision control on Cenozoic juvenile K-rich magmatism in the South Armenian block, Lesser Caucasus

Abstract

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.