Middle Jurassic to Cenozoic evolution of arc magmatism during Neotethys subduction and arc-continent collision in the Kapan Zone, southern Armenia

Abstract

The Kapan Zone in southern Armenia is a Middle Jurassic to Upper Cretaceous island-arc domain at the southern margin of Eurasia that collided with the Gondwana-derived South-Armenian Block in the Upper Cretaceous–Paleocene. Igneous rocks of Middle Jurassic to Quaternary age record the geodynamic evolution of the Kapan Zone, whose tectonic setting changed from an active arc environment, related to the northeastwards subduction of the Neotethys, to a post-collisional setting. U–Pb zircon ages of intrusive rocks from the Kapan Zone confirm the division of the stratigraphic column into the Middle Jurassic, Upper Jurassic–Lower Cretaceous and Paleogene magmatic complexes. Middle Jurassic tonalite was sampled from pebble dykes and dated at 165.6±1.4Ma, voluminous plutons intruded in the Kapan Zone between 137.7±1.6Ma and 131.5±2.1Ma within a time span of less than 10Ma and Paleogene gabbro was dated at 50.82±0.51Ma. Volcanic rocks in the Kapan Zone range from basaltic to rhyolitic composition with andesite being the most common rock type. Intrusive and volcanic rocks from the three magmatic complexes and Quaternary basanite have subduction-related signatures. They are enriched in fluid-mobile LILE such as K, Rb and Ba and are depleted in HFSE such as Zr, Nb and Ta, which suggest their derivation from a metasomatized mantle source as old crustal material is virtually absent in the Kapan Zone. The subduction-related signature is conserved in post-collisional rocks from the Paleogene magmatic complex and the Quaternary basanite, indicating their origin from a mantle source which was metasomatized during preceding subduction. REE patterns from igneous rocks of the Kapan Zone are characterized by increased slope in younger rocks. Flat patterns in Middle Jurassic rocks indicate melting from a depleted mantle reservoir whereas REE patterns with increased slope in rocks from the Upper Jurassic–Lower Cretaceous magmatic complex, the Paleogene magmatic complex and the Quaternary basanite suggest either smaller amounts of partial melting and/or input of less depleted mantle material in the source region. The initial isotopic signature of rocks from the Kapan Zone supports melting of a primitive mantle source with limited contribution of assimilated crustal material. The more crustal isotopic signature of rocks from the Middle Jurassic and Paleogene magmatic complexes relative to rocks from the Upper Jurassic–Lower Cretaceous magmatic complex indicates subduction of continent-derived sedimentary material and interaction with old basement units at depth. Reported coeval subduction-related Middle Jurassic to Cretaceous intrusive and volcanic rocks from the Sanandaj–Sirjan Zone in Iran are in good agreement with our results from the Kapan Zone in southern Armenia. This suggests a continuation of the magmatic arc of the Lesser Caucasus to the southwest of the Kapan Zone, indicating continuous arc magmatic activity along the Eastern Pontides, the Lesser Caucasus and the Sanandaj–Sirjan Zone.