Geochemistry and tectonics of Cenozoic volcanism in the Lesser Caucasus (Azerbaijan) and the peri-Arabian region: collision-induced mantle dynamics and its magmatic fingerprint

Abstract

The Lesser Caucasus occurs in the hinterland of the Arabia–Eurasia collision zone in the broad Alpine–Himalayan orogenic belt and includes Cenozoic plutonic and volcanic sequences that provide important clues for collision-driven continental magmatism and mantle dynamics. Two main magmatic episodes (Eocene and late Miocene–Quaternary) formed the volcanic landscape and the igneous assemblages in the Lesser Caucasus of Azerbaijan. (1) The Eocene sequence consists of trachybasalt and basaltic trachyandesite with subordinate tephrite-basanite, basaltic andesite, and trachyandesite, showing shoshonitic and mildly alkaline compositions. The Miocene–Quaternary magmatic episode is represented by (2a) an early phase of upper Miocene–lower Pliocene andesite, trachyandesite, trachydacite, dacite and rhyolite lavas, and by (2b) a late phase of upper Pliocene–Quaternary trachybasalt, basaltic trachyandesite, basaltic andesite, trachyandesite, trachyte, and rhyolite flows. The rocks of the early phase have high-K calc-alkaline compositions, whereas those of the late phase show high-K shoshonitic compositions, defining an alkaline trend and a K2O-enriched melt source. All three volcanic associations show variant troughs in Nb, Ta, Hf, and Zr, strong enrichment in Rb, Ba, Th, La, and depletion in Ti, Yb, Y relative to mid-ocean ridge basalt N-(MORB) in their multi-element patterns. The enrichment of incompatible elements and K suggests derivation from a metasomatized mantle source, whereas the troughs in Nb and Ta indicate a subduction influence in the mantle melt sources. Mantle-derived magmas were modified by AFC/FC processes for all three volcanic sequences. These geochemical features are similar to those of coeval volcanic associations in the peri-Arabian region, and indicate the existence of subduction-metasomatized lithospheric mantle beneath the Lesser Caucasus during the Cenozoic. Partial melting of this subduction-modified subcontinental lithospheric mantle in the peri-Arabian region was triggered initially by slab breakoff following discrete continental collision events in the early Eocene. The heat source for the later Miocene–Quaternary volcanism in the entire peri-Arabian region was provided by asthenospheric upwelling, which itself was caused by delamination of the mantle lithosphere following the final Arabia–Eurasia collision at ∼13 Ma. Increased alkalinity of successively younger units in the Plio-Quaternary volcanic associations resulted from the input of enriched asthenospheric melt during the last stages of post-collisional magmatism. Active, crustal-scale and orogen-parallel, transtensional fault systems in the peri-Arabian region facilitated the formation of fissure eruptions and stratovolcanoes in the latest Cenozoic.