Alkaline magmas in zones of continental convergence: The Tezhsar volcano-intrusive ring complex, Armenia

Abstract

Alkaline igneous rocks are relatively rare in settings of tectonic convergence and little is known about their petrogenesis in these settings. This study aims to contribute to a better understanding of the formation of alkaline igneous rocks by an investigation of the Tezhsar volcano-intrusive alkaline ring complex (TAC) in the Armenian Lesser Caucasus, which is located between the converging Eurasian and Arabian plates. We present new petrological, geochemical and Sr-Nd isotope data for the TAC to constrain magma genesis and magma source characteristics. Moreover, we provide a new 40Ar/39Ar age of 41.0±0.5 Ma on amphibole from a nepheline syenite that is integrated into the regional context of ongoing regional convergence and widespread magmatism. The TAC is spatially concentric and measures ~10 km in diameter representing the relatively shallow plumbing system of a major stratovolcano juxtaposed by ring faulting with its extrusive products. The plutonic units comprise syenites and nepheline syenites, whereas the extrusive units are dominated by trachytic-phonolitic rocks. The characteristic feature of the TAC is the development of pseudomorphs after leucite in all types of the volcanic, subvolcanic and intrusive alkaline rocks.
40 Whole-rock major element data show a metaluminous (Alkalinity Index = 0-0.1), alkalic and silica-undersaturated (Feldspathoid Silica-Saturation Index <0) character of the TAC. The general trace element enrichment and strong fractionation of REEs (LaN/YbN up to 70) indicate a relatively enriched magma source and small degrees of partial melting. All TAC rocks show a negative Nb-Ta anomalies typical of subduction zone settings. The initial 87Sr/86Sr ratios (0.704-0.705) and positive eNd values (+3 to +5) indicate an isotopically depleted upper mantle and lack of significant crustal influence, which in turn suggests the TAC magma has formed via differentiation from lithospheric mantle melts. Regionally, the age of ~41 Ma places the 48 TAC amid a Lesser Caucasian Eocene period of dominantly calc-alkaline magmatism. The TAC’s arc-like geochemical signatures are interpreted to result from prior subduction of the Tethyan slab beneath the Eurasian continental margin. The alkaline character, distinct from regional trends, is attributed to Neotethyan slab rollback causing extension and inducing small degrees of decompression melting of metasomatised lithospheric mantle.