Geochemistry and Petrogenesis of Shoshonitic Dyke Swarm in the Northeast of Meshkinshahr, NW Iran

Abstract

The study area is located in Ardabil province in the northeast of Meshkinshahr city. More than 200 small and large Eocene-age dykes form outcrops in this area. Laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) U–Pb zircon analyses yield a consistent age of 44.3 ± 1.8 Ma for the dyke swarms. These dykes include tephritic, andesitic and basaltic compositions, and show enrichment in LREEs (relative to HREEs) and are characterized by enrichment in LILEs and depletion in HFSEs. Petrological observations, along with major, rare earth and trace elements geochemistry, suggest that the dykes have a shoshonitic signature. All the rocks are highly enriched in incompatible trace elements and have variable Sr–Nd isotopes. Enrichment in incompatible elements and other geochemical features for the dyke swarm rocks suggest that a metasomatized subcontinental lithospheric mantle is the magma source. The negative Nb–Ta–Ti anomalies in the rocks are comparable with the features of subduction-related magmatism and contamination with ancient crustal components. The radiogenic 87Sr/86Sr isotopic values of the rocks imply the involvement of slab terrigenous sediments and/or a continental lithosphere. Isotopically, the volcanic rocks exhibit a binary trend, representing 1–5% mixing between the primary mantle and sediment melts. Our melting models suggest that there are residual garnet + spinel in the source, which are incompatible with the partial melting of amphibole- and/or phlogopite-bearing lherzolites. The geochronological, geochemical and isotopic data for the northeast Meshkinshahr dyke swarms suggest that these Late Eocene magmas were derived from a small degree of partial melting of a subduction-metasomatized lithospheric mantle source in a post-collisional setting.