Ahmad Jahangiri

1. Late Mesozoic (Jurassic-Early Cretaceous) northward movement of Africa and India towards the southwestern Cimmeria-Eurasia, initiated a subduction zone along the southern Iran margin. This subdu...

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.

Gabbro and diorite xenoliths occur within the alkaline trachyandesites in the northern part of Uromieh-Dokhtar magmatic belt (NW Iran). The host volcanic rocks are located on the upper Miocene sediments; therefore, they are probably Plio-Quaternary in age. The texture of xenoliths is granular, and the gabbro xenoliths are classified into two groups based on their petrographic characteristics and mineral chemistry. Based on chemical analyses, xenoliths show a subalkaline to slightly alkaline magmatic trend. Thermobarometric results indicate that gabbro and diorite xenoliths formed within the middle crustal level. All xenolith samples are light rare earth elements (LREEs) enriched and depleted in Ta, Zr, and Nb. These features can be explained by the subduction of the Neotethys. The LREE enrichment as well as negative Th, Ti, Nb, and Zr anomalies and positive Pb anomaly are in agreement with an origin through partial melting of an enriched mantle previously affected by metasomatism processes linked to mantle wedge-derived melts (fluids). Different REE concentration levels and patterns for Group 1 gabbro, Group 2 gabbro, and diorite xenoliths point out that their parental melts were not the same. Those forming the Group 1 gabbro xenoliths originated from melting of garnet mantle lherzolite, whereas the parental melts from Group 2 gabbro and diorite xenoliths originated from melting of spinel-garnet mantle lherzolite. The ⁸⁶Sr/⁸⁷Sr and ¹⁴³Nd/¹⁴⁴Nd ratios and chemical composition of minerals are different for host volcanic rocks and xenoliths; therefore, we may conclude that the parental melts of the different groups of xenoliths and host volcanic rocks differ in composition.