Original paper

Petrogenesis of the Eocene post-collisional alkaline volcanism in Meshginshahr, NW Iran

Zamani, Reza; Emami, Mohammad Hashem; Abedini, Mansour Vosoughi; Somarin, Alireza Karimzadeh

Abstract

The Eocene alkaline volcanic rocks exposed in NE Meshginshahr (NW Iran) are part of the Alborz-Azerbaijan magmatic belt and comprise an analcime-bearing (mafic series) and an analcime-free series (felsic series). The mafic series consists of tephriphonolithe, phonotephrite and basaltic trachyandesite with porphyritic and rare vitrophyric textures of feldspar, clinopyroxene and analcime phenocrysts set in a microlithic groundmass of feldspar, rare clinopyroxene, analcime and volcanic glass. The rocks of the felsic series are trachyandesites composed of feldspar and amphibole phenocrysts set in a microlithic groundmass of plagioclase, alkaline feldspar and glass. In Harker-type diagrams, major elements such as MgO, TiO2, FeOt, CaO and Na2O display negative correlations with increasing SiO2, whereas K2O and Al2O3 show a positive correlation. A compositional gap or bimodality between the two series exists. Incompatible elements (LILE, LREE and Th and U) are enriched compared to primitive mantle but show negative Ti, Nb, Ta and Zr anomalies. The chondrite-normalized REE pattern indicates that all rock types are strongly enriched in LREE relative to HREE. The petrogenetic studies reveal that, concomitantly with subduction of Neo-Tethys crust beneath the Iranian plate in the south, a north-dipping subduction zone was active in the north that closed the Sevan-Akera-Qaradagh Ocean. Subduction was followed by continent-continent collision and welding of the north western part of central Iran to the Lesser Caucasus in the Late Cretaceous-Early Paleocene. These events were followed by the establishing of an extensional environment in a post-collisional setting following hot asthenosphere upwelling which caused partial melting in the lithospheric mantle. The magmas formed during this event evolved and were possibly contaminated during their passage through the thick Iranian crust, thus producing various types of alkaline rocks in this magmatic belt.

Keywords

iranalkaline volcanic rockspost-collisionmeshginshahr