Petrogenesis of the Cenozoic magmatism in north-western Iran

A petrological study has been conducted on three volcanic districts (Tafresh, Nowbaran and Bijar-Qorveh) of NW Iran with the purpose to give a contribution to the scientific community about the tectono-magmatic framework of the Arabia-Eurasia collision zone. These volcanoes belong to the so-called Urumieh-Dokthar Magmatic Arc (UDMA) running from NW to SE along the western margin of Iran, whose volcanism is related to the NE-ward Neotethys Ocean subduction beneath the Iranian plate since Early Cretaceous time, evolved into Arabia-Iran continental collision during early Cenozoic. Tafresh investigated rocks mainly range from basaltic andesites to rhyolites and are probably linked to closed-system magma evolutionary processes. Such processes involved fractionation of i) mainly ferromagnesian minerals and plagioclase, followed by ii) removal of plagioclase and lesser amphibole (with minor clinopyroxene) and finally iii) lesser alkali feldspar and minor amphibole in the most evolved terms. LILE-enriched and HFSE-depleted geochemical signature (likely originated from a hydrous primitive melt equilibrated with a spinel-bearing peridotite source) inferred the typical subduction-related trend emplaced in a subduction-related setting. Strongly evolved rocks are supposed to be derived from crustal anatexis of a mixed meta-sedimentary source. One sample shows distinctive adakitic signature (high La/Yb and Sr/Y ratios, low Yb and HFSE contents) which are interpreted as the product of the melting of a meta-mafic source rock (i.e., subducted oceanic slabs) with residual garnet and amphibole. Differently, Nowbaran Quaternary melanephelinites result by far the most peculiar igneous rocks of the entire Bitlis-Zagros Orogen. The absence of feldspars and melilite is coupled with extremely low SiO2 content and very high CaO and Al2O3 abundances, leading to ultracalcic compositions. Moreover, these rocks show high Mg# and very high Ni and Cr values, which likely suggest a primitive character of these melts. Isotopic ratios and primitive-mantle normalized pattern indicate hybrid sources, as their trend exhibits subduction-related imprinting mixed with HiMU-OIB features. Such uncommon compositions are thought to be unlikely derived from a classic four-phase (i.e. C-H-free) peridotitic mantle or from digestion of carbonatic compositions. More likely, they are generated from carbonated apatite-hornblendite-rich metasomes which are considered as the products of interaction between peridotitic matrix and partial melts derived from arc cumulates (formed by crystallization of hydrous and CO2-bearing magmas generated during previous subduction-related arc). On the other hand, two main volcanic cycles have been recognized in Bijar-Qorveh area. Upper Miocene (~9.2-8.3 Ma) compositions mostly range from trachy-andesites to trachytes (with minor rhyolitic terms) whereas Pleistocene (~1.3-0.5 Ma) samples are mostly represented by trachybasalts and tephrites, with lesser alkali basalts. Major and trace elements of Miocene rocks (i.e., Dehgolan and Qorveh) likely suggest fractional crystallization of ferromagnesian minerals then followed by removal of plagioclase and amphibole. These rocks exhibit high-K calcalkaline affinity, as also shown by primitive mantle-normalized patterns characterized by strong LILE and LREE enrichments, typical of subduction-related magmas. Only three Dehgolan samples show by far higher K2O-TiO2-P2O5 and Rb contents, which are thought to be related to a Ti-phlogopite-rich source rock. Quaternary samples (i.e., Qezelke Kand, Bijar and Takab) consist of alkaline series showing mainly sodic to strictly potassic (i.e. Takab) affinity. They show high Mg#, Ni and Cr content which probably is indicative that primary magma was not affected by extensive fractionation. Moreover, isotopic ratios and incompatible multielemental patterns show mixed features of both intraplate-like and subduction-related end members. Takab rocks always cluster distinctly, showing higher K2O and Rb abundances which could be linked to a phlogopite-rich mantle source. Noteworthy, Qezelke Kand outcrops are characterized by the association of basic products with four high silica rocks which have been referred to as adakites, likely deriving from the melting of subducted basalts in eclogite facies. Bijar-Qorveh Miocene high-K volcanic products represent the final magmatic phase of the calcalkaline volcanism occurring in Central Iran during Tertiary period, whereas Quaternary basic magmatism is to be considered as the expression of regional tensional tectonics or related to variations in the source mineralogy during melting processes.