Post-collisional calc alkaline-adakites from a metasomatized subcontinental lithospheric mantle: Sr-Nd isotopes and bulk-rock geochemistry of Miocene volcanic rocks from the Varzaghan area, NW Iran

ABSTRACT

The NW Iran is located at the western termination of the Alborz Mountain range and continuation of the Sanandaj-Sirjan magmatic-metamorphic zone at the NW. The volcanic rocks in the Varzaghan area are distributed over different compositional ranges from high-K basalt, andesite, dacite, and trachyandesite, dacite, to rhyolite with adakitic signatures. Chondrite-normalized REE diagrams show flat MREE and HREE patterns for calc-alkaline volcanic rocks and enrichment in LREE, indicating a garnet-free source. Adakitic rock sample chondrite-normalized REE patterns are relatively steep with significant enrichment in LREE and depletion in HREE, indicating a garnet-bearing source or garnet fractionation process. The arc trace element signatures, narrow range of initial⁸⁷Sr/⁸⁶Sr_(i) = 0.70425–0.70487, different initial¹⁴³Nd/¹⁴⁴Nd_(i) = 0.51264–0.51286 and the presence of adakitic rocks reveal that the volcanic rock sources were not the same and can be related to the partial melting of metasomatized mantle and lower continental crust sources. Slab break-off and upwelling of hot asthenosphere in a post-collisional tectonic regime resulted in the melting of the subcontinental lithosphere and lower continental crust which produced calc-alkaline-adakitic volcanic rocks in the Varzaghan area. The main petrological processes involved in the volcanic rocks petrogenesis were fractional crystallization and crustal assimilation.

GRAPHICAL ABSTRACT

Summarized figure. The northwest Varzaghan volcanic rocks formed as domes are mafic to felsic. Parental magma was derived from different sources in an extensional geodynamic setting that developed due to slab break-off and subsequent lithospheric heating and thinning. The primary magmas were modified by fractional crystallization and crustal assimilation. Extension related to lithospheric thinning induced decompression melting of the metasomatized subcontinental lithospheric mantle that was modified by slab fluid and sediment components from the Neotethys subducted oceanic slab before the collision event between Arabia-Iran plates. The rocks formed as island-arc-andesite-dacite-rhyolite lavas originated from mantle source, and the adakites relate to both mantle and lower crust sources.

KEYWORDS:

• Calc-alkaline
• adakite
• subcontinental lithosphere
• post-collision

Acknowledgments

We thank Shuangquan Zhang from the Department of Earth Sciences at Carleton University for his collaboration in isotopic analyses. This paper is a part of the Ph.D. dissertation of the second author in the University of Tabriz.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Supplementary material

Supplemental data for this article can be accessed online at https://doi.org/10.1080/00206814.2024.2346788

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Funding

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