Late Eocene–Oligocene post-collisional monzonitic intrusions from the Alborz magmatic belt, NW Iran. An example of monzonite magma generation from a metasomatized mantle source

• Potassic Cenozoic magmatism developed batholith-scale associations in Alborz magmatic belt.
• Monzonitic magmas are identified as primary melts from metasomatized mantle.
• Slab-derived adakitic melts infiltrated the mantle before melting.
• The source of post-collision magmas was cooked by a previous stage of subduction.

A potassic magmatic association in the Zagros hinterland of the Tethyan orogen in Iran is identified and characterized for relevant geochronologic and petrologic features. New data, including a combination of field relations, U–Pb zircon geochronology and rock geochemistry, come from seven plutons (Khankandi, Shaivar-Dagh, Yuseflu, Mizan, Saheb-Divan, Roudbar and Abhar) that form the Arasbaran–Taroum batholith (ATB), which forms part of the Alborz magmatic belt (AMB) of NW Iran. Zircon SHRIMP ages range from 38.32 ± 0.17 Ma, 38.94 ± 0.42 Ma and 37.78 ± 0.28 Ma for magma pulses of the Abhar pluton, at the East of the batholith, to 24.51 ± 0.27 Ma and 23.55 ± 0.47 Ma for pulses of the Mizan pluton at the West. Considering these ages and the previously published ones together, emplacement of the batholith took place during Late Eocene and Oligocene, from 38 to 23 Ma, with an age progression from SE to NW at a rate of 2 cm/year. The whole batholith is characterized by potassic rocks with K2O > 2 wt.% in gabbros and diorites (SiO2 < 50 wt.%). Higher contents of K2O, of up to > 6 wt.%, are normally found in rocks with intermediate silica contents of about 60 wt.% SiO2. These intermediate silica rocks are truly monzonites and are the most abundant in each pluton. With regard to trace elements, the monzonitic rocks of the ATB show some of the typical signatures of arc magmatism (depletion in Nb and Ti). Most samples contain moderate contents of Sr (500–800 ppm), close to similar potassic magmas forming Cenozoic complexes in Central Iran. The relatively moderate Sr/Y and La/Yb ratios suggest that ATB magmas retain some adakitic signatures from the source region. Geochemical modeling is performed by using melt compositions and phase relations calculated with MELTS software, combined with experimental data and trace element signatures. We conclude that monzonitic and shoshonitic magmas of some plutons of the ATB (Shaivar-Dagh, Kahnkandi and Yuseflu) have an adakitic signature inherited from early melts that metasomatized the peridotite mantle. Decompression melting at a relatively low pressure of a metasomatized mantle source is the most plausible explanation for the generation of potassic magmas of the ATB. The lack of adakitic signature in three plutons supports the hypothesis that trace-element features are related to processes in the source. The tectonic implications are straightforward: First, subduction is needed to cook a metasomatized mantle source by fluxing of calc-alkaline melts (adakitic or not) from the subduction zone. Second, lithosphere extension is needed to carry metasomatized mantle to melt and to generate the potassic magmatism that formed the Arasbaran–Taroum batholith, as well as other Cenozoic potassic magmas of the Alborz magmatic belt in NW Iran.