Ages, geochemistry and Sr-Nd-Pb isotopes of alkaline potassic volcanic rocks from the Ahar-Arasbaran region (NW Iran): Evidence for progressive evolution of mantle sources during the Neotethyan subduction system

The volcanism of the Arasbaran region, northwest Iran, is characterized by multiple magmatic pulses from Cretaceous to Quaternary related to the consumption of the Neotethys oceanic basin and the subsequent continental collision between the Arabia and the Eurasian plates. In this paper we focus on the Eocene igneous products, which show wide compositional variations, ranging from shoshonite to tephrite and phonolite. They may be further grouped into leucite (analcime)-bearing and leucite-free rock types on the basis of their rock-forming minerals. Leucite-bearing and leucite-free Eocene magmatic rocks are geographically separate outcropping in the WNW and ESE part of the Ahar-Arasbaran region, respectively. Geochemical and isotopic compositions, coupled with the strong silica-undersaturated character, of leucite-bearingrocks suggest in their mantle source the involvement of metasomatizing partial melts from subducted altered oceanic crust and subordinate carbonate-bearing sediments. On the other hand, the compositions of leucite-free igneous rocks are compatible with the involvement of a relatively higher contribution of partial melts from terrigenous (carbonate-poor) subducted sediments. The close spatial association and the relative geographical/stratigraphic position of these products indicate diachronous metasomatic events in the mantle wedge underlying the Arasbaran area that could have been originated by the late arrival of carbonate-rich sediments at depth during slab steepening and incipient roll-back preceding the continental collision. K-Ar dating indicates that the Arasbaran magmatism was triggered by a late geodynamic event, during middle Eocene, plausibly consisting of re-adjusting of isotherms that heated the veined mantle wedge following the slab migration after roll-back. The slightly younger age of leucite-bearing rocks with respect to leucite-free rocks, coupled with the lower melting degree of the former may suggest an evolution of the local thermal regime with the progressive involvement of portions of the mantle wedge closer to the subducted plate.