Geochemistry and tectonomagmatic setting of the volcanic and Sub-volcanic rocks of the south Gilan, Iranian Alborz mountains: Evidence for Adakite-like and normal arc magmatism

As part of the Alborz magmatic zone, Tertiary volcanic and subvolcanic rocks are exposed in southern Gilan province (Rudbar and Rostamabad) and have been evaluated for the first time in this study. Field and petrographic studies indicate a compositional spectrum from basalt, andesite to trachyandesite, dacite to trachy-dacite, rhyodacite. Geochemically, the rocks were divided into two distinct groups: adakitic rocks with high Sr, Sr/Y and La/Yb and low MgO, Y and Yb, and typical calc-alkaline rocks of arc nature. All rocks show a parallel enrichment trend from LREE to HREE. Both partial melting and crystal fractionation play important role in controlling the compositional diversity and composition of the main magma of the samples. Both rock groups formed in a subduction zone environment and were produced with different degrees of melting in a slab window environment. All data indicate that the rocks were formed during the subduction to the south of the Para-Tethys oceanic crust. A “ridge-trench” model for the tectonomagmatic scenario has been proposed, suggesting that these lavas were formed in a Para-Tethys supersubduction zone. According to this model, a slab window was formed after the subduction of the Para-Tethys oceanic crust. Partial melting of this crust at depth, with different degrees of partial melting in some areas, coinciding with the formation of a tectonic window in the subduction zone, led to the production of adakitic magma. Typical arc rocks with higher degrees of melting and at shallower depths formed further away from the subduction zone.