Petrogenesis and tectonic setting of an basalt-Trachyte-Rhyolite suite in the Spilli area (south of Siahkal), north of Iran: evidences of continental rift-related bimodal magmatism in Alborz

The spilli volcanic rocks suite consisting of Basalt- Trachyte- Rhyolite with upper Cretaceous¡ outcrop in the northern part of Alborz and south of Siahkal area (east of the Guilan province). Based on geochemical data¡ the studied suite attributed to transitional to alkali series. Negative correlation of Al2O3¡ CaO¡ P2O5 and positive correlation of Rb and Th versus SiO2 reveal the occurrence of fractional crystallization process. Also¡ the negative correlation of Sr versus Y¡ Sr/Zr versus Sr and CaO/Al2O3 versus SiO2 show that fractionation of plagioclase has played an important role in petrogenesis of the spilli Suite. The hypotheses is supported by the negative anomalies of Eu¡ Ba and Sr. The overall geochemical evidences indicate that the basic rocks belong to intra-continental rift zone whereas the felsic rocks are classified as A1 type derived from parent basaltic magmas via fractional crystallization in an anorogenic setting. The studied magmatism share many similarities with bimodal magmatism in continental rift zones.