Late Cretaceous basaltic magmatism in south of Lahijan: a significant magmatic event in Central Alborz

The volcanics of Kojid area (south of Lahijan) belong to Central Alborz. Stratigraphic relationships show that they are Late Cretaceous in age. The volcanics cover an area of about 500 km2 and include pillowed to massive lavas, associated volcanoclastic units and numerous cross-cutting dykes. Geochemically, the volcanics are mainly alkaline basalts. Furthermore, an arc-related probably post-Cretaceous (?) basalts occur in association with alkaline basalts. In alkaline basalts, plagioclase, clinopyroxene and olivine are the major phenocrystic phases. Clinopyroxenes are augite to diopside in composition characterized by high TiO2 (0.96- 5.13 wt.%) and Na2O (0.24-0.68 wt.%) contents. Based on whole rock geochemical data, the alkaline basalts are characterized by light rare earth element (LREE) enrichment with (La/Yb)N ratio of 5.9- 18.2 and similar to oceanic island basalts (OIB). The major and trace element geochemical signatures are consistent with intra-plate alkaline basalts generated in an extensional tectonic regime (aborted continental rift). Also, the relatively deep submarine environment of melt extrusion indicates a well-developed intra-continental rift. Our finding is in contrast with earlier studies suggesting ophiolitic affinity of these rocks. Furthermore, we think that the high volume of Late- Cretaceous volcanics of Central Alborz is probably related to a significant thermal perturbation associated with asthenosphere upwelling and hot spot/plume activity which could be considered as a key feature in Cretaceous evolution of Alborz Zone.