Geochemistry and geodynamic setting of Paleogene volcanic rocks from south Ardestan, Iran

South Ardestan Paleogene volcanic rocks comprise a succession of, in order of abundances, rather silica-rich felsic rocks, andesites and rare basaltic andesites. Major and trace element geochemistry of andesites and basaltic andesites were studied in details (Yeganehfar, 2007). The former (e.g., covariation of FeO/MgO-SiO2 and Alkalis-FeO-MgO) indicated a calcalkaline affinity while, the letter (i.e., enrichment in LILE and depletion in HFSE) pointed to a subduction-related signature. The andesites and basaltic andesites are porphyritic in different degrees. Plagioclase, pyroxene, olivine remnants and Fe-Ti oxides are the most abundant phenocrysts in the basic rocks. Normalized trace element abundances and patterns of the andesites and basaltic andesites show similarities to both island arc volcanics and continental margin volcanics. South Ardestan andesites and basaltic andesites are likely produced during northeastward subduction of Neotethyan oceanic slab beneath central Iranian Plate in Paleogene (mainly Eocene). Based on quantitative trace element modeling, south Ardestan basic volcanic rocks are derived from 15 to 30 percent partial melting of a spinel lherzolitic mantle rock. The presence of a basaltic flow with tholeiitic affinity at the base of south Ardestan volcanic succession and the presence of two dykes of tholeiitic affinity that cut the upper part of succession, are noteworthy. The basal basaltic flow is consistent with the chronology of events in island arc, since early stage magmatism is tholeiitic in island arc. Tholeiitic dykes that correspond to the latter stage of magmatism in the study area, would be considered as indications of a change in tectonic regime. Assuming a horizontal component for subduction (oblique subduction) or dominance of a rather stable (relaxation) period is the most probable causes (alternatives). Based on petrological and geochemical investigation of rather basic volcanic rocks, south Ardestan Paleogene volcanic succession would be attributed to an island arc with indications of evolving to an active continental margin. Selected References: Yeganehfar, H., 2007. Geochemistry and petrology of the volcanic rocks from south Ardestan. M.Sc thesis. Tarbiat Modares University, Iran. 115 p.