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In the northern hillside of Central Alborz, in south Marzanabad, there are outcrops of basic volcanic rocks which attributed to Cretaceous time .In this study, effective processes on evolution of these volcanic rocks are surveyed to add on our knowledge about Mesozoic magmatisim in Central Alborz. Based on petrographical and geochemical studies, south Marzanabad basic rocks could be classified into 2 groups of basalt and andesite basalt. Geochemical diagrams imply that the magma nature is alkaline and generated in intracontinetal extensional environment. Microscopic studies, mineral chemical composition and the observed trends in geochemical diagrams of whole rocks, all are inline with fractional crystallization as a main magmatic process. Fractionation began with crystallization of olivine and pyroxene and minor plagioclase in basalts and continued with crystallization of clinopyroxene and plagioclase in andesite basalt. In addition to petrographical, mineralogical and chemical evidences, the occurrence of fractional crystallization process is confirmed by the geochemical modeling with Melts software which also let us to reconstruct the condition of fractionation process. Based on the fractional crystallization modeling in south Marzanabad basic volcanic rocks, basalts are generated by 40 percent of fractional crystallization from primary magma. Basaltic andesites also produced after the fractionation of basalts, during 70 percent fractional crystallization from the same magma. Olivine thermobarometric studies also confirm the accuracy of data which is obtained by the Melts software.

Keywords: basalt, Marzan Abad, fractional crystallization, Modeling, Melts software

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