Source properties and tectonic setting of the basic magmatism in the Lower Red Formation, north of Garmsar (Semnan, Central Iran)

In north and northwest of Garmsar in Semnan province¡ some basic igneous rocks with the Oligocene age outcropped as dyke¡ sill and very small stocks in the Lower Red Formation. These rocks have basic composition and alkaline nature. They show various textures such as porphyric¡ glomeroporphyric¡ ophitic¡ subophitic¡ intergranular and granular. Plagioclase and clinopyroxene as the main minerals¡ olivine¡ opaque (magnetite¡ titanomagnetite)¡ apatite and phelogopite as the minor minerals and chlorite¡ prehnite¡ epidote¡ serpentine sericite and calcite are the secondary minerals in these rocks. Primitive mantle and chondrite normalized diagrams of these rocks show strong enrichments in light rare earth elements (LREEs) and relative depletions in heavy rare earth elements (HREEs). Also¡ parallel trends of the samples in these diagrams indicate a common source for these rocks and the role of the differentiation crystallization in their genesis. Investigated rocks are plotted in back-arc basin setting field in discrimination tectonic diagrams. Geochemical studies show magma forming of these rocks have been formed from 11- 16% partial melting of an enriched modified garnet lherzolitic mantle source in 90-100 km depths. This magma intruded via deep fractures and faults in the incipient extensional back-arc basin resulted from northward subduction of Arabian plate beneath the Central Iran in the Oligocene-Miocene time.