Abstarct: In Jamyl area, about120 kilometers south of theShahrood city, Semnan province, there are a few outcrops of mafic igneous rocks.P recise investigations indicate that these are the of extrusives and intrusives. Field observations, petrographic sinvestigationand geochemical diagrams show that the Jamyl igneous rocks, have basaltic and gabbroic nature. Plagioclase and pyroxene are essential minerals. Olivine, apatite, opaque and minerals (pyrite and magnetite baxsed on polished sections) are accesories. Chlorite, calcite, epidote, titanite and zeolite are secondary minerals. Porphyritic Glomeroporphyritic, intergranular and trachity are the most important textures. Main constituents of the gabbroic and gabbrodioritic rocks are plagioclase, green hornblend, opaque minerals and clinopyroxens. Apatite and quartz are accessory minerals and epidot, cholorite, calcite and serecite are secondary minerals. Intergranular, Sub ophitic, ophitic and microgranular textures are main texture of gabroic rocks. Jurassic basaltic rocks geochemical investigations of Jamyl area show that these rocks have alkaline nature. According to tectonic setting discrimination diagrams these rocks related to intercontineuted rifting. Trace element changes in samples indicate that an enritchment in LREE and high incompatible elements and a depletion in HREE. Consistent pattern of LREE of LILE in these diagrams and variation patterns of the oxides of main element and compatible and incompatible element with respect to each other, indicate the same origin for the magmatic rocks in this area and show the role of fractional crystallization in magmatic evolutions. According to geochemical and petrogenetic survays, the alkaline mafic rocks original magma of the baxse of equivalet Shemshak Formation resulted from low degree partial melting (7 to 14 percent) of an enriched mantle beneath the continental lithosphere which has a garnet – lerzolite composition at depth of 100 to 110 kilometers. This magma originated in an inter continental extentional environment. Geochemical evidencs cleary show that crustal contamination has no or very insignificant role during it is evolution.