The gabbro - diorite plutonic complex on which the Karaj dam is constructed, essentially is composed of coarse and isogranular igneous rocks. This complex is intruded in the Assara - shale horizon, between upper and middle members of volcano - sedimentary character of clastic tuffites of Karaj formation. Mapping of the area under study, proved that this complex is likely, as a <> body, that is sunken in its central part, with a trend of west north - west to east south - east direction, very much similar to the trend of Aderan syncline axis, about 1 kilometer south of the dam. Detailed petrographical and geochmical studies indicate that the body is a > which was differenciated from a gabbro - monzonitic mass located underneath. The time of intrusion of this complex is believed to be post Eocene, corresponding to the time of orogenic folding of the tuffite beds of this region. The tectonic activity of the area commenced by this intrusion. Many of the important faults in the area, such as <> have been traced out. These faults are accompined with 20 meters of <> crushed zone without any trace of magmatic mineralization activties.
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