Igneous rocks of the Karaj Dam basement are mainly
composed of gabbro, diorite and monzodiorite-monzonite which are
locally penetrated by the quartz-monzonitic dikes. The upper and
lower margins of the pluton, which has gabbroic composition, show
porphyritic texture (chilled margin). The porphyritic texture in
chilled margin gradually changes to the equigranular gabbro, diorite
and monzodiorite-monzonite. The mineral chemistry (electron
microprobe analysis) of the plagioclase and pyroxene in various rock
samples from lower contact chilled towards the upper parts
suggesting differentiation processes. The Mg of the pyroxene and
An% of plagioclase of the contact chilled samples can be used as an
indication of the original magma and plotted between the gabbro and
monzonitic samples. In addition, increasing of the Mg within the
whole rock samples from the upper of contact chilled, in comparison
to the lower one, suggests elements differentiation by the gravity
diffusion. Moreover, Na2O, K2O and incompatible elements increase
and MgO, Fe2O3 (tot), CaO and compatible elements decrease with
the progress of the magma differentiation. The samples in the Log
Ni-Log La and Log Ni-Log Zr diagram plot almost as straight
horizontal line, which confirms occurrence of the magma
differentiation.