Geology & geochemistry of Igneous Rocks in base of Shemshak Formation in Easterm Alborz

Category Petrology
Group GSI.IR
Location 21th symposium on geosciences
Holding Date 21 May 2008

Abstract:
      In the base of Shemshak formation in eastern Alborz zone, basic igneous rocks have been reported as lava. But precise investigation of geological outcrops in Shahmirzad, Cheshmehali, Tazareh and Gheshlagh regions in around of Shahroud, Damghan and Semnan show that this rocks in Shahmirzad and Cheshmehali regions are lava and in Tazereh and Gheshlagh regions are in the form of intrusive igneous and sill. In Gheshlagh outcrop this complex is a differentiated large sill with 200 meters thickness. Its differentiated spectrum changes, from bottom to top, from olivine gabbros to monzonite.
   Petrographical and geochemical studies show that these rocks had alkaline nature and were formed in continental intra-plate setting. This tectonic setting corresponds to tensional environment, in which Shemshak formation was formed.

   General:
     In the base of Shemshak formation in eastern Alborz zone basic igneous rocks have been reported as lava. But precise investigation of geological outcrops in Shahmirzad, Cheshmehali, Tazareh and Gheshlagh regions in around of Shahroud, Damghan and Semnan show that this rocks in Shahmirzad and Cheshmehali regions are lava and in Tazereh and Gheshlagh regions are in the form of intrusive igneous and sill. In Gheshlagh outcrop this complex is a differentiated large sill with 200 meters thickness. Its differentiated spectrum changes, from bottom to top, from olivine gabbros to monzonite.
   Petrographical and geochemical studies show that these rocks had alkaline nature and were formed in continental intra- plate setting. This tectonic setting corresponds to tensional environment, in which Shemshak formation was formed. Mong the kinds of basic to intermediate rocks (A- assemblage) and semi-acidic to acidic rocks (B- assemblage) plotted in the Harker diagrams which indicates complexity of the processes of petrogenesis such as, fractional crystallization, magma mixing and alteration, and existence of the bi-modality magmatism (basic and acidic magmatism) is ine of the most significant evidences

tags: TEHRAN