Writers : Sahabi.F and Saffarzadeh.A.H
Refference : Geosciences Scientific Quarterly Journal,No.:21-22,P.:32
Publishing Year : 1375

Abstract :
Gatehdeh subvolcanic dome has been intruded to the surface through a faulting -network within the pyroclastics of the Karaj Formation (Eocene). The dome - forming rocks are alkali feldspar- microgranite in nature and characterized by fine- grained, aplitic texture and consisting of orthoclase (80- 85 percent), and quartz (10 percent). Phenocrysts of Pelagioclase, Celsian, Biotite and Zircon are very few throughout the specimens. The orthoclase content of the rocks has been noticeably increased by formation of brown pigments during metasomatic alterations. Materials such as Si02 , CaCO3, BaSO and Fe2O formed in the course of secondary mineralization as a cavity filling cement in the brecciated rocks and fractures. Richness of K2O (12-16 percent) and poorness of CaO, MgO, Na2O, and TiO2 (totally less than 0/5 percent) are the most dominant geochemical characteristics of these rocks. However they can not be categorised as potassic or ultrapotassic rocks due to lack of minimum requirement of GaO and Na 2O.
It is thought at this stage that the Gatehdeh subvolcanic mass was originated from a sialic crust, melted
by conjugation of olivine- basaltic magma In the course of crustal melting process.
Subject List : Petrogenesis