PETROLOGY OF IGNEOUS ROCKS IN SHIRKUH SW OF YAZD AS A UNIQUE GROWTH PLACE FOR A MEDICAL PLANT “NEPTA ASTROTRICHA”

Petrology of igneous rocks in the Shirkuh Mountains SW of Yazd as an exclusive growth place for Nepeta astrotricha, a spearmint family and medical plant has investigated in this study. Petrological study indicates granitoid rocks (granodiorite, two mica granite and monzogranite) in the growing plant place. Quartz and Feldspar (perthitic alkali-feldspar and plagioclase with albite twining) are the essential minerals while biotite and muscovite are the secondary minerals. Zircon crystals with pleochroic halos exist within biotites. Accessory minerals such as chlorite and sericite are the alteration products of biotite and plagioclase. Sosuritization is another type of plagioclase alteration. Anortitic parts has changed to epidote (Clinozoisite, zoisite) while residual plagioclases produced albite. There are a collection of carbonate minerals, muscovite, garnet and epidote (Pistacite & Zoisite) in the rock samples which have enclaves. The existence of garnet crystals indicates an anatexis origin of the plutonic mass.

Shirkuh granite with Late Jurassic-Early Cretaceous age occurred at the Late Cimmerian orogenic phase during the subduction of Neo-Tethys oceanic crust beneath the Central Iran plate. Later, Shirkuh granite covered with gray conglomerate and red sandstone (Sangestan Formation) in Upper Jurassic-Lower Cretaceous. Finally, Taft limestone covered all previous rocks and formed the tops of the Shirkuh Mountains. There is one of the highest snowy peaks of the Shirkuh Mountains in the study area which is called Barfkhaneh Tezerjan.

The calc-alkaline granitic rocks are per-aluminuos, rich in Al and Ti and the K amount is medium to high. Alteration of granitic rocks as well as the alteration of upstream rocks (conglomerate and limestone) has produced the necessary elements for growing the special plant “Nepeta astrotricha”.

The essential gathered bio-ecological factors are the high altitude, sufficient moisture, cold water resulting of snow melting and the stony sandy soil. Nepeta astrotricha is found exclusively in the Shirkuh Mountains slop, just beside the cold streams and might have been the result of harshness increasing in its granitic soil habitat at higher altitude and reduction in biological activity.