Chemical compositions and optical characters of sodic–calcic and calcic amphiboles in basic schists of Shahneshin area, southwest of Zanjan

Shahneshin area is located in southwest of Zanjan at the border between Zanjan and Kurdestan provinces. Based on petrographic investigation, Shahneshin basic schists could be divided into actinolite schist, actinolite pyroxene schist, actinolite pyroxene garnet schist and actinolite epidote schist. Actinolite schists are the most abundant rocks among basic schists and are composed of amphibole, muscovite, chlorite and albite as major minerals. Amphiboles occurred in two different fabric including inclined and parallel to the main schistosity. Optical characters of amphibole changes from core towards rim in which birefringence and extinction angle decrease and ppl color changes from green-blue to the pale green. In the other hand, chemical analysis of the same amphibole from core towards rim shows that core has richerite-winchite composition while rim has actinolite composition. Therefore, there is clear relation between optical and chemical characters in which reverse glaucophane substitution (CaM4 MgOctAlOct) and plagioclase substitution (CaM4 AlT) occurred. P-T calculation using chemical composition of amphibole cores show that they formed under 14 kbar pressure and 440 OC temperature while their rims formed under pressure of 4 kbar and temperature of 520 OC. Thus optical and chemical zoning observed in amphiboles in Shahneshin basic schists could be formed due to changes in pressure and temperature of the metamorphism.