Mineralogy of the Lower Oligocene alkali basalts and their sillimanitite xenoliths (SW of Jandaq, Isfahan Province, Central Iran)

Along Toveireh Fault, in the northwest of Central-East Iranian Microcontinent, the Oligocene alkali-basalt outcrop was investigated for petrography and mineral chemistry. They include phenocrystals of chrysolite (Fo70–90), augite-diopside (Mg#=0.82) and labradorite (An30-70) in a fine-grained matrix of the same minerals as pheoncrysts, together with sanidine and secondary minerals (serpentine and zeolite), as well as opaque minerals. These rocks contain sillimanitite xenoliths which are mostly composed of fibrous and prismatic sillimanites, together with minor amount of corundum, and secondary minerals (chlorite and scapolite). Xenoliths show granoblastic, nematoblastic, poikiloblastic, and corona textures. These high-Al and moderately silica-rich xenoliths were probably formed by metamorphism of continental crust sedimentary rocks, conveyed up to the earth's surface by ascending alkali basalt magma. Fibrolite formation around some of the prismatic sillimanits and Al-rich spinels (Mg#=0.4-0.5) formation in the contact of xenolith and host basalt indicate that pressure-temperature decreased during magma ascending and reaction of basaltic magma with xenolith.