Petrography and mineral chemistry of the Eocene volcanic rocks in the Posht-e-Badam block

The Eocene volcanic rocks of the Posht-e-Badam block represent very good exposures in Saghand (Khoshoumi Mountain)، Alahabad and Chapedony areas (Yazd province). These rocks were developed during five volcanic phases: 1- Volcanic rocks of Alahabad area which are formed during three various phases: 1-1: The central andesitic phase; 1-2: The first andesitic fissure phase; and 1-3: The second andesibasaltic fissure phase. 2- The Chapedony andesite to andesibasaltics. 3- The Saghand (Khoshoumi Mountain) andesic rocks. On the base of field observations، the evidences of contamination are clearly noticed in the volcanic rocks of Saghand area which point to slower rate of magma ascending. The lowest volume of Eocene volcanic rocks exposed in Alahabad area. These rocks developed by two fissures and one central eruption. The presence of the highest volume of the Eocene volcanic rocks in Chapedony area displays active Chapedony fault during the Eocene time. The absence of contamination evidences in the rocks of Chapedony and Alahabad areas indicate the higher rate of magma ascending in compare to that of Saghand area. The occurrence of xenoliths and xenocrysts، the formation of fine grained clinopyroxenes around xenocrystic quartz، the sieved and anti-rapakivi textures، the gulf corrosion of quartz and the oscillatory zoning of plagioclases reveal magmatic contamination in Saghand area. The chemistry of clinopyroxenes and biotites display that these rocks، in nature، are similar to the calc-alkaline continental volcanic arc rocks.