A study of various occurrences of the Al2SiO5 polymorphs in the rocks/veins of the Hamedan region, Iran: with special reference to origin of quartz-kyanite veins

The study area is a part of so-called Sanandaj-Sirjan zone or Zagros imbricate zone of the Zagros Orogen, Iran. The Hamedan region comprises a metamorphic sequence of low- to high-grade regional and contact metamorphic rocks intruded by mafic, intermediate and felsic plutonic bodies. The sequence comprises pelitic, psammitic, mafic, calc-pelitic and calc-silicate rocks.  Pelitic rocks are the most abundant rocks in the region which are mostly composed of slates, phyllites, mica schists, garnet-mica schists, garnet-andalusite-(±sillimanite/±kyanite) schists, garnet-staurolite schists, mica hornfelses, garnet hornfelses, garnet-andalusite-(± fibrolite) hornfelses, cordierite-(±andalusite) hornfelses, cordierite-K-feldspar hornfelses and sillimanite-K-feldspar hornfelses. Several interlayers of amphibolites and amphibole schists alternate with pelitic rocks in the sequence. Various types of Al2SiO5-bearing silicic veins occur in the region.  Isotope characteristics indicate metamorphic origin for some of them.  In this study, we have defined three different generations of kyanite according to their various geological characteristics, including: (1) metamorphic (randomly distributed and commonly co-existed with sillimanite/andalusite), (2) stress-(shear)-induced (pseudomorphs of kyanite after andalusite in the shear zones) and (3) late-stage (hydrothermal or metasomatic) kyanites.  The quartz-kyanite veins/pods are late structures that are created by interactions with wall rocks. Circulating Al- and Si-rich fluids may be responsible for formation of these veins/pods.