Feather-like hornblende aggregates in the phyllites from the southern Sanandaj-Sirjan zone, Iran; their origin and mode of formation

Several outcrops of spectacular feather-like hornblende aggregates occur in the phyllites of the Gol-e-Gohar complex (southeast of Iran) and form special Garbenschiefer rock types. The Gol-e-Gohar complex, as a part of the southern Sanandaj-Sirjan metamorphic zone, contains a succession of metabasites, phyllites, and slates intruded by dioritic intrusions. There are two types of hornblendes in the phyllites; the first one is concentrated around the fractures and the second one is randomly distributed in the rocks and forms radial feather-like hornblende aggregates. Petrographical and chemical characteristics of these two shapes of hornblendes are the same, except the latter is developed parallel to the foliation planes. The hornblendes occur as unstrained needle-shaped porphyroblasts with oriented quartz and feldspar inclusions as well as polygonal grains in the matrix with mosaic texture, which implies the matrix has been recrystallized. On the basis of field observations, petrography, and chemical compositions of the hornblendes, we inferred that the Garbenschiefer phyllites formed during hydrothermal metamorphism in association with penetration of hot fluids. The compositions of hornblende aggregates are similar to those formed in hydrothermal systems and differ from regional and thermal metamorphic amphiboles. All evidence shows that, in the studied area, ascending of dioritic intrusions increases fluid temperature, the hot fluids leach some elements from the metabasites, and finally the enriched fluids flow upward via the fractures. In the upper levels, the fluids penetrate into the phyllites along their foliation planes and, with a decrease in temperature and pressure, they crystallize hornblende aggregates under static hydrothermal conditions.