PETROGENESIS OF ANDALUSITE-KYANITE-SILLIMANITE VEINS, SANANDAJ-SIRJAN METAMORPHIC BELT, IRAN

Veins or leucosomes that contain all three Al2SiO5 polymorphs are extremely rare. Three coexisting polymorphs are known from metapelitic rocks and quartzites, but reports of Al2SiO5-bearing veins describe at most two polymorphs. Quartz-rich veins and vein marginal zones in metapelitic schists of the Sanandaj-Sirjan metamorphic belt, Hamadan region, Iran, commonly contain two polymorphs, and, in one case, three coexisting Al2SiO5 polymorphs. For veins in andalusite and/or sillimanite-bearing schists, the types of polymorphs in veins correlate with Al2SiO5 polymorph(s) in the host rocks. Veins contain andalusite + quartz in low- to medium-grade andalusite schists, and sillimanite + quartz + plagioclase in the highest-grade, sillimanite-bearing rocks. One sample, located at a vein-host margin, contains andalusite + kyanite + sillimanite + staurolite + muscovite + quartz. Textural relations among Al2SiO5 phases in samples containing two or three polymorphs indicate the crystallization sequence andalusite to sillimanite to kyanite.

The distribution of kyanite is not correlated with host rock mineralogy. Kyanite occurs in veins that cross-cut rocks with or without Al2SiO5 in the groundmass; e.g., andalusite-sillimanite schist, garnet-staurolite schist, and a granite that intruded the sequence and produced a low-pressure - high-temperature contact aureole. Because kyanite veins crosscut the granite, the kyanite-forming event post-dated intrusion of the granite into the metapelitic sequence. Kyanite also occurs in both andalusite- and sillimanite-bearing veins.

We propose a model of early generation of andalusite ± sillimanite veins by syn- to late-metamorphic reaction of metapelitic rocks with aqueous fluids, and later formation of kyanite in veins by metamorphism of pre-existing veins or leucosomes. The late kyanite veins formed at higher pressures and/or lower temperatures than the conditions of formation of the earlier (andalusite-sillimanite) veins. The tectonic-metamorphic events that drove regional and contact metamorphism, including vein formation, involved contraction of continental margin basins and development of a continental arc, followed by collision and closure of a Neo-Tethyan seaway that drove further burial and heating of the metasedimentary sequence.