Quartz-rich veins in metapelitic schists of the Sanandaj-Sirjan belt, Hamadan region, Iran, commonly contain two Al 2 SiO 5 polymorphs, and, more rarely, three coexisting Al 2 SiO 5 polymorphs. In most andalusite and sillimanite schists, the types of polymorphs in veins correlate with Al 2 SiO 5 polymorph(s) in the host rocks, although vein polymorphs are texturally and compositionally distinct from those in adjacent host rocks; e.g. vein andalusite is enriched in Fe 2 O 3 relative to host rock andalusite. Low-grade rocks contain andalusite + quartz veins, medium-grade rocks contain andalusite + sillimanite + quartz ± plagioclase veins, and high-grade rocks contain sillimanite + quartz + plagioclase veins ⁄ leucosomes. Although most andalusite and sillimanite-bearing veins occur in host rocks that also contain Al 2 SiO 5 , kyanite-quartz veins crosscut rocks that lack Al 2 SiO 5 (e.g. staurolite schist, granite). A quartz vein containing andalusite + kyanite + sillimanite + staurolite + muscovite occurs in andalusite-sillimanite host rocks. Textural relationships in this vein indicate the crystallization sequence andalusite to kyanite to sillimanite. This crystallization sequence conflicts with the observation that kyanite-quartz veins post-date andalusite-sillimanite veins and at least one intrusive phase of a granite that produced a low-pressure-high-temperature contact aureole; these relationships imply a sequence of andalusite to sillimanite to kyanite. Varying crystallization sequences for rocks in a largely coherent metamorphic belt can be explained by P-T paths of different rocks passing near (slightly above, slightly below) the Al 2 SiO 5 triple point, and by overprinting of multiple metamorphic events in a terrane that evolved from a continental arc to a collisional orogen.
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