CRYSTALLIZATION SEQUENCE AND TECTONIC SIGNIFICANCE OF ANDALUSITE, KYANITE, AND SILLIMANITE "TRIPLE POINT" LOCALITIES (Invited Presentation)
A new locality with coexisting andalusite, kyanite, and sillimanite is near Lesjaverk, Norway. In addition to the three Al2SiO5 polymorphs, the rock contains quartz, plagioclase, muscovite, and rutile. Petrographic observation indicates that andalusite (dark red in hand sample) formed first, followed by major growth of coarse, blue kyanite, with subsequent crystallization of fibrolitic sillimanite; i.e., andalusite àkyanite à sillimanite. Metamorphic temperatures were determined using Zr-in-rutile thermometry; results of 580-650°C calculated for an estimated range of moderate pressures indicate conditions near the kyanite-sillimanite boundary. Deformation, evidenced by subgrains in andalusite and quartz, kink bands in kyanite, and alignment of muscovite, may have facilitated polymorphic transformation without being severe enough to obliterate early-formed polymorphs. The preservation of all three polymorphs may also be related in part to the quartz-rich rock composition and lack of minerals such as biotite that are involved in Al2SiO5-consuming reactions in a typical metapelitic rock.