CRYSTALLIZATION SEQUENCE AND TECTONIC SIGNIFICANCE OF ANDALUSITE, KYANITE, AND SILLIMANITE "TRIPLE POINT" LOCALITIES (Invited Presentation)

Rocks containing three Al₂SiO₅ polymorphs are uncommon; only 9 localities have previously been reported and documented sufficiently to demonstrate the coexistence of andalusite, kyanite, and sillimanite in the same rock (thin section scale). The crystallization sequence of coexisting polymorphs, typically inferred from petrographic observation, provides information about tectono-metamorphic history. For the 9 well-documented localities, as well as a new site in Norway, two distinct crystallization sequences have been proposed: kyanite -> sillimanite -> andalusite (Idaho and New Mexico, US Cordillera; Betic Cordillera, Spain; Abukuma, Japan), and andalusite -> kyanite -> sillimanite (Santander massif, Colombia; Sivrihisar massif, Turkey; Hamedan region, Iran; Yenisei Ridge, Russia; Pyeongan complex, South Korea; Lesjaverk, Norway). The main difference between these two sequences is whether andalusite is the first or last-formed polymorph, and therefore whether low-pressure / moderate- to high-temperature metamorphism preceded or postdated kyanite-sillimanite zone conditions. The low-pressure prograde sequence of kyanite -> andalusite -> sillimanite is rare/unknown.

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.