Crystallization sequences of coexisting andalusite, kyanite, and sillimanite, and a report on a new locality: Lesjaverk, Norway

Rocks containing three Al₂SiO₅ polymorphs – andalusite (And), kyanite (Ky), and sillimanite (Sil) – represent disequilibrium resulting from polymorph instability driven by changing pressure–temperature (P–T) conditions during related or unrelated tectonometamorphic events. There are six possible crystallization sequences. The two most common sequences of seventeen localities surveyed are: (1) Ky → Sil → And (six localities), and (2) And → Ky → Sil (six localities), including one locality for which two different sequences have been proposed. The two most common crystallization sequences have very distinct P–T conditions and paths and differ in whether low-P/moderate- to high-T (And zone) metamorphism preceded or postdated Ky-Sil zone conditions. Ky → Sil → And sequences may form by a continuum of tectonometamorphic events that drive isothermal decompression from Ky zone conditions through the Sil zone to shallow (And zone) crustal levels, such as during extension ± magmatism that ultimately results in thin, hot crust. In sequences with early andalusite (And → Ky → Sil), formation of andalusite is typically unrelated to the later Barrovian metamorphism that formed Ky-Sil at moderate P–T conditions. Two other sequences have been reported but are much rarer: (3) Ky → And → Sil (three localities), and (4) And → Sil → Ky (two, possibly three localities). In these, Ky-zone metamorphism occurred as a distinct tectonometamorphic episode before or after the one that formed And-Sil. In cases in which Al₂SiO₅ polymorphs occur in veins or shear zones, the effects of deformation and fluids, including the possibility of non-lithostatic pressure effects, must be considered. Three-Al₂SiO₅ polymorph-bearing rocks from Lesjaverk, Norway, have not previously been described in the literature, and are interpreted here as forming by the sequence And → Ky → Sil. Metamorphic conditions determined using Zr-in-rutile thermometry indicate peak T = 600–640 °C at moderate P near the Ky-Sil boundary, similar to the peak conditions reported for other localities with the same crystallization sequence. These results are consistent with the finding that And → Ky → Sil rocks record lower P–T conditions than the other reported sequences.